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arXiv: 26 January 2012

2012-01-26T07:30:00.009-08:00

Hubble flow variance and the cosmic rest frame

Authors: David L. Wiltshire, Peter R. Smale, Teppo Mattsson, Richard Watkins

We characterize the radial and angular variance of the Hubble flow in the COMPOSITE sample of 4534 galaxy distances. Independent of any cosmological assumptions other than the existence of a suitably averaged linear Hubble law, we find with decisive Bayesian evidence (ln B >> 5) that the Hubble constant averaged in spherical radial shells is closer to its global value when referred to the rest frame of the Local Group rather than to the standard rest frame of the Cosmic Microwave Background (CMB) radiation. Angular averages reveal a dipole structure in the Hubble flow variance, correlated with structures within a sphere of radius 30/h - 60/h Mpc. Furthermore, the angular map of Hubble flow variance is found to coincide with the angular map of the residual CMB temperature dipole in the Local Group rest frame, with correlation coefficient -0.92. This suggests a new mechanism for the origin of the CMB dipole: in addition to a local boost it is generated by differences in the distance to the surface of last scattering, of a maximum +/- 0.35/h Mpc, which arise from foreground structures within 60/h Mpc, a 0.6% effect. The dipole feature is accounted for by our position in a filamentary sheet between Local Voids and Local Walls, producing a foreground density gradient on scales up to 60/h Mpc on opposite sides of the sky. This result potentially eliminates problems of interpretation of "bulk flows". Furthermore, anomalies associated with large angles in the CMB anisotropy spectrum, and also the dark flow inferred from the kinetic Sunyaev-Zel'dovich effect on small angular scales, need to be critically re-examined.

The Direction of Gravity

Authors: Eric V. Linder

arXiv:1201.5127v1

Gravity directs the paths of light rays and the growth of structure. Moreover, gravity on cosmological scales does not simply point down: it accelerates the universal expansion by pulling outward, either due to a highly negative pressure dark energy or an extension of general relativity. We examine methods to test the properties of gravity through cosmological measurements. We then consider specific possibilities for a sound gravitational theory based on the Galileon shift symmetry. The evolution of the laws of gravity from the early universe to the present acceleration to the future fate -- the paths of gravity -- carries rich information on this fundamental force of physics and on the mystery of dark energy.

arXiv: 25 January 2012

2012-01-26T07:30:00.007-08:00

Measuring cosmological distances by coalescing binaries

Authors: Ivan De Martino, Salvatore Capozziello, Mariafelicia De Laurentis, Michelangelo Formisano

arXiv:1201.5087v1

Gravitational waves detected from well-localized inspiraling binaries would allow us to determine, directly and independently, binary luminosity and redshift. In this case, such systems could behave as "standard candles" providing an excellent probe of cosmic distances up to z <0.1 and complementing other indicators of cosmological distance ladder.

arXiv: 24 January 2012

2012-01-26T07:30:00.005-08:00

Evidence for Quadratic Tidal Tensor Bias from the Halo Bispectrum

Authors: Tobias Baldauf, Uros Seljak, Vincent Desjacques, Patrick McDonald

arXiv:1201.4827v1

The relation between the clustering properties of luminous matter in the form of galaxies and the underlying dark matter distribution is of fundamental importance for the interpretation of ongoing and upcoming galaxy surveys. The so called local bias model, where galaxy density is a function of local matter density, is frequently discussed as a means to infer the matter power spectrum or correlation function from the measured galaxy correlation. However, gravitational evolution generates a term quadratic in the tidal tensor and thus non-local in the density field, even if this term is absent in the initial conditions (Lagrangian space). Because the term is quadratic, it contributes as a loop correction to the power spectrum, so the standard linear bias picture still applies on large scales, however, it contributes at leading order to the bispectrum for which it is significant on all scales. Such a term could also be present in Lagrangian space if halo formation were influenced by the tidal field. We measure the corresponding coupling strengths from the matter-matter-halo bispectrum in numerical simulations and find a non-vanishing coefficient for the tidal tensor term. We find no scale dependence of the bias parameters up to k=0.1 h/Mpc and that the tidal effect is increasing with halo mass. While the Lagrangian bias picture is a better description of our results than the Eulerian bias picture, our results suggest that there might be a tidal tensor bias already in the initial conditions. We also find that the coefficients of the quadratic density term deviate quite strongly from the theoretical predictions based on the spherical collapse model and a universal mass function. Both quadratic density and tidal tensor bias terms must be included in the modeling of galaxy clustering of current and future surveys if one wants to achieve the high precision cosmology promise of these datasets.

Singularity phenomena in viable f(R) gravity

Authors: Chung-Chi Lee, Chao-Qiang Geng, Louis Yang

arXiv:1201.4546v2

The curvature singularity in viable f(R) gravity models is examined when the background density is dense. This singularity could be eliminated by adding the $R^{2}$ term in the Lagrangian. Some of cosmological consequences, in particular the source for the scalar mode of gravitational waves, are discussed.

arXiv: 23 Januaru 2012

2012-01-26T07:30:00.003-08:00

The hybrid inflation waterfall and the primordial curvature perturbation

Authors: David H. Lyth

arXiv:1201.4312v1

Without demanding a specific form for the inflaton potential, we obtain an estimate of the contribution to the curvature perturbation generated during the linear era of the hybrid inflation waterfall. The spectrum of this contribution peaks at some wavenumber $k=k_*$, and goes like $k^3$ for $k\ll k_*$, making it typically negligible on cosmological scales. The scale $k_*$ can be outside the horizon at the end of inflation, in which case $\zeta=- (g^2 - \vev{g^2})$ with $g$ gaussian. Taking this into account, the cosmological bound on the abundance of black holes is likely to be satisfied if the curvaton mass $m$ much bigger than the Hubble parameter $H$, but is likely to be violated if $m\lsim H$. Coming to the contribution to $\zeta$ from the rest of the waterfall, we are led to consider the use of the `end-of-inflation' formula, giving the contribution to $\zeta$ generated during a sufficiently sharp transition from nearly-exponential inflation to non-inflation, and we state for the first time the criterion for the transition to be sufficiently sharp. Our formulas are applied to supersymmetric GUT inflation and to supernatural/running-mass inflation

arXiv: 20 January 2012

2012-01-26T07:30:00.001-08:00

Testing gravity with halo density profiles observed through gravitational lensing

Authors: Tatsuya Narikawa, Kazuhiro Yamamoto

arXiv:1201.4037v1

We present a new test of the modified gravity endowed with the Vainshtein mechanism with the density profile of a galaxy cluster halo observed through gravitational lensing. A scalar degree of freedom in the galileon modified gravity is screened by the Vainshtein mechanism to recover Newtonian gravity on high-density regions, however it might not be completely hidden on the outer side of a cluster of galaxies. Then the modified gravity might yield an observational signature in a surface mass density of a cluster of galaxies measured through gravitational lensing, since the scalar field could contribute to the lensing potential. We investigate how the breaking of the Vainshtein mechanism affects the surface mass density observed through gravitational lensing, assuming that the density profile of a cluster of galaxies follows the original Navarro-Frenk-White (NFW) profile, the generalized NFW profile and the Einasto profile. We compare the theoretical predictions with observational results of the surface mass density reported recently by other researchers. We obtain constraints on the amplitude and the typical scale of the breaking of the Vainshtein mechanism in a subclass of the generalized galileon model.

Title: The density profiles of Dark Matter halos in Spiral Galaxies

Authors: Gianluca Castignani, Noemi Frusciante, Daniele Vernieri, Paolo Salucci

arXiv:1201.3998v1

In spiral galaxies, we explain their non-Keplerian rotation curves (RCs) by means of a non-luminous component embedding the stellar-gaseous disks. Understanding the detailed properties of this component (labelled Dark Matter, DM) is one of the most pressing issues of Cosmology. We investigate the recent relationship (claimed by Walker et al. 2010) between $r $, the galaxy radial coordinate, and the dark halo contribution to the circular velocity at $r$, {\it a}) in the framework of the Universal Rotation Curve (URC) paradigm and directly {\it b}) by means of the kinematics of a large sample of Dark matter dominated spirals. We find a general agreement between the W+10 claim, the distribution of DM emerging from URC and that inferred in the (low luminosity) objects. We show that the emerging phenomenology, linking the spiral's luminosity, radii and circular velocities, implies an evident inconsistency with (naive) predictions in the $\Lambda$CDM scenario.

Dark Matter: A Brief Review

Authors: Annika H. G. Peter

arXiv:1201.3942v1

From astronomical observations, we know that dark matter exists, makes up 23% of the mass budget of the Universe, clusters strongly to form the load-bearing frame of structure for galaxy formation, and hardly interacts with ordinary matter except gravitationally. However, this information is not enough to identify the particle specie(s) that make up dark matter. As such, the problem of determining the identity of dark matter has largely shifted to the fields of astroparticle and particle physics. In this talk, I will review the current status of the search for the nature of dark matter. I will provide an introduction to possible particle candidates for dark matter and highlight recent experimental astroparticle- and particle-physics results that constrain the properties of those candidates. Given the absence of detections in those experiments, I will advocate a return of the problem of dark-matter identification to astronomy, and show what kinds of theoretical and observational work might be used to pin down the nature of dark matter once and for all. This talk is intended for a broad astronomy audience.

A closer look at CMB constraints on WIMP dark matter

Authors: Aravind Natarajan

arXiv:1201.3939v1

We use Cosmic Microwave Background data from the WMAP, SPT, BICEP, and QUaD experiments to obtain constraints on the dark matter particle mass $m_\chi$, and show that the combined data requires $m_\chi > 7.6$ GeV at the 95% confidence level for the $\chi \chi \rightarrow b \bar b$ channel. We examine whether the bound on $m_\chi$ is sensitive to $\sigma_8$ measurements made by galaxy cluster observations. The large uncertainty in $\sigma_8$ and the degeneracy with $\Omega_{\rm m}$ allow only small improvements in the dark matter mass bound. Increasing the number of effective neutrino-like degrees of freedom to $N_{\rm eff} = 3.85$ improves the mass bound to $m_\chi > 8.6$ GeV at 95% confidence, for the $\chi \chi \rightarrow b \bar b$ channel. We also study models in which dark matter halos at $z<60$ reionize the Universe. We compute the Ostriker-Vishniac power resulting from partial reionization at intermediate redshifts $10<z<60$, but find the effect to be small. We discuss the importance of the large angle polarization as a complementary probe of dark matter annihilation. By performing Monte Carlo simulations, we show that future experiments that measure the $EE$ power spectrum from $20 < l < 50$ can exclude $m_\chi \sim$ 10 GeV at the 2 (3) $\sigma$ level provided the error bars are smaller than 4 (3) $\times$ cosmic variance.

arXiv: 30 December 2011

2012-01-02T06:31:00.001-08:00

Viability of the cluster mass function formalism in parametrised modified gravity

Authors: Daniel B. Thomas, Carlo R. Contaldi

arXiv:1112.6378v1

Model-independent parametrisations for examining departures from General Relativity have been increasingly studied over the past few years. Various observables have been used to constrain the parameters and forecasts for future surveys have been carried out. In one such forecast, galaxy cluster counts were used to constrain the parameters. Here, we carry out a limited set of $N$-body simulations, with a modified Poisson equation, to examine the accuracy of existing mass functions for modified gravity cosmologies. As well as altering the gravitational calculation, we include the effect of a screening scale to ensure consistency of the theory with solar system tests. Our results suggest that if a screening scale exists its effect can be taken into account in the cluster count calculation through its effect on the linear matter power spectrum. If this is done, the accuracy of the standard mass function formalism in modified gravity theories with reasonably small departures from General Relativity, as tested in this work, is comparable to the standard case.

Discrimination between Lambda-CDM, quintessence, and modified gravity models using wide area surveys

Authors: Houri Ziaeepour

arXiv:1112.6025v1

In the past decade or so observations of supernovae, Large Scale Structures (LSS), and Cosmic Microwave Background (CMB) have confirmed the presence of what is called dark energy, and measured its density as well as the value of other cosmological parameters according to concordance - Lambda-CDM model with few percent uncertainties. Next generation of surveys will allow to distinguish between a Lambda-CDM and alternative models such as modified gravity and (interacting)-quintessence models. In this work we parametrize homogeneous and anisotropic components of matter density in the context of interacting dark energy models with the goal discriminating between f(R) modified gravity and its generalization, and interacting dark energy models, for which we also propose a phenomenological description of energy-momentum conservation equations inspired by particle physics. It is based on the fact that the simplest interactions between particles/fields are elastic scattering and decay. The parametrization of growth rate proposed here is nonetheless general and can be used to constrain other interactions. We also present a crude estimation of the accuracy of the measurement of these parameters using Euclid and Planck surveys data.

Measuring CMB non-Gaussianity as a probe of Inflation and Cosmic Strings

Authors: D.M. Regan

arXiv:1112.5899v1

The leading candidate for the very early universe is described by a period of rapid expansion known as inflation. While the standard paradigm invokes a single slow-rolling field, many different models may be constructed which fit the current observational evidence. In this work we outline theoretical and observational studies of non-Gaussian fluctuations produced by models of inflation and by cosmic strings - topological defects that may be generated in the very early universe during a phase transition. In particular, we consider the imprint of cosmic strings on the cosmic microwave background (CMB) and describe a formalism for the measurement of general four-point correlation functions, or trispectra, using the CMB. In addition we describe the application of our methodology to non-Gaussian signals imprinted in the large scale structure of the universe. Such deviations from Gaussianity are generally expressed in terms of the so-called bispectrum and trispectrum.

arXiv: 26 December 2011

2011-12-27T09:26:00.001-08:00

Formation of primordial black holes from non-Gaussian perturbations produced in waterfall transition

Authors: Edgar Bugaev, Peter Klimai

arXiv:1112.5601v1

We consider the process of primordial black hole (PBH) formation originated from primordial curvature perturbations produced during waterfall transition (with tachyonic instability), at the end of hybrid inflation. It is known that in such inflation models, rather large values of curvature perturbation amplitudes can be reached, which can potentially cause a significant PBH production in the early Universe. The probability distributions of density perturbation amplitudes in this case can be strongly non-Gaussian, which requires a special treatment. We calculated PBH abundances and PBH mass spectra for the model, and analyzed their dependence on model parameters. We obtained the constraints on the parameters of the inflationary potential, using the available limits on $\beta_{PBH}$.

Dark Matter Halo Profiles of Massive Clusters: Theory vs. Observations

Authors: Suman Bhattacharya, Salman Habib, Katrin Heitmann (Argonne/ KICP/ U. Chicago)

arXiv:1112.5479v1

We study dark matter halo profiles using a suite of numerical simulations. We carry out (gravity-only) simulations of the current concordance LCDM cosmology, covering a halo mass range of 2.10^(12) to 2.10^(15) solar masses and a redshift range of z=0-2, dictated primarily by cluster observation considerations. We find that the shape of the concentration-mass (c-M) relation flattens at high redshift and this flattening of the slope is naturally expressed if c is written as a function of the peak height parameter, \nu. Although the logarithmic slope of the c-M relation changes with redshift, that of the (c-\nu) relation is effectively constant over the redshift range z=0-2. The amplitude of c(\nu) varies by about 30% from z=0-2 over the mass range for massive clusters. The (c-\nu) relation is, however, not universal. We use a large suite of simulations covering the currently allowed wCDM parameter space and show that the (c-\nu) relation varies by about +/- 20 % when cosmological parameters are varied. We find that the distribution of the concentrations can be well-fit by a Gaussian with variance, \sigma_c=0.33c, where the ratio of the variance to the mean, \sigma_c/c, is independent of the radius at which the concentration is defined, the dynamical state of the halo, and the underlying cosmology. We compare our simulation predictions with current results obtained from (primarily low) redshift observations and find good agreement with the observational data for massive clusters of mass > 4.10^(14) solar masses, but there are disagreements at lower masses. Because of uncertainty in observational systematics and modeling of baryonic physics, the significance of these discrepancies remains to be understood. (Abridged)

arXiv: 23 December 2011

2011-12-27T09:24:00.001-08:00

Cosmological Constraints from Sunyaev-Zel'dovich-Selected Clusters with X-ray Observations in the First 178 Square Degrees of the South Pole Telescope Survey

Authors: B. A. Benson, T. de Haan, J. P. Dudley, C. L. Reichardt, K. A. Aird, K. Andersson, R. Armstrong, M. Bautz, M. Bayliss, G. Bazin, L. E. Bleem, M. Brodwin, J. E. Carlstrom, C. L. Chang, H. M. Cho, A. Clocchiatti, T. M. Crawford, A. T. Crites, S. Desai, M. A. Dobbs, R. J. Foley, W. R. Forman, E. M. George, M. D. Gladders, N. W. Halverson, F. W. High, G. P. Holder, W. L. Holzapfel, S. Hoover, J. D. Hrubes, C. Jones, M. Joy, R. Keisler, L. Knox, A. T. Lee, E. M. Leitch, J. Liu, M. Lueker, D. Luong-Van, A. Mantz, D. P. Marrone, M. McDonald, J. J. McMahon, J. Mehl, S. S. Meyer, L. Mocanu, J. J. Mohr, T. E. Montroy, S. S. Murray, T. Natoli, S. Padin, T. Plagge, C. Pryke, A. Rest, J. Ruel, J. E. Ruhl, B. R. Saliwanchik, A. Saro, K. K. Schaffer, L. Shaw, E. Shirokoff, J. Song, H. G. Spieler, B. Stalder,

Z. Staniszewski, A. A. Stark, K. Story, C. W. Stubbs, R. Suhada, A. van Engelen, K. Vanderlinde, J. D. Vieira, A. Vikhlinin, R. Williamson, O. Zahn, A. Zenteno

et al. (12 additional authors not shown)

arXiv:1112.5435v1

We use measurements from the South Pole Telescope (SPT) Sunyaev Zel'dovich (SZ) cluster survey in combination with X-ray measurements to constrain cosmological parameters. We present a statistical method that fits for the scaling relations of the SZ and X-ray cluster observables with mass while jointly fitting for cosmology. The method is generalizable to multiple cluster observables, and self-consistently accounts for the effects of the cluster selection and uncertainties in cluster mass calibration on the derived cosmological constraints. We apply this method to a data set consisting of an SZ-selected catalog of 18 galaxy clusters at z > 0.3 from the first 178 deg2 of the 2500 deg2 SPT-SZ survey, with 14 clusters having X-ray observations from either Chandra or XMM. Assuming a spatially flat LCDM cosmological model, we find the SPT cluster sample constrain sigma_8 (Omega_m/0.25)^0.30 = 0.785 +- 0.037. In combination with measurements of the CMB power spectrum from the SPT and the seven-year WMAP data, the SPT cluster sample constrain sigma_8 = 0.795 +- 0.016 and Omega_m = 0.255 +- 0.016, a factor of 1.5 improvement on each parameter over the CMB data alone. We consider several extensions beyond the LCDM model by including the following as free parameters: the dark energy equation of state (w), the sum of the neutrino masses (sum mnu), the effective number of relativistic species (Neff), and a primordial non-Gaussianity (fNL). We find that adding the SPT cluster data significantly improves the constraints on w and sum mnu beyond those found when using measurements of the CMB, supernovae, baryon acoustic oscillations, and the Hubble constant. Considering each extension independently, we best constrain w=-0.973 +- 0.063 and the sum of neutrino masses sum mnu < 0.28 eV at 95% confidence, a factor of 1.25 and 1.4 improvement, respectively, over the constraints without clusters. [abbrev.]

The shapes of Milky Way satellites: looking for signatures of tidal stirring

Authors: Ewa L. Lokas, Steven R. Majewski, Stelios Kazantzidis, Lucio Mayer, Jeffrey L. Carlin, David L. Nidever, Leonidas A. Moustakas

arXiv:1112.5336v1

We study the shapes of Milky Way satellites in the context of the tidal stirring scenario for the formation of dwarf spheroidal galaxies. The standard procedures used to measure shapes involve smoothing and binning of data and thus may not be sufficient to detect subtle structural properties like bars. Taking advantage of the fact that in nearby dwarfs photometry of individual stars is available we introduce discrete measures of shape based on the two-dimensional inertia tensor and the Fourier bar mode. We apply these measures of shape first to a variety of simulated dwarf galaxies formed via tidal stirring of disks embedded in dark matter halos and orbiting the Milky Way. In addition to strong mass loss and randomization of stellar orbits, the disks undergo morphological transformation which typically involves the formation of a triaxial bar after the first pericenter passage. These tidally induced bars persist for a few Gyr before being shortened towards a more spherical shape if the tidal force is strong enough. We test this prediction by measuring in a similar way the shape of nearby dwarf galaxies, satellites of the Milky Way. We detect inner bars in Ursa Minor, Sagittarius, LMC and possibly Carina. In addition, six out of eleven studied dwarfs show elongated stellar distributions in the outer parts which may signify transition to the tidal tails. We thus find the shapes of Milky Way satellites to be consistent with the predictions of the tidal stirring model.

Standard Model false vacuum Inflation: correlating the tensor-to-scalar ratio to the top and Higgs masses

Authors: Isabella Masina, Alessio Notari

arXiv:1112.5430v1

For a narrow band of values of the top quark and Higgs boson masses, the Standard Model Higgs potential develops a false minimum at energies of about $10^{16}$ GeV, where primordial Inflation could have happened. A graceful exit to a radiation dominated era is provided e.g. by scalar-tensor gravity models. We pointed out that if Inflation happened in this false minimum, the Higgs boson mass has to be in the range $126.0 \pm 3.5$ GeV, where ATLAS and CMS subsequently reported excesses of events. Here we show that for these values of the Higgs mass, the inflationary gravitational wave background has be discovered with a tensor-to-scalar ratio at hand of future experiments. We suggest that combining cosmological observations with measurements of the top and Higgs masses represents a further test of the hypothesis that the Standard Model false minimum was the source of Inflation in the Universe.

arXiv: 22 December 2011

2011-12-27T09:19:00.001-08:00

An Anomaly in the Angular Distribution of Quasar Spectra

Authors: Michael J. Longo

arXiv:1112.5045v1

Quasars provide our most distant view of the Universe. The Sloan Survey now contains over 100,000 quasar candidates. A careful look at the angular distribution of quasar spectra shows a surprising blue shift toward (alpha, delta) ~ (190{\deg}, 0{\deg}). The angular distribution of the shift appears to be consistent with a large peculiar velocity toward that direction. However, the size of the shift would suggest our peculiar velocity is ~0.2 c, which is two orders of magnitude larger than measures of our peculiar velocity from nearby galaxies and cosmic microwave background (CMB) measurements. It is too large to explain as a systematic error in the quasar magnitudes. The direction is consistent with that of the reported anomalies in the CMB, the so-called "axis of evil". The angular pattern of the blue shift appears to be consistent with the existence of an expanding bubble universe in that direction, which could also explain the CMB anomalies.

The WiggleZ Dark Energy Survey: Cosmological neutrino mass constraint from blue high-redshift galaxies

arXiv:1112.4940v1

The absolute neutrino mass scale is currently unknown, but can be constrained from cosmology. The WiggleZ high redshift star-forming blue galaxy sample is less sensitive to systematics from non-linear structure formation, redshift-space distortions and galaxy bias than previous surveys. We obtain a upper limit on the sum of neutrino masses of 0.60eV (95% confidence) for WiggleZ+Wilkinson Microwave Anisotropy Probe. Combining with priors on the Hubble Parameter and the baryon acoustic oscillation scale gives an upper limit of 0.29eV, which is the strongest neutrino mass constraint derived from spectroscopic galaxy redshift surveys.

Primordial Magnetic Field Effects on the CMB and Large Scale Structure

Authors: Dai G. Yamazaki, Kiyotomo Ichiki, Toshitaka Kajino, Grant J. Mathew

arXiv:1112.4922v1

Magnetic fields are everywhere in nature and they play an important role in every astronomical environment which involves the formation of plasma and currents. It is natural therefore to suppose that magnetic fields could be present in the turbulent high temperature environment of the big bang. Such a primordial magnetic field (PMF) would be expected to manifest itself in the cosmic microwave background (CMB) temperature and polarization anisotropies, and also in the formation of large- scale structure. In this review we summarize the theoretical framework which we have developed to calculate the PMF power spectrum to high precision. Using this formulation, we summarize calculations of the effects of a PMF which take accurate quantitative account of the time evolution of the cut off scale. We review the constructed numerical program, which is without approximation, and an improvement over the approach used in a number of previous works for studying the effect of the PMF on the cosmological perturbations. We demonstrate how the PMF is an important cosmological physical process on small scales. We also summarize the current constraints on the PMF amplitude $B_\lambda$ and the power spectral index $n_B$ which have been deduced from the available CMB observational data by using our computational framework.

New Constraints on Isospin-Violating Dark Matter

Authors: Jason Kumar, David Sanford, Louis E. Strigari

arXiv:1112.4849v1

We derive bounds on the dark matter annihilation cross-section for low-mass (5-20 GeV) dark matter annihilating primarily to up or down quarks, using the Fermi-LAT bound on gamma-rays from Milky Way satellites. For models in which dark matter-Standard Model interactions are mediated by particular contact operators, we show that these bounds can be directly translated into bounds on the dark matter-proton scattering cross-section. For isospin-violating dark matter, these constraints are tight enough to begin to constrain the parameter-space consistent with experimental signals of low-mass dark matter. We discuss possible models that can evade these bounds.

arXiv: 21 December 2011

2011-12-21T19:33:00.001-08:00

Nonlinear cosmological consistency relations and effective matter stresses

Authors: Guillermo Ballesteros, Lukas Hollenstein, Rajeev Kumar Jain, Martin Kunz

arXiv:1112.4837v1

We propose a completely nonlinear framework to construct consistency relations for testing generic cosmological scenarios using the evolution of large scale structure. It is based on the covariant approach in combination with a frame that is purely given by the metric, the normal frame. As an example, we apply this framework to the LCDM model, extending the usual first order conditions on the metric potentials to second order. We argue that working in the normal frame is not only a practical choice but one that closely resembles our actual situation as observers. In this frame, effective pressures and anisotropic stresses appear at second order in perturbation theory, even for dust. We quantify this effect and compare it, for illustration, with the pressure of a generic clustering dark energy fluid and the anisotropic stress in the DGP model. Besides, we also discuss the effect of a mismatch of the potentials on the determination of galaxy bias.

The Fine-Tuning of the Universe for Intelligent Life

Authors: Luke A. Barnes

arXiv:1112.4647v1

The fine-tuning of the universe for intelligent life has received a great deal of attention in recent years, both in the philosophical and scientific literature. The claim is that in the space of possible physical laws, parameters and initial conditions, the set that permits the evolution of intelligent life is very small. I present here a review of the scientific literature, outlining cases of fine-tuning in the classic works of Carter, Carr and Rees, and Barrow and Tipler, as well as more recent work. To sharpen the discussion, the role of the antagonist will be played by Victor Stenger's recent book The Fallacy of Fine-Tuning: Why the Universe is Not Designed for Us. Stenger claims that all known fine-tuning cases can be explained without the need for a multiverse. Many of Stenger's claims will be found to be highly problematic. We will touch on such issues as the logical necessity of the laws of nature; objectivity, invariance and symmetry; theoretical physics and possible universes; entropy in cosmology; cosmic inflation and initial conditions; galaxy formation; the cosmological constant; stars and their formation; the properties of elementary particles and their effect on chemistry and the macroscopic world; the origin of mass; grand unified theories; and the dimensionality of space and time. I also provide an assessment of the multiverse, noting the significant challenges that it must face. I do not attempt to defend any conclusion based on the fine-tuning of the universe for intelligent life. This paper can be viewed as a critique of Stenger's book, or read independently.

arXiv: 20 December 2011

2011-12-19T19:12:00.003-08:00

Cosmic shear bispectrum from second-order perturbations in General Relativity

Authors: Francis Bernardeau, Camille Bonvin, Nicolas Van de Rijt, Filippo Vernizzi

arXiv:1112.4430v1

Future lensing surveys will be nearly full-sky and reach an unprecedented depth, probing scales closer and closer to the Hubble radius. This motivates the study of the cosmic shear beyond the small-angle approximation and including general relativistic corrections that are usually suppressed on sub-Hubble scales. The complete expression of the reduced cosmic shear at second order including all relativistic effects was derived in [1]. In the present paper we compute the resulting cosmic shear bispectrum when all these effects are properly taken into account and we compare it to primordial non-Gaussianity of the local type. The new general relativistic effects are generically smaller than the standard non-linear couplings. However, their relative importance increases at small multipoles and for small redshifts of the sources. The dominant effect among these non standard corrections is due to the inhomogeneity of the source redshift. In the squeezed limit, its amplitude can become of the order of the standard couplings when the redshift of the sources is below 0.5. Moreover, while the standard non-linear couplings depend on the angle between the short and long mode, the relativistic corrections do not and overlap almost totally with local type non-Gaussianity. We find that they can contaminate the search for a primordial local signal by f_NL>10.

Comment on "The Real Problem with MOND" by Scott Dodelson, arXiv:1112.1320

Authors: J. W. Moffat, V. T. Toth

arXiv:1112.4386v1

We comment on arXiv:1112.1320 and point out that baryonic oscillations of the matter power spectrum, while predicted by theories that do not incorporate collisionless cold dark matter, are strongly suppressed by the statistical window function that is used to process finite-sized galaxy samples. We assert that with present-day data sets, the slope of the matter power spectrum is a much stronger indicator of a theory's validity. We also argue that MOND should not be used as a strawman theory as it is not in general representative of modified gravity theories; some theories, notably our scalar-vector-tensor MOdified Gravity (MOG), offer much more successful predictions of cosmological observations.

The Statistics of Cosmological Lyman-alpha Absorption

Authors: Dipak Munshi, Peter Coles, Matteo Viel

arXiv:1112.4334v1

We study the effect of the non-Gaussianity induced by gravitational evolution upon the statistical properties of absorption in quasar (QSO) spectra. Using the generic hierarchical ansatz and the lognormal approximation we derive the analytical expressions for the one-point PDF as well as for the joint two-point probability distribution (2PDF) of transmitted fluxes in two neighbouring QSOs. These flux PDFs are constructed in 3D as well as in projection (i.e. in 2D). The PDFs are constructed by relating the lower-order moments, i.e. cumulants and cumulant correlators, of the fluxes to the 3D neutral hydrogen distribution which is, in turn, expressed as a function of the underlying dark matter distribution. The lower-order moments are next modelled using a generating function formalism in the context of a {\em minimal tree-model} for the higher-order correlation hierarchy. These different approximations give nearly identical results for the range of redshifts probed, and we also find a very good agreement between our predictions and outputs of hydrodynamical simulations. The formalism developed here for the joint statistics of flux-decrements concerning two lines of sight can be extended to multiple lines of sight, which could be particularly important for the 3D reconstruction of the cosmic web from QSO spectra (e.g. in the BOSS survey). These statistics probe the underlying projected neutral hydrogen field and are thus linked to "hot-spots" of absorption. The results for the PDF and the bias presented here use the same functional forms of scaling functions that have previously been employed for the modelling of other cosmological observation such as the Sunyaev-Zel'dovich effect.

What Do Dark Matter Properties Tell Us About Their Mass Assembly Histories?

Authors: Anson W. C. Wong, James E. Taylor

arXiv:1112.4229v1

Individual dark matter halos in cosmological simulations vary widely in their detailed structural properties such as shape, rotation, substructure and degree of internal relaxation. Recent non-parametric (principal component) analyses suggest that a few principal components explain a large fraction of the scatter in halo properties. The main principal component is closely linked with concentration, which in turn is known to be related to the mass accretion history of the halo. Here we examine more generally the connection between mass accretion history and structural parameters. The space of mass accretion histories has principal components of its own. We find that the strongest two can be interpreted as the overall age of the halo and the acceleration or deceleration of growth at late times. These two components only account for $\sim70$%\ of the scatter in mass accretions histories however, due to the stochastic effect of major mergers. Relating structural parameters to formation history, we find that concentration correlates strongly with the early history of the halo, while relaxation correlates with the late history. We examine the inferences about formation history that can be drawn by splitting haloes into subsamples, based on observable properties such as concentration and shape at some final time. This approach suggests interesting possibilities, such as the possibility of defining young and old samples of galaxy clusters in a rigorous, quantitative way, or testing the dynamical assumptions of galaxy formation models empirically.

Modified Newtonian Dynamics: A Review

Authors: Benoit Famaey, Stacy McGaugh

arXiv:1112.3960v1

A wealth of astronomical data indicate the presence of mass discrepancies in the Universe. The motions observed in a variety of classes of extragalactic systems exceed what can be explained by the mass visible in stars and gas. Either (i) there is a vast amount of unseen mass in some novel form - dark matter - or (ii) the data indicate a breakdown of our understanding of gravity on the relevant scales, or (iii) both. Here, we first review a few outstanding challenges for the dark matter interpretation of mass discrepancies in galaxies, purely based on observations and independently of any alternative theoretical framework. We then show that many of these puzzling observations can be summarized by one single scaling relation - Milgrom's law - involving an acceleration constant (or a characteristic surface density) of the order of the square-root of the cosmological constant in natural units. This relation can at present most easily be interpreted as the effect of a single universal force law resulting from a modification of Newtonian dynamics (MOND) on galactic scales. We exhaustively review the current observational successes and problems of this alternative paradigm at all astrophysical scales, and summarize the various theoretical attempts (TeVeS, GEA, BIMOND, and others) made to effectively embed this modification of Newtonian dynamics within a generally covariant theory of gravity.

arXiv: 19 December 2011

2011-12-19T19:12:00.001-08:00

Inflationary magnetic fields spoil the homogeneity and isotropy of the Universe

Authors: Camille Bonvin, Chiara Caprini, Ruth Durrer

arXiv:1112.3897v1

We show that magnetic fields generated during inflation gives rise to a constant mode in the Bardeen potential after inflation, in the radiation era, which is proportional to the magnetic scalar anisotropic stress. The ratio of this constant mode of the Bardeen potential to the background curvature grows in the radiation era, with the fatal consequence of spoiling the homogeneity and isotropy of the Friedmann Lemaitre (FL) Universe. This happens even if back-reaction on the background metric is negligible during inflation, and severely constrains magnetogenesis mechanisms operating during inflation.

arXiv: 16 December 2011

2011-12-19T19:11:00.001-08:00

Vacuum Structure and the Arrow of Time

Authors: Raphael Bousso

arXiv:1112.3341v1

We find ourselves in an extended era of entropy production. Unlike most other observations, the arrow of time is usually regarded as a constraint on initial conditions. I argue, however, that it primarily constrains the vacuum structure of the theory. I exhibit simple scalar field potentials in which low-entropy initial conditions are not necessary, or not sufficient, for an arrow of time to arise. I argue that the string theory landscape gives rise to an arrow of time independently of the initial entropy, assuming a plausible condition on the lifetime of its metastable vacua. In particular, a theory of initial conditions that favors large initial entropy, such as the Hartle-Hawking proposal, is not ruled out by observation. The dynamical resolution of the arrow of time problem arises from the same structural properties of the string landscape that allow it to solve the cosmological constant problem without producing an empty universe, particularly its high dimensionality and the large difference in vacuum energy between neighboring vacua.

Cycles in the Multiverse

Authors: Matthew C. Johnson, Jean-Luc Lehners

arXiv:1112.3360v1

Eternal inflation is a seemingly generic consequence of theories that give rise to accelerated expansion of the universe and possess multiple vacuum states. Making predictions in an eternally inflating universe is notoriously difficult because one must compare infinite quantities, and a wide variety of regulating procedures yield radically different results. This is the measure problem of eternal inflation. In this paper, we analyze models of eternal inflation which allow for the possibility of cyclic bubble universes: in each bubble, standard cosmological evolution is re-played over and over again. Eternal inflation can generically arise in cyclic models that include a dark energy dominated phase. In such models, several problematic consequences of standard regulating procedures, such as the youngness and Boltzmann Brain problems, are substantially alleviated. We discuss the implications for making predictions in cyclic models, as well as some general implications for understanding the measure problem in eternal inflation.

arXiv: 15 December 2011

2011-12-19T19:09:00.001-08:00

Seeing in the dark -- II. Cosmic shear in the Sloan Digital Sky Survey

Authors: Eric M. Huff, Tim Eifler, Christopher M. Hirata, Rachel Mandelbaum, David Schlegel, Uros Seljak

arXiv:1112.3143v1

Statistical weak lensing by large-scale structure -- cosmic shear -- is a promising cosmological tool, which has motivated the design of several large upcoming surveys. Here, we present a measurement of cosmic shear using coadded Sloan Digital Sky Survey (SDSS) imaging in 168 square degrees of the equatorial region, with r<23.5 and i<22.5, a source number density of 2.2 galaxies per square arcminute and median redshift of 0.52. These coadds were generated using a new method described in the companion Paper I that was intended to minimise systematic errors in the lensing measurement due to coherent PSF anisotropies that are otherwise prevalent in the SDSS imaging data. We present measurements of cosmic shear out to angular separations of 2 degrees, along with systematics tests that (combined with those from Paper I on the catalogue generation) demonstrate that our results are dominated by statistical rather than systematic errors. Assuming a cosmological model corresponding to WMAP7 and allowing only the amplitude of matter fluctuations to vary, we find a best-fit value of sigma_8=0.636 +0.109 -0.154 (1-sigma); without systematic errors this would be sigma_8=0.636 +0.099 -0.137 (1-sigma). Assuming a flat LCDM model, the combined constraints with WMAP7 are sigma_8=0.784 +0.028 -0.026 (1-sigma), +0.055 -0.054 (2-sigma) and Omega_m h^2=0.1303 +0.0047 -0.0048 (1-sigma)+0.009 -0.009 (2-sigma); the 2-sigma error ranges are respectively 14 and 17 per cent smaller than WMAP7 alone. Aside from the intrinsic value of such cosmological constraints from the growth of structure, we identify some important lessons for upcoming surveys that may face similar issues when combining multi-epoch data to measure cosmic shear.

Surface mass density of the Einasto family of dark matter haloes: Are they Sersic-like?

Authors: Barun Kumar Dhar (1), Liliya L.R. Williams (1) ((1) School of Physics and Astronomy, University of Minnesota, Minneapolis, USA)

arXiv:1112.3116v1

Recent advances in N-body simulations of dark matter halos have shown that three-parameter models, in particular the Einasto profile characterized by d ln {\rho}(r)/d ln r / r with a shape parameter {\alpha} < 0.3, are able to produce better fits to the 3D spatial density profiles than two-parameter models like the Navarro, Frenk and White (NFW), and Moore et al. profiles.
In this paper, we present for the first time an analytically motivated form for the 2D surface mass density of the Einasto family of dark matter haloes, in terms of the 3D spatial density parameters for a wide range of the shape parameter 0.1 < {\alpha} < 1. Our model describes a projected (2D) Einasto profile remarkably well between 0 and (3 - 5) r_{200}, with errors less than 0.3 per cent for {\alpha} < 0.3 and less than 2 per cent for {\alpha} as large as 1. This model (in 2D) can thus be used to fit strong and weak lensing observations of galaxies and clusters whose total spatial (3D) density distributions are believed to be Einasto-like. Further, given the dependence of our model on the 3D parameters, one can reliably estimate structural parameters of the spatial (3D) density from 2D observations. We also consider a Sersic-like parametrization for the above family of projected Einasto profiles and observe that fits with a Sersic profile are sensitive to whether one fits the projected density in linear scale or logarithmic scale and yield widely varying results. Structural parameters of Einasto-like systems, inferred from fits with a Sersic profile, should be used with caution.

arXiv: 14 December 2011

2011-12-19T19:08:00.005-08:00

The Impact of Assuming Flatness in the Determination of Neutrino Properties from Cosmological Data

Authors: Aaron Smith, Maria Archidiacono, Asantha Cooray, Francesco De Bernardis, Alessandro Melchiorri, Joseph Smidt

arXiv:1112.3006v1

Recent cosmological data have provided new constraints on the number of neutrino species and on the neutrino mass. However these constraints depend on assumptions related to the underlying cosmology. Since a correlation is expected between the number of effective neutrinos, N_{eff}, the neutrino mass \sum M_\nu, and the curvature of the universe, \Omega_k, it is useful to investigate the current constraints in the framework of a non-flat universe. In this paper we update the constraints on neutrino parameters by making use of the latest Cosmic Microwave Background (CMB) data from the ACT and SPT experiments and consider the possibility of a universe with non-zero curvature. We first find a negative correlation between curvature and N_{eff} with a correlation coefficient of -0.36 and we place new constraints on N_{eff} and \Omega_k, with N_{eff} = 4.03 +/- 0.45 and 10^3, \Omega_k = -4.46 +/- 5.24. Thus, even when \Omega_k is allowed to vary, N_{eff} = 3 is still disfavored at 95% confidence. The correlation between neutrino mass and curvature is much stronger, with a correlation coefficient of 0.78 that shifts the 95% upper limit of \sum M_\nu < 0.446 eV to \sum M_\nu < 0.948 eV. Thus, the impact of assuming flatness in neutrino cosmology is significant and an essential consideration with future experiments.

Is Dark Energy Falsifiable?

Authors: Carl H. Gibson (University of California at San Diego), Rudolph E. Schild (Harvard University)

arXiv:1112.2758v1

Is the accelerating expansion of the Universe true, inferred through observations of distant supernovae, and is the implied existence of an enormous amount of anti-gravitational dark energy material driving the accelerating expansion of the universe also true? To be physically useful these propositions must be falsifiable; that is, subject to observational tests that could render them false, and both fail when viscous, diffusive, astro-biological and turbulence effects are included in the interpretation of observations. A more plausible explanation of negative stresses producing the big bang is turbulence at Planck temperatures. Inflation results from gluon viscous stresses at the strong force transition. Anti-gravitational (dark energy) turbulence stresses are powerful but only temporary. No permanent dark energy is needed. At the plasma-gas transition, viscous stresses cause fragmentation of plasma proto-galaxies into dark matter clumps of primordial gas planets, each of which falsifies dark-energy cold-dark-matter cosmologies. Clumps of these planets form all stars, and explain the alleged accelerating expansion of the universe as a systematic dimming error of Supernovae Ia by light scattered in the hot turbulent atmospheres of evaporated planets surrounding central white dwarf stars.

arXiv: 12 December 2011

2011-12-19T19:08:00.003-08:00

The Paths of Gravity in Galileon Cosmology

Authors: Stephen A. Appleby, Eric V. Linder

arXiv:1112.1981v1

Galileon gravity offers a robust gravitational theory for explaining cosmic acceleration, having a rich phenomenology of testable behaviors. We explore three classes of Galileon models -- standard uncoupled, and linearly or derivatively coupled to matter -- investigating the expansion history with particular attention to early time and late time attractors, as well as the linear perturbations. From the relativistic and nonrelativistic Poisson equations we calculate the generalizations of the gravitational strength (Newton's constant), deriving its early and late time behavior. By scanning through the parameters we derive distributions of the gravitational strength at various epochs and trace the paths of gravity in its evolution. Using ghost-free and stability criteria we restrict the allowed parameter space, finding in particular that the linear and derivative coupled models are severely constrained by classical instabilities in the early universe.

What if ... General Relativity is not the theory?

Authors: Orfeu Bertolami

arXiv:1112.2048v1

The nature of gravity is fundamental to understand the scaffolding of the Universe and its evolution. Einstein's general theory of relativity has been scrutinized for over ninety five years and shown to describe accurately all phenomena from the solar system to the Universe. However, this success is achieved in the case of the largest scales provided one admits contributions to energy-momentum tensor involving dark components such as dark energy and dark matter. Moreover, the theory has well known shortcomings, such as the problem of singularities, the cosmological constant problem and the well known initial conditions problems for the cosmological description. Furthermore, general relativity also does not fit the well known procedures that allow for the quantization of the other fundamental interactions. In this discussion we briefly review the experimental bounds on the foundational principles of general relativity, and present three recent proposals to extend general relativity or, at least, to regard it under different perspectives.

What Drives the Growth of Black Holes?

Authors: David M. Alexander (Durham), Ryan C. Hickox (Durham, Dartmouth)

arXiv:1112.1949v1

Massive black holes (BHs) are at once exotic and yet ubiquitous, residing in the centers of massive galaxies in the local Universe. Recent years have seen remarkable advances in our understanding of how these BHs form and grow over cosmic time, during which they are revealed as active galactic nuclei (AGN). However, despite decades of research, we still lack a coherent picture of the physical drivers of BH growth, the connection between the growth of BHs and their host galaxies, the role of large-scale environment on the fueling of BHs, and the impact of BH-driven outflows on the growth of galaxies. In this paper we review our progress in addressing these key issues, motivated by the science presented at the "What Drives the Growth of Black Holes?" workshop held at Durham on 26th-29th July 2010, and discuss how these questions may be tackled with current and future facilities

arXiv: 9 December 2011

2011-12-19T19:08:00.001-08:00

Constraints on the CMB temperature redshift dependence from SZ and distance measurements

Authors: A. Avgoustidis, G. Luzzi, C. J. A. P. Martins, A. M. R. V. L. Monteiro

arXiv:1112.1862v1

The relation between redshift and the CMB temperature, $T_{CMB}(z)=T_0(1+z)$ is a key prediction of standard cosmology, but is violated in many non-standard models. Constraining possible deviations to this law is an effective way to test the $\Lambda$CDM paradigm and search for hints of new physics. We present state-of-the-art constraints, using both direct and indirect measurements. In particular, we point out that in models where photons can be created or destroyed, not only does the temperature-redshift relation change, but so does the distance duality relation, and these departures from the standard behaviour are related, providing us with an opportunity to improve constraints. We show that current datasets limit possible deviations of the form $T_{CMB}(z)=T_0(1+z)^{1-\beta}$ to be $\beta=0.004\pm0.016$ up to a redshift $z\sim 3$. We also discuss how, with the next generation of space and ground-based experiments, these constraints can be improved by more than one order of magnitude.

The kinetic Sunyaev-Zel'dovich signal from inhomogeneous reionization: a parameter space study

Authors: Andrei Mesinger, Matthew McQuinn, David Spergel

arXiv:1112.1820v1

[ABRIDGED] Inhomogeneous reionization acts as a source of arcminute-scale anisotropies in the cosmic microwave background (CMB), the most important of which is the kinetic Sunyaev-Zel'dovich (kSZ) effect. Observational efforts with the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) are poised to detect this signal for the first time. Indeed, recent SPT measurements place a bound on the dimensionless kSZ power spectrum around a multipole of l=3000 of P_tot < 2.8 (6) micro K^2 at 95% C.L., by ignoring (allowing) correlations between the thermal Sunyaev-Zel'dovich (tSZ) effect and the cosmic infrared background (CIB). To interpret these and upcoming observations, we compute the kSZ signal from a suite of ~ 100 reionization models using the publicly available code 21cmFAST. Our physically motivated reionization models are parameterized by the ionizing efficiency of high-redshift galaxies, the minimum virial temperature of halos capable of hosting stars, and the ionizing photon mean free path. We predict the contribution of patchy reionization to be P_patchy = 1.5-3.5 micro K^2. Therefore, even when conservatively adopting a low estimate of the post-reionization signal, P_OV ~ 2 micro K^2, none of our models are consistent with the aggressive 2sigma SPT bound that does not include correlations. This implies that either: (i) the early stages of reionization occurred in a much more homogeneous manner than suggested by the stellar-driven scenarios we explore, such as would be the case if, e.g., very high energy X-rays or exotic particles contributed significantly; and/or (ii) that there is a significant correlation between the CIB and the tSZ. On the other hand, the conservative SPT bound is compatible with all of our models, and is on the boarder of constraining reionization.

Fables of reconstruction: controlling bias in the dark energy equation of state

Authors: Robert G. Crittenden, Gong-Bo Zhao, Levon Pogosian, Lado Samushia, Xinmin Zhang

arXiv:1112.1693v1

We develop an efficient, non-parametric Bayesian method for reconstructing the time evolution of the dark energy equation of state w(z) from observational data. Of particular importance is the choice of prior, which must be chosen carefully to minimise variance and bias in the reconstruction. Using a principal component analysis, we show how a correlated prior can be used to create a smooth reconstruction and also avoid bias in the mean behaviour of w(z). We test our method using Wiener reconstructions based on Fisher matrix projections, and also against more realistic MCMC analyses of simulated data sets for Planck and a future space-based dark energy mission. While the accuracy of our reconstruction depends on the smoothness of the assumed w(z), the relative error for typical dark energy models is <10% out to redshift z=1.5.

arXiv: 5 December 2011

2011-12-12T06:50:00.001-08:00

Nonlinear Evolution of Cosmological Structures in Warm Dark Matter Models

Authors: Aurel Schneider, Robert E. Smith, Andrea V. Maccio, Ben Moore

arXiv:1112.0330v1

The dark energy dominated warm dark matter (WDM) model is a promising alternative cosmological scenario. We explore large-scale structure formation in this paradigm. We do this in two different ways: with the halo model approach and with the help of an ensemble of high resolution N-body simulations. Combining these quasi-independent approaches, leads to a physical understanding of the important processes which shape the formation of structures. We take a detailed look at the halo mass function, the concentrations and the linear halo bias of WDM. In all cases we find interesting deviations with respect to CDM. In particular, the concentration-mass relation displays a turnover for group scale dark matter haloes, for the case of WDM particles with masses of the order ~0.25 keV. This may be interpreted as a hint for top-down structure formation on small scales. We implement our results into the halo model and find much better agreement with simulations. On small scales the WDM halo model now performs as well as its CDM counterpart.

Probing dark energy with the next generation X-ray surveys of galaxy clusters

Authors: B. Sartoris, S. Borgani, P. Rosati, J. Weller

arXiv:1112.0327v1

We present forecasts on the capability of future wide-area high-sensitivity X-ray surveys of galaxy clusters to yield constraints on the parameters defining the Dark Energy (DE) equation of state (EoS). Our analysis is carried out for future X-ray surveys which have enough sensitivity to provide accurate measurements of X-ray mass proxies and Fe-line based redshifts for about 2x10^4 clusters. We base our analysis on the Fisher Matrix formalism, by combining information on the cluster number counts and power spectrum, also including, for the first time in the analysis of the large scale cluster distribution, the effect of linear redshift-space distortions (RSDs). This study is performed with the main purpose of dissecting the cosmological information provided by geometrical and growth tests, which are both included in the analysis of number counts and clustering of galaxy clusters. We compare cosmological constraints obtained by assuming different levels of prior knowledge of the parameters which define the observable-mass X-ray relation. This comparison further demonstrates the fundamental importance of having a well calibrated observable-mass relation and, most importantly, its redshift evolution. Such a calibration can be achieved only by having at least $\sim 10^3$ net photon counts for each cluster included in the survey. We show that RSDs in the power spectrum analysis carry important cosmological information also when traced with galaxy clusters and the DE FoM increases by a factor of 8. Besides confirming the potential that large cluster surveys have in constraining the nature of DE, our analysis emphasizes that a full exploitation of the cosmological information carried by such surveys requires not only a large statistic but also a robust measurement of the mass proxies and redshifts for a significant fraction of the cluster sample, derived from the same X-ray survey data.

The Dark Matter Density Profile of the Fornax Dwarf

Authors: John Jardel, Karl Gebhardt

arXiv:1112.0319v1

We construct axisymmetric Schwarzschild models to measure the mass profile of the local group dwarf galaxy Fornax. These models require no assumptions to be made about the orbital anisotropy of the stars, as is the case for commonly used Jeans models. We test a variety of parameterizations of dark matter density profiles and find cored models with uniform density rho_c = (1.6 +/- 0.1) x 10^-2 M_sun pc^-3 fit significantly better than the cuspy halos predicted by cold dark matter simulations. We also construct models with an intermediate-mass black hole, but are unable to make a detection. We place a 1-sigma upper limit on the mass of a potential intermediate-mass black hole at M_BH < 3.2 x 10^4 M_sun.

The Cosmic History of Black Hole Growth from Deep Multiwavelength Surveys

Authors: Ezequiel Treister (U. de Concepcion), C. Megan Urry (Yale)

arXiv:1112.0320v1

Significant progress has been made in the last few years on understanding how supermassive black holes form and grow. In this paper, we begin by reviewing the spectral signatures of Active Galactic Nuclei (AGN) ranging from radio to hard X-ray wavelengths. We then describe the most commonly used methods to find these sources, including optical/UV, radio, infrared and X-ray emission and optical emission lines. We then describe the main observational properties of the obscured and unobscured AGN population. Finally, we summarize the cosmic history of black hole accretion, i.e., when in the history of the Universe supermassive black holes were getting most of their mass. We finish with a summary of open questions and a description of planned and future observatories that are going to help answer them.

arXiv: 6 December 2011

2011-12-12T06:49:00.001-08:00

Constraining Thawing Dark Energy using Galaxy Cluster Number Counts

Authors: N. Chandrachani Devi, T. Roy Choudhury, Anjan A. Sen

arXiv:1112.0728v1

We study the formation of galaxy clusters in the presence of thawing class of scalar field dark energy. We consider cases where the scalar field has canonical as well non canonical kinetic term in its action. We also consider various forms for the potential of the scalar field e.g, linear, quadratic, inverse quadratic, exponential as well as Pseudo-Nambu-Goldstone Boson (PNGB) type. Moreover we investigate situation where dark energy is homogeneous as well as situation where dark energy takes part in the virialization process. We use the Sheth-Torman formalism while calculating the number density of galaxy clusters. Our results show that cluster number density for different dark energy models have significant deviation from the corresponding value for the \Lambda CDM case. The deviation is more for higher redshifts. Moreover the tachyon type scalar field with linear potential has the highest deviation from the \Lambda CDM case. For the total cluster number counts, different dark energy models can have substantial deviation from \Lambda CDM and this deviation is most significant around $z \sim 0.5 \sim 1$ for all the models we considered.

Lensing of 21-cm Fluctuations by Primordial Gravitational Waves

Authors: Laura Book, Marc Kamionkowski, Fabian Schmidt

arXiv:1112.0567v1

Weak-gravitational-lensing distortions to the intensity pattern of 21-cm radiation from the dark ages can be decomposed geometrically into curl and curl-free components. Lensing by primordial gravitational waves induces a curl component, while the contribution from lensing by density fluctuations is strongly suppressed. Angular fluctuations in the 21-cm background extend to very small angular scales, and measurements at different frequencies probe different shells in redshift space. There is thus a huge trove of information with which to reconstruct the curl component of the lensing field, allowing tensor-to-scalar ratios conceivably as small as r ~ 10^{-9} - far smaller than those currently accessible - to be probed.

Influence of Microlensing on Spectral Anomaly of Lensed Objects

Authors: Sasa Z. Simic, Luka C. Popovic, Predrag Jovanovic

arXiv:1112.0646v1

Here we consider the influence of the microlensing on the spectrum of a lensed object taking into account that composite emission is coming from different regions arranged subsequently around the central source. We assumed that the lensed object has three regions with the black body emission; first the innermost with the highest temperature of $10^4K$, second and third (located around the central) with slightly lower temperatures $7.5\cdot10^3$ and $5\cdot10^3$K, respectively. Than we explore the flux anomaly in lensed object due to microlensing. We compare U,V and B spectra of a such source. This results show that, due to microlensing, in a spectroscopically stratified object a flux anomaly is present.

arXiv: 29 Novemeber 2011

2011-12-04T17:24:00.001-08:00

Cosmic microwave background constraints on the duration and timing of reionization from the South Pole Telescope

arXiv:1111.6386v1

The epoch of reionization is a milestone of cosmological structure formation, marking the birth of the first objects massive enough to yield large numbers of ionizing photons. The mechanism and timescale of reionization remain largely unknown. Measurements of the CMB Doppler effect from ionizing bubbles embedded in large-scale velocity streams (the patchy kinetic Sunyaev-Zel'dovich effect) can constrain the duration of reionization. When combined with large-scale CMB polarization measurements, the evolution of the ionized fraction can be inferred. Using new multi-frequency data from the South Pole Telescope (SPT), we show that the ionized fraction evolved relatively rapidly. For our basic foreground model, we find the kinetic Sunyaev-Zel'dovich power sourced by reionization at l=3000 to be <= 2.1 micro K^2 at 95% CL. Using reionization simulations, we translate this to a limit on the duration of reionization of Delta z <= 4.4 (95% CL). We find that this constraint depends on assumptions about the angular correlation between the thermal Sunyaev-Zel'dovich power and the cosmic infrared background (CIB). Introducing the degree of correlation as a free parameter, we find that the limits on kSZ power weaken to <= 4.9 micro K^2, implying Delta z <= 7.9 (95% CL). We combine the SPT constraint on the duration of reionization with the WMAP7 measurement of the integrated optical depth to probe the cosmic ionization history. We find that reionization ended with 95% CL at z > 7.2 under the assumption of no tSZ-CIB correlation, and z>5.8 when correlations are allowed. Improved constraints from the full SPT data set in conjunction with upcoming Herschel and Planck data should detect extended reionization at >95% CL provided Delta z >= 4. (abbreviated)

arXiv: 28 November 2011

2011-11-27T19:19:00.001-08:00

Back Reaction from Walls

Authors: Enea Di Dio, Marc Vonlanthen, Ruth Durrer

arXiv:1111.5764v1

We study the distance-redshift relation in a universe filled with 'walls' of pressure-less dust separated by under dense regions. We show that as long as the density contrast of the walls is small, or the diameter of the under dense regions is much smaller than the Hubble scale, the distance-redshift relation remains close to what is obtained in a Friedmann universe. However, when arbitrary density contrasts are allowed, every prescribed distance-redshift relation can be reproduced with such models.

Halo expansion in cosmological hydro simulations: towards a baryonic solution of the cusp/core problem in massive spirals

Authors: Andrea V. Maccio', Greg Stinson, Chris B. Brook, James Wadsley, H.M.P. Couchman, Sijing Shen, Brad K. Gibson, Tom Quinn

arXiv:1111.5620v1

A clear prediction of the Cold Dark Matter model is the existence of cuspy dark matter halo density profiles on all mass scales. This is not in agreement with the observed rotation curves of spiral galaxies, challenging on small scales the otherwise successful CDM paradigm. In this work we employ high resolution cosmological hydro-dynamical simulations to study the effects of dissipative processes on the inner distribution of dark matter in Milky-Way like objects (M~1e12 Msun). Our simulations include supernova feedback, and the effects of the radiation pressure of massive stars before they explode as supernovae. The increased stellar feedback results in the expansion of the dark matter halo instead of contraction with respect to N-body simulations. Baryons are able to erase the dark matter cuspy distribution creating a flat, cored, dark matter density profile in the central several kpc of a massive Milky-Way like halo. The profile is well fit by a Burkert profile, with fitting parameters consistent with the observations. In addition, we obtain flat rotation curves as well as extended, exponential stellar disk profiles. While the stellar disk we obtain is still partially too thick to resemble the MW thin disk, this pilot study shows that there is enough energy available in the baryonic component to alter the dark matter distribution even in massive disc galaxies, providing a possible solution to the long standing problem of cusps vs. cores.

The Shapes and Alignments of Dark Matter Halos

Authors: Michael D. Schneider, Carlos S. Frenk, Shaun Cole

arXiv:1111.5616v1

We present measurements of the triaxial dark matter halo shapes and alignment correlation functions in the Millennium and Millennium-2 dark matter N-body simulations. These two simulations allow us to measure the distributions of halo shapes down to 10% of the virial radius over a halo mass range of 6E9 - 2E14 M_sun/h. We largely confirm previous results on the distributions of halo axis ratios as a function of halo mass, but we find that the median angle between halo major axes at different halo radii can vary by a factor of 2 between the Millennium-1 and 2 simulations because of the different mass resolution. Thus, error in the shape determinations from limited resolution is potentially degenerate with the misalignment of halo inner and outer shapes used to constrain Brightest Cluster Galaxy alignments in previous works. We also present simplifying parameterizations for the 3-D halo-mass alignment correlation functions that are necessary ingredients for triaxial halo models of large-scale structure and models of galaxy intrinsic alignments as contaminants for cosmic shear surveys. We measure strong alignments between halos of all masses and the surrounding dark matter overdensities out to several tens of Mpc/h, in agreement with observed shear-galaxy and cluster shape correlations. We use these measurements to forecast the contribution to the weak lensing signal around galaxy clusters from correlated mass along the line-of-sight. For prolate clusters with major axes aligned with the line-of-sight the fraction of the weak lensing signal from mass external to the cluster can be twice that predicted if the excess halo alignment correlation is assumed to be zero.

Gravitino cosmology with a very light neutralino

Authors: Herbi K. Dreiner (Bonn), Marja Hanussek (Bonn), Jong-Soo Kim (Dortmund, Adelaide), Subir Sarkar (Oxford)

arXiv:1111.5715v1

It has been shown that very light or even massless neutralinos are consistent with all current experiments, given non-universal gaugino masses. Furthermore, a very light neutralino is consistent with astrophysical bounds from supernov{\ae} and cosmological bounds on dark matter. Here we study the cosmological constraints on this scenario from Big Bang nucleosynthesis taking gravitinos into account and find that a very light neutralino is even favoured by current observations.

arXiv: 24 November 2011

2011-11-25T05:53:00.005-08:00

The polytropic approximation and X-ray scaling relations: constraints on gas and dark matter profiles for galaxy groups and clusters

Authors: Pedro R. Capelo, Paolo S. Coppi, Priyamvada Natarajan

arXiv:1111.5573v1

We constrain gas and dark matter (DM) parameters of galaxy groups and clusters, by comparing X-ray scaling relations to theoretical expectations, obtained assuming that the gas is in hydrostatic equilibrium with the DM and follows a polytropic relation. We vary four parameters: the gas polytropic index Gamma, its temperature at large radii T_xi, the DM logarithmic slope at large radii zeta and its concentration c_vir. When comparing the model to the observed mass-temperature (M-T) relation of local clusters, our results are independent of both T_xi and c_vir. We thus obtain constraints on Gamma, by fixing the DM profile, and on zeta, by fixing the gas profile. For an NFW DM profile, we find that 6/5<Gamma<13/10, which is consistent with numerical simulations and observations of individual clusters. Taking 6/5<Gamma<13/10 allows the DM profile to be slightly steeper than the NFW profile at large radii. Upon including local groups, we constrain the mass-dependence of Gamma and the value of T_xi. Interestingly, with Gamma=6/5 and zeta=-3, we reproduce the observed steepening/breaking of the M-T relation at low M, if 10^6 K<T_xi<10^7 K, consistent with simulations and observations of the warm-hot intergalactic medium. When extrapolated to high redshift z, the model with a constant Gamma reproduces the expected self-similar behaviour. We also account for the observed, non-self-similar relations provided by some high-z clusters, as they provide constraints on the evolution of Gamma. Comparing our model to the observed luminosity-temperature relation, we discriminate between different M-c_vir relations: a weak dependence of c_vir on M is currently preferred by data. This simple theoretical model accounts for much of the complexity of recent, improved X-ray scaling relations, provided that we allow for a mild dependence of Gamma on M or for T_xi consistent with intercluster values. [abridged]

arXiv: 23 November 2011

2011-11-25T05:53:00.003-08:00

Axions in Cold Dark Matter and Inflation Models

Authors: Luca Visinelli

arXiv:1111.5281v1

The subjects of this thesis are the invisible axion and the more general family of axion-like particles.
The invisible axion is a hypothetical elementary particle and a cold dark matter candidate. I present an improved computation of the constraints on the parameter space of the cold dark matter axion in the standard cosmology, that includes the contributions from anharmonicities in the axion potential and from the decay of axionic strings. In this scenario, I update the value of the mass of the cold dark matter axion, finding the value $(67\pm17){\rm \mu eV}$, approximately one order of magnitude larger than previous computations.
The effect of nonstandard cosmological scenarios on the parameter space of axion cold dark matter is studied for the first time. In particular, I consider the cases of low-temperature reheating and kination cosmologies, and I show that the mass of the cold dark matter axion can differ from the value in the standard cosmological scenario by orders of magnitude.
Finally, I consider the family of axion-like particles, assuming that these particles serve as the inflaton in the context of warm inflation. I find that the axion energy scale $f$, which in the standard inflation scenario is of the order of the Planck mass, can be lowered to the much safer Grand Unification Theory scale $f \sim 10^{16}{\rm GeV}$. I also constrain the parameter space and the amount of gravitational waves from this model, using results from the Wilkinson Microwave Anisotropy Probe 7-year data.

Cross-correlating Sunyaev-Zel'dovich and Weak Lensing Maps

Authors: Dipak Munshi, Shahab Joudaki, Peter Coles, Joseph Smidt

arXiv:1111.5010v1

We present novel statistical tools to cross-correlate frequency cleaned thermal Sunyaev-Zel'dovich (tSZ) maps and tomographic weak lensing (wl) convergence maps. Moving beyond the lowest order cross-correlation, we introduce a hierarchy of mixed higher-order statistics, the cumulants and cumulant correlators, to analyze non-Gaussianity in real space, as well as corresponding polyspectra in the harmonic domain. Using these moments, we derive analytical expressions for the joint two-point probability distribution function (2PDF) for smoothed tSZ (y_s) and convergence (\kappa_s) maps. The presence of tomographic information allows us to study the evolution of higher order {\em mixed} tSZ-weak lensing statistics with redshift. We express the joint PDFs p_{\kappa y}(\kappa_s,y_s) in terms of individual one-point PDFs (p_{\kappa}(\kappa_s), p_y(y_s)) and the relevant bias functions (b_{\kappa}(\kappa_s), b_y(y_s)). Analytical results for two different regimes are presented that correspond to the small and large angular smoothing scales. Results are also obtained for corresponding {\em hot spots} in the tSZ and convergence maps. In addition to results based on hierarchical techniques and perturbative methods, we present results of calculations based on the lognormal approximation. The analytical expressions derived here are generic and applicable to cross-correlation studies of arbitrary tracers of large scale structure including e.g. that of tSZ and soft X-ray background.

arXiv: 22 November 2011

2011-11-25T05:53:00.001-08:00

One Gravitational Potential or Two? Forecasts and Tests

Authors: Edmund Bertschinger

arXiv:1111.4659v1

The metric of a perturbed Robertson-Walker spacetime is characterized by three functions: a scale-factor giving the expansion history and two potentials which generalize the single potential of Newtonian gravity. The Newtonian potential induces peculiar velocities and, from these, the growth of matter fluctuations. Massless particles respond equally to the Newtonian potential and to a curvature potential. The difference of the two potentials, called the gravitational slip, is predicted to be very small in general relativity but can be substantial in modified gravity theories. The two potentials can be measured, and gravity tested on cosmological scales, by combining weak gravitational lensing or the Integrated Sachs-Wolfe effect with galaxy peculiar velocities or clustering.

An improved fitting formula for the dark matter bispectrum

Authors: Héctor Gil-Marín, Christian Wagner, Frantzeska Fragkoudi, Raul Jimenez, Licia Verde

arXiv:1111.4477v1

In this paper we present an improved fitting formula for the dark matter bispectrum motivated by the previous phenomenological approach of Scoccimarro & Couchman (2001). We use a set of LCDM simulations to calibrate the fitting parameters in the k-range of 0.03 h/Mpc<k<0.4 h/Mpc and in the redshift range of 0<z<1.5. This new proposed fit describes well the BAO-features although it was not designed to. The deviation between the simulations output and our analytic prediction is typically less than 5% and in the worst case is never above 10%. We envision that this new analytic fitting formula will be very useful in providing reliable predictions for the non-linear dark matter bispectrum for LCDM models.

arXiv: 21 November 2011

2011-11-25T05:52:00.001-08:00

Straightening the Density-Displacement Relation with a Logarithmic Transform

Authors: Bridget L. Falck, Mark C. Neyrinck, Miguel A. Aragon-Calvo, Guilhem Lavaux, Alexander S. Szalay

arXiv:1111.4466v1

We investigate the use of a logarithmic density variable in estimating the Lagrangian displacement field, motivated by the success of a logarithmic transformation in restoring information to the matter power spectrum. The logarithmic relation is an extension of the linear relation, motivated by the continuity equation, in which the density field is assumed to be proportional to the divergence of the displacement field; we compare the linear and logarithmic relations by measuring both of these fields directly in a cosmological N-body simulation. The relative success of the logarithmic and linear relations depends on the scale at which the density field is smoothed. Thus we explore several ways of measuring the density field, including Cloud-In-Cell smoothing, adaptive smoothing, and the (scale-independent) Delaunay tessellation, and we use both a Fourier space and a geometrical tessellation approach to measuring the divergence. We find that the relation between the divergence of the displacement field and the density is significantly tighter with a logarithmic density variable, especially at low redshifts and for very small (~2 Mpc/h) smoothing scales. We find that the grid-based methods are more reliable than the tessellation-based method of calculating both the density and the divergence fields, though in both cases the logarithmic relation works better in the appropriate regime, which corresponds to nonlinear scales for the grid-based methods and low densities for the tessellation-based method.

Detection of branon dark matter with gamma ray telescopes

Authors: J.A.R. Cembranos, A. de la Cruz-Dombriz, V. Gammaldi, A.L. Maroto

arXiv:1111.4448v1

Branons are new degrees of freedom that appear in flexible brane-world models corresponding to brane fluctuations. These new fields can behave as standard weakly interacting massive particles (WIMPs) with a significant associated thermal relic density. We analyze the present constraints from their spontaneous annihilations into photons for EGRET, Fermi-LAT and MAGIC, and the prospects for detection in future Cherenkov telescopes. In particular, we focus on possible signals coming from the Galactic Center and different dwarf spheroidals, such as Draco, Sagittarius, Canis Major and SEGUE 1. We conclude that for those targets, present observations are below the sensitivity limits for branon detection by assuming standard dark matter distributions and no additional boost factors. However, future experiments such as CTA could be able to detect gamma-ray photons coming from the annihilation of branons with masses higher than 150 GeV.

Limits on Self-Interacting Dark Matter

Authors: Chris Kouvaris

arXiv:1111.4364v1

We impose new severe constraints on the self-interactions of fermionic asymmetric dark matter based on observations of nearby old neutron stars. WIMP self-interactions mediated by Yukawa- type interactions can lower significantly the number of WIMPs necessary for gravitational collapse of the WIMP population accumulated in a neutron star. Even nearby neutron stars located at regions of low dark matter density can accrete sufficient number of WIMPs that can potentially collapse, form a mini black hole, and destroy the host star. Based on this, we derive constraints on the WIMP self-interactions which in some cases are by several orders of magnitude stricter than the ones from the bullet cluster (which are currently considered the most stringent).

NGC 2419 does not challenge MOND, Part 2

Authors: R.H. Sanders

arXiv:1111.4334v1

I argue that, despite repeated claims of Ibata et al., the globular cluster NGC 2419 does not pose a problem for modified Newtonian dynamics (MOND). I present a new polytropic model with a running polytropic index. This model provides an improved representation of the radial distribution of surface brightness while maintaining a reasonable fit to the velocity dispersion profile. Although it may be argued that the differences with these observations remain large compared to the reported random errors, there are several undetectable systematic effects which render a formal likelihood analysis irrelevant. I comment generally upon these effects and upon the intrinsic limitations of pressure supported objects as tests of gravity.

arXiv: 17 November 2011

2011-11-24T14:28:00.001-08:00

Tracing the Dark Matter Sheet in Phase Space

Authors: Tom Abel, Oliver Hahn, Ralf Kaehler (KIPAC/Stanford/SLAC)

arXiv:1111.3944v1

The primordial dark matter velocity dispersion is small compared to the velocities attained during structure formation. Its initial density distribution is close to uniform and it occupies an initial sheet in phase space that is single valued in velocity space. Because of gravitational forces this three dimensional manifold evolves in phase space without ever tearing, conserving phase-space volume and preserving the connectivity of nearby points. N-body simulations already follow the motion of this sheet in phase space. This fact can be used to extract full fine-grained phase-space-structure information from existing cosmological N-body simulations. Particles are considered as the vertices of an unstructured three dimensional mesh, moving in six dimensional phase-space. On this mesh, mass density and momentum are uniquely defined. We show how to obtain the space density of the fluid, detect caustics, and count the number of streams as well as their individual contributions to any point in configuration-space. We calculate the bulk velocity, local velocity dispersions, and densities from the sheet - all without averaging over control volumes. This gives a wealth of new information about dark matter fluid flow which had previously been thought of as inaccessible to N-body simulations. We outline how this mapping may be used to create new accurate collisionless fluid simulation codes that may be able to overcome the sparse sampling and unphysical two-body effects that plague current N-body techniques.

Constraining Dynamical Dark Energy Models through the Abundance of High-Redshift Supermassive Black Holes

Authors: Alessandra Lamastra, Nicola Menci, Fabrizio Fiore, Cinzia Di Porto, Luca Amendola

arXiv:1111.3800v1

We compute the number density of massive Black Holes (BHs) at the centre of galaxies at z=6 in different Dynamical Dark Energy (DDE) cosmologies, and compare it with existing observational lower limits, to derive constraints on the evolution of the Dark Energy equation of state parameter w. Our approach only assumes the canonical scenario for structure formation from the collapse of overdense regions of the Dark Matter dominated primordial density field on progressively larger scales; the Black Hole accretion and merging rate have been maximized in the computation so as to obtain robust constraints on w and on its look-back time derivative w_a. Our results provide independent constraints complementary to those obtained by combining Supernovae, Cosmic Microwave Background and Baryonic Acoustic Oscillations; while the latter concern combinations of w_0 and w_a leaving the time evolution of the state parameter w_a highly unconstrained, the BH abundance mainly provide upper limits on w_a, only weakly depending on w_0. Combined with the existing constraints, our results significantly restrict the allowed region in DDE parameter space, ruling out DDE models not providing cosmic time and fast growth factor large enough to allow for the building up of the observed abundance of BHs; in particular, models with -1.2 \leq w_0 \leq -1 and positive redshift evolution w_a > 0.8 - completely consistent with previous constraints - are strongly disfavoured by our independent constraints from BH abundance. Such range of parameters corresponds to "Quintom" DDE models, with w crossing -1 starting from larger values

Magnification by Galaxy Group Dark Matter Halos

Authors: Jes Ford, Hendrik Hildebrandt, Ludovic Van Waerbeke, Alexie Leauthaud, Peter Capak, Alexis Finoguenov, Masayuki Tanaka, Matthew R. George, Jason Rhodes

arXiv:1111.3698v1

We report on the detection of gravitational lensing magnification by a population of low-mass galaxy groups, at a significance level of 4.8 sigma. Using X-ray selected groups in the COSMOS 1.64 deg^2 field, and high-redshift Lyman-break galaxies as sources, we measure a lensing induced angular cross-correlation between the samples. After satisfying consistency checks that demonstrate we have indeed detected a magnification signal, and are not suffering from contamination by physical overlap of samples, we proceed to implement an optimally-weighted cross-correlation function to further boost the signal-to-noise of the measurement. Interpreting this optimally weighted measurement allows us to study properties of the lensing groups. We find that the group mass profiles are well fit by the Singular Isothermal Sphere (SIS) model, and we implement a multi-SIS fit that recovers a distribution of lens masses consistent with the values that have already been well measured using the weak lensing shear technique. We argue that future weak lensing studies will need to incorporate magnification along with shear, both to reduce residual systematics and to make full use of all available source information, in an effort to maximize scientific yield of the observations.

arXiv: 15 November 2011

2011-11-21T06:29:00.001-08:00

Power Spectrum Estimation from Peculiar Velocity Catalogues

Authors: Edward Macaulay, Hume A. Feldman, Pedro G. Ferreira, Andrew H. Jaffe, Shankar Agarwal, Michael J. Hudson, Richard Watkins

arXiv:1111.3338v1

The peculiar velocities of galaxies are an inherently valuable cosmological probe, providing an unbiased estimate of the distribution of matter on scales much larger than the depth of the survey. Much research interest has been motivated by the high dipole moment of our local peculiar velocity field, which suggests a large scale excess in the matter power spectrum, and can appear to be in some tension with the LCDM model. We use a composite catalogue of 4,537 peculiar velocity measurements with a characteristic depth of 33 h-1 Mpc to estimate the matter power spectrum. We compare the constraints with this method, directly studying the full peculiar velocity catalogue, to results from Macaulay et al. (2011), studying minimum variance moments of the velocity field, as calculated by Watkins, Feldman & Hudson (2009) and Feldman, Watkins & Hudson (2010). We find good agreement with the LCDM model on scales of k > 0.01 h Mpc-1. We find an excess of power on scales of k < 0.01 h Mpc-1, although with a 1 sigma uncertainty which includes the LCDM model. We find that the uncertainty in the excess at these scales is larger than an alternative result studying only moments of the velocity field, which is due to the minimum variance weights used to calculate the moments. At small scales, we are able to clearly discriminate between linear and nonlinear clustering in simulated peculiar velocity catalogues, and find some evidence (although less clear) for linear clustering in the real peculiar velocity data.

Chameleon Effects on Small Scale Structure and the Baryonic Jeans Mass

Authors: Katherine Jones-Smith

arXiv:1111.3227v1

In the framework of Newtonian cosmology or general relativity it is simple to derive a mass scale below which collapsed structures are relatively devoid of baryons. We examine how the inclusion of a chameleon scalar field affects this baryonic Jeans mass, bearing in mind both the canonical case of a gravitational-strength coupling between the scalar field and matter, as well as the strong coupling regime wherein the coupling is very large. We find that baryon effects persist down to smaller scales in a chameleon theory than they do in ordinary general relativity, especially in the case of strong coupling. Several potentially observable consequences of this are identified.

The effects of baryons on the halo mass function

Authors: Weiguang Cui, Stefano Borgani, Klaus Dolag, Giuseppe Murante, Luca Tornatore

arXiv:1111.3066v1

We present an analysis of the effects of baryon physics on the halo mass function. The analysis is based on simulations of a cosmological volume. Besides a Dark Matter (DM) only simulation, we also carry out two other hydrodynamical simulations. We identified halos using a spherical overdensity algorithm and their masses are computed at three different overdensities (with respect to the critical one), $\Delta_c=200$, 500 and 1500. We find the fractional difference between halo masses in the hydrodynamical and in the DM simulations to be almost constant, at least for halos more massive than $\log (M_{\Delta_c} / \hMsun)\geq 13.5$. In this range, mass increase in the hydrodynamical simulations is of about 4-5 per cent at $\Delta_c=500$ and $\sim 1$ - 2 per cent at $\Delta_c=200$. Quite interestingly, these differences are nearly the same for both radiative and non-radiative simulations. Such variations of halo masses induce corresponding variations of the halo mass function (HMF). At $z=0$, the HMFs for GH and CSF simulations are close to the DM one, with differences of $\mincir 3$ per cent at $\Delta_c = 200$, and $\simeq 7$ per cent at $\Delta_c=500$, with $\sim 10$ - 20 per cent differences reached at $\Delta_c = 1500$. At this higher overdensity, the increase of the HMF for the radiative case is larger by about a factor 2 with respect to the non--radiative case. Assuming a constant mass shift to rescale the HMF from the hydrodynamic to the DM simulations, brings the HMF difference with respect to the DM case to be consistent with zero. Our results have interesting implications to bracket uncertainties in the mass function calibration associated to the uncertain baryon physics, in view of cosmological applications of future large surveys of galaxy clusters. (Abridged)

Search for Unknown Dark Matter Satellites of the Milky Way

Authors: Alex Drlica-Wagner, Ping Wang, Elliott Bloom, Louis Strigari, for the Fermi-LAT Collaboration

arXiv:1111.3358v1

We present a search for Galactic dark matter (DM) satellites using the Large Area Telescope (LAT). N-body simulations based on the Lambda-CDM model of cosmology predict a large number of as yet unobserved Galactic DM satellites. These satellites could potentially produce gamma rays through the self-annihilation of DM particles. Some DM satellites are expected to have hard gamma-ray spectra, finite angular extents, and a lack of counterparts at other wavelengths. We searched for LAT sources with these characteristics. We found no candidate DM satellites matching these criteria in one year of LAT data and interpreted this result in the context of N-body simulations.

Newtonian and Relativistic Cosmologies

Authors: Stephen R. Green, Robert M. Wald

arXiv:1111.2997v1

Cosmological N-body simulations are now being performed using Newtonian gravity on scales larger than the Hubble radius. It is well known that a uniformly expanding, homogeneous ball of dust in Newtonian gravity satisfies the same equations as arise in relativistic FLRW cosmology, and it also is known that a correspondence between Newtonian and relativistic dust cosmologies continues to hold in linearized perturbation theory in the marginally bound/spatially flat case. Nevertheless, it is far from obvious that Newtonian gravity can provide a good global description of an inhomogeneous cosmology when there is significant nonlinear dynamical behavior at small scales. We investigate this issue in the light of a perturbative framework that we have recently developed, which allows for such nonlinearity at small scales. We propose a relatively straightforward "dictionary"---which is exact at the linearized level---that maps Newtonian dust cosmologies into general relativistic dust cosmologies, and we use our "ordering scheme" to determine the degree to which the resulting metric and matter distribution solve Einstein's equation. We find that Einstein's equation fails to hold at "order 1" at small scales and at "order $\epsilon$" at large scales. We then find the additional corrections to the metric and matter distribution needed to satisfy Einstein's equation to these orders. While these corrections are of some interest in their own right, our main purpose in calculating them is that their smallness should provide a criterion for the validity of the original dictionary (as well as simplified versions of this dictionary). We expect that, in realistic Newtonian cosmologies, these additional corrections will be very small; if so, this should provide strong justification for the use of Newtonian simulations to describe relativistic cosmologies, even on scales larger than the Hubble radius.

Universality of the Volume Bound in Slow-Roll Eternal Inflation

Authors: Sergei Dubovsky, Leonardo Senatore, Giovanni Villadoro

arXiv:1111.1725v1

It has recently been shown that in single field slow-roll inflation the total volume cannot grow by a factor larger than e^(S_dS/2) without becoming infinite. The bound is saturated exactly at the phase transition to eternal inflation where the probability to produce infinite volume becomes non zero. We show that the bound holds sharply also in any space-time dimensions, when arbitrary higher-dimensional operators are included and in the multi-field inflationary case. The relation with the entropy of de Sitter and the universality of the bound strengthen the case for a deeper holographic interpretation. As a spin-off we provide the formalism to compute the probability distribution of the volume after inflation for generic multi-field models, which might help to address questions about the population of vacua of the landscape during slow-roll inflation.

arXiv: 14 Novemebr 2011

2011-11-14T06:17:00.001-08:00

Physics of Dark Matter in the Light of Dark Atoms

Authors: Maxim Yu. Khlopov

arXiv:1111.2838v1

Direct searches for dark matter lead to serious problems for simple models with stable neutral Weakly Interacting Massive Particles (WIMPs) as candidates for dark matter. A possibility is discussed that new stable quarks and charged leptons exist and are hidden from detection, being bound in neutral dark atoms of composite dark matter. Stable -2 charged particles $O^{--}$ are bound with primordial helium in O-helium (OHe) atoms, being specific nuclear interacting form of composite Warmer than Cold dark matter. Slowed down in the terrestrial matter, OHe is elusive for direct methods of underground dark matter detection based on the search for effects of nuclear recoil in WIMP-nucleus collisions. The positive results of DAMA experiments can be explained as annual modulation of radiative capture of O-helium by nuclei. In the framework of this approach test of DAMA results in detectors with other chemical content becomes a nontrivial task, while the experimental search of stable charged particles at LHC or in cosmic rays acquires a meaning of direct test for composite dark matter scenario.

The Cosmic Abundance of Classical Milky Way Satellites

Authors: Louis E. Strigari, Risa H. Wechsler

arXiv:1111.2611v1

We study the abundance of satellites akin to the brightest, classical dwarf spheroidals around galaxies similar in magnitude and isolation to the Milky Way and M31 in the Sloan Digital Sky Survey. From a combination of photometric and spectroscopic redshifts, we bound the mean and the intrinsic scatter in the number of satellites down to ten magnitudes fainter than the Milky Way. Restricting to magnitudes brighter than Sagittarius, we show that the Milky Way is not a significant statistical outlier in its population of classical dwarf spheroidals. At fainter magnitudes, we find an upper limit of 13 on the mean number of satellites brighter than the Fornax dwarf spheroidal. Methods to improve these limits that utilize full photometric redshift distributions hold promise, but are currently limited by incompleteness at the very lowest redshifts. Theoretical models are left to explain why the majority of dark matter subhalos that orbit Milky Way-like galaxies are inefficient at making galaxies at the luminosity scale of the brightest dwarf spheroidals, or why these subhalos predicted by Lambda-CDM do not exist.

Halos and Voids in f(R) Gravity

Authors: Baojiu Li (Durham and Cambridge), Gong-Bo Zhao (Portsmouth), Kazuya Koyama (Portsmouth)

arXiv:1111.2602v1

In this paper, we study the distribution of dark matter halos and voids using high resolution simulations in f(R) gravity models with the chameleon mechanism to screen the fifth force in dense environment. For dark matter halos, we show that the semi-analytic thin shell condition, with a suitably-defined environment, provides a good approximation to describe the mass and environmental dependence of the screening of the fifth force in halos. Due to stronger gravity, there are far more massive halos and large voids in f(R) models compared with the \Lambda CDM model. The numbers of voids with an effective radius of 15Mpc/h are twice and four times as many as those in \Lambda CDM for f(R) models with |f_{R0}|=1e-5 and 1e-4 respectively. This provides a new means to test the models using the upcoming observational data. We also find that halos inside voids are all unscreened in our simulations, which are ideal objects for the gravity test.

Searching For Dark Matter Subhalos In the Fermi-LAT Second Source Catalog

Authors: Alexander V. Belikov, Dan Hooper, Matthew R. Buckley

arXiv:1111.2613v1

The dark matter halo of the Milky Way is expected to contain an abundance of smaller subhalos. These subhalos can be dense and produce potentially observable fluxes of gamma rays. In this paper, we search for dark matter subhalo candidates among the sources in the Fermi-LAT Second Source Catalog which are not currently identified or associated with counterparts at other wavelengths. Of the nine high-significance, high-latitude (|b|>60 degrees), non-variable, unidentified sources contained in this catalog, only one or two are compatible with the spectrum of a dark matter particle heavier than approximately 50-100 GeV. The majority of these nine sources, however, feature a spectrum that is compatible with that predicted from a lighter (~5-40 GeV) dark matter particle. This population is consistent with the number of observable subhalos predicted for a dark matter candidate in this mass range and with an annihilation cross section of a simple thermal relic (sigma v~3x10^{-26} cm^3/s). Observations in the direction of these sources at other wavelengths will be necessary to either reveal their astrophysical nature (as blazars or other active galactic nuclei, for example), or to further support the possibility that they are dark matter subhalos by failing to detect any non-gamma ray counterpart.

arXiv: 11 November 2011

2011-11-13T13:55:00.001-08:00

The sensitivity of BAO Dark Energy Constraints to General Isocurvature Perturbations

Authors: S. Muya Kasanda, C. Zunckel, K. Moodley, B. A. Bassett, P. Okouma

arXiv:1111.2572v1

Baryon Acoustic Oscillation (BAO) surveys will be a leading method for addressing the dark energy challenge in the next decade. We explore in detail the effect of allowing for small amplitude admixtures of general isocurvature perturbations in addition to the dominant adiabatic mode. We find that non-adiabatic initial conditions leave the sound speed unchanged but instead excite different harmonics. These harmonics couple differently to Silk damping, altering the form and evolution of acoustic waves in the baryon-photon fluid prior to decoupling. This modifies not only the scale on which the sound waves imprint onto the baryon distribution, which is used as the standard ruler in BAO surveys, but also the shape, width and height of the BAO peak. We discuss these effects in detail and show how more general initial conditions impact our interpretation of cosmological data in dark energy studies. We find that the inclusion of these additional isocurvature modes leads to an increase in the Dark Energy Task Force Figure of merit by 140% and 60% for the BOSS and ADEPT experiments respectively when considered in conjunction with Planck data. We also show that the incorrect assumption of adiabaticity has the potential to bias our estimates of the dark energy parameters by $3\sigma$ ($1\sigma$) for a single correlated isocurvature mode, and up to $8\sigma$ ($3\sigma$) for three correlated isocurvature modes in the case of the BOSS (ADEPT) experiment. We find that the use of the large scale structure data in conjunction with CMB data improves our ability to measure the contributions of different modes to the initial conditions by as much as 100% for certain modes in the fully correlated case.

arXiv: 10 November 2011

2011-11-13T13:46:00.001-08:00

Dark matter seeding and the kinematics and rotation of neutron stars

Authors: M. Angeles Perez-Garcia, Joseph Silk

arXiv:1111.2275v1

Self-annihilation of dark matter accreted from the galactic halo in the inner regions of neutron stars may affect their kinematical properties, namely velocity kicks and rotation patterns. We find that if a stable long-lived single or multiple strangelet off-center seed forms, there is an associated change in momentum and torque that may affect the kinematical observables of the star.

Observational Constraints on the Averaged Universe

Authors: Chris Clarkson, Timothy Clifton, Alan Coley, Rockhee Sung

arXiv:1111.2214v1

Averaging in general relativity is a complicated operation, due to the general covariance of the theory and the non-linearity of Einstein's equations. The latter of these ensures that smoothing spacetime over cosmological scales does not yield the same result as solving Einstein's equations with a smooth matter distribution, and that the smooth models we fit to observations need not be simply related to the actual geometry of spacetime. One specific consequence of this is a decoupling of the geometrical spatial curvature term in the metric from the dynamical spatial curvature in the Friedmann equation. Here we investigate the consequences of this decoupling by fitting to a combination of HST, CMB, SNIa and BAO data sets. We find that only the geometrical spatial curvature is tightly constrained, and that our ability to constrain dark energy dynamics will be severely impaired until we gain a thorough understanding of the averaging problem in cosmology.

The Milky Way's bright satellites as an apparent failure of LCDM

Authors: Michael Boylan-Kolchin, James S. Bullock, Manoj Kaplinghat

arXiv:1111.2048v1

We use the Aquarius simulations to show that the most massive subhalos in galaxy-mass dark matter halos in LCDM are grossly inconsistent with the dynamics of the brightest Milky Way dwarf spheroidal galaxies. While the best-fitting hosts of the dwarf spheroidals all have 12 < Vmax < 25 km/s, LCDM simulations predict at least ten subhalos with Vmax > 25 km/s. These subhalos are also among the most massive at earlier times, and significantly exceed the UV suppression mass back to z ~ 10. No LCDM-based model of the satellite population of the Milky Way explains this result. The problem lies in the satellites' densities: it is straightforward to match the observed Milky Way luminosity function, but doing so requires the dwarf spheroidals to have dark matter halos that are a factor of ~5 more massive than is observed. Independent of the difficulty in explaining the absence of these dense, massive subhalos, there is a basic tension between the derived properties of the bright Milky Way dwarf spheroidals and LCDM expectations. The inferred infall masses of these galaxies are all approximately equal and are much lower than standard LCDM predictions for systems with their luminosities. Consequently, their implied star formation efficiencies span over two orders of magnitude, from 0.2% to 20% of baryons converted into stars, in stark contrast with expectations gleaned from more massive galaxies. We explore possible solutions to these problems within the context of LCDM and find them to be unconvincing. In particular, we use controlled simulations to demonstrate that the small stellar masses of the bright dwarf spheroidals make supernova feedback an unlikely explanation for their low inferred densities.

arXiv: 9 November 2011

2011-11-09T17:08:00.007-08:00

Power spectrum for the Bose-Einstein condensate dark matter

Authors: Hermano Velten, Etienne Wamba

arXiv:1111.2032v1

We assume that dark matter is composed of scalar particles that form a Bose-Einstein condensate (BEC) at some point during the cosmic evolution. Afterwards, cold dark matter is in the form of a condensate and behaves slightly different from the standard dark matter component. We study the large scale perturbative dynamics of the BEC dark matter in a model where this component coexists with baryonic matter and cosmological constant. The perturbative dynamics is studied using neo- Newtonian cosmology (where the pressure is dynamically relevant for the homogeneous and isotropic background) which is assumed to be correct for small values of the sound speed. We show that BEC dark matter effects can be seen in the matter power spectrum if the mass of the condensate particle lies in the range 15meV < m < 700meV leading to a small, but perceptible, excess of power at large scales.

Cluster Density Profiles as a Test of Modified Gravity

Authors: Lucas Lombriser, Fabian Schmidt, Tobias Baldauf, Rachel Mandelbaum, Uros Seljak, Robert E. Smith

arXiv:1111.2020v1

We present a new test of gravitational interactions at the r\simeq(0.2-20)Mpc scale, around the virial radius of dark matter halos measured through cluster-galaxy lensing of maxBCG clusters from the Sloan Digital Sky Survey (SDSS). We employ predictions from self-consistent simulations of f(R) gravity to find an upper bound on the background field amplitude of f_R0<3.5x10^-3 at the 1D-marginalized 95% confidence level. We also constrain the amplitude F_0 of a phenomenological fit modeled on the profile enhancement induced by f(R) gravity when not including effects from the increased cluster abundance in f(R). In both scenarios, dark-matter-only simulations of the concordance model corresponding to f_R0=0 and F_0=0 are consistent with the lensing measurements at the 68% confidence level.

Supernova Simulations and Strategies For the Dark Energy Survey

Authors: J. P. Bernstein, R. Kessler, S. Kuhlmann, R. Biswas, E. Kovacs, G. Aldering, I. Crane, D. A. Finley, J. A. Frieman, T. Hufford, M. J. Jarvis, A. G. Kim, J. Marriner, P. Mukherjee, R. C. Nichol, P. Nugent, D. Parkinson, R. R. R. Reis, M. Sako, H. Spinka, M. Sullivan

arXiv:1111.1969v1

We present an analysis of supernova light curves simulated for the upcoming Dark Energy Survey (DES) supernova search. The simulations employ a code suite that generates and fits realistic light curves in order to obtain distance modulus/redshift pairs that are passed to a cosmology fitter. We investigated several different survey strategies including field selection, supernova selection biases, and photometric redshift measurements. Using the results of this study, we chose a 30 square degree search area in the griz filter set. We forecast 1) that this survey will provide a homogeneous sample of up to 4000 Type Ia supernovae in the redshift range 0.05<z<1.2, and 2) that the increased red efficiency of the DES camera will significantly improve high-redshift color measurements. The redshift of each supernova with an identified host galaxy will be obtained from spectroscopic observations of the host. A supernova spectrum will be obtained for a subset of the sample, which will be utilized for control studies. In addition, we have investigated the use of combined photometric redshifts taking into account data from both the host and supernova. We have investigated and estimated the likely contamination from core-collapse supernovae based on photometric identification, and have found that a Type Ia supernova sample purity of up to 98% is obtainable given specific assumptions. Furthermore, we present systematic uncertainties due to sample purity, photometric calibration, dust extinction priors, filter-centroid shifts, and inter-calibration. We conclude by estimating the uncertainty on the cosmological parameters that will be measured from the DES supernova data.

Statistics of Substructures in Dark Matter Haloes

Authors: E. Contini, G. De Lucia, S. Borgani

arXiv:1111.1911v1

We study the amount and distribution of dark matter substructures within dark matter haloes, using a large set of high-resolution simulations ranging from group size to cluster size haloes, and carried our within a cosmological model consistent with WMAP 7-year data. In particular, we study how the measured properties of subhaloes vary as a function of the parent halo mass, the physical properties of the parent halo, and redshift. The fraction of halo mass in substructures increases with increasing mass. There is, however, a very large halo-to-halo scatter that can be explained only in part by a range of halo physical properties, e.g. concentration. At given halo mass, less concentrated haloes contain significantly larger fractions of mass in substructures because of the reduced strength of tidal disruption. Most of the substructure mass is located at the outskirts of the parent haloes, in relatively few massive subhaloes. This mass segregation appears to become stronger at increasing redshift, and should reflect into a more significant mass segregation of the galaxy population at different cosmic epochs. When haloes are accreted onto larger structures, their mass is significantly reduced by tidal stripping. Haloes that are more massive at the time of accretion (these should host more luminous galaxies) are brought closer to the centre on shorter time-scales by dynamical friction, and therefore suffer of a more significant stripping. The halo merger rate depends strongly on the environment with substructure in more massive haloes suffering more important mergers than their counterparts residing in less massive systems. This should translate into a different morphological mix for haloes of different mass.

Optimizing future dark energy surveys for model selection goals

Authors: Catherine Watkinson, Andrew R. Liddle, Pia Mukherjee, David Parkinson

arXiv:1111.1870v1

We demonstrate a methodology for optimizing the ability of future dark energy surveys to answer model selection questions, such as `Is acceleration due to a cosmological constant or a dynamical dark energy model?'. Model selection Figures of Merit are defined, exploiting the Bayes factor, and surveys optimized over their design parameter space via a Monte Carlo method. As a specific example we apply our methods to generic multi-fibre baryon acoustic oscillation spectroscopic surveys, comparable to that proposed for SuMIRe PFS, and present implementations based on the Savage-Dickey Density Ratio that are both accurate and practical for use in optimization. It is shown that whilst the optimal surveys using model selection agree with those found using the Dark Energy Task Force (DETF) Figure of Merit, they provide better informed flexibility of survey configuration and an absolute scale for performance; for example, we find survey configurations with close to optimal model selection performance despite their corresponding DETF Figure of Merit being at only 50% of its maximum. This Bayes factor approach allows us to interpret the survey configurations that will be good enough for the task at hand, vital especially when wanting to add extra science goals and in dealing with time restrictions or multiple probes within the same project.

Beyond the power spectrum: primordial and secondary non-Gaussianity in the microwave background

Authors: Kendrick M. Smith (Princeton)

arXiv:1111.1783v1

Cosmic microwave background observations are most commonly analyzed by estimating the power spectrum. In the limit where the CMB statistics are perfectly Gaussian, this extracts all the information, but the CMB also contains detectable non-Gaussian contributions from secondary, and possibly primordial, sources. We review possible sources of CMB non-Gaussianity and describe statistical techniques which are optimized for measuring them, complementing the power spectrum analysis. The machinery of $N$-point correlation functions provides a unifying framework for optimal estimation of primordial non-Gaussian signals or gravitational lensing. We review recent results from applying these estimators to data from the WMAP satellite mission.

arXiv: 8 November 2011

2011-11-09T17:08:00.005-08:00

MOND--particularly as modified inertia

Authors: Mordehai Milgrom

arXiv:1111.1611v1

After a succinct review of the MOND paradigm--with its phenomenology, and its various underlying theories--I concentrate on so called modified inertia (MI) formulations of MOND, which have so far received only little attention. These share with all MOND theories the salient MOND predictions, such as asymptotically flat rotation curves, and the universal mass-asymptotic-speed relation. My emphasis here is, however, on the fact that MI theories can differ substantially from their "modified-gravity" (MG) kin in predicting other phenomena. Because MI theories are non local in time, MOND effects depend on the full trajectory of a system, not only on its instantaneous state, as in MG theories. This may lead to rather different predictions for, e.g., the external-field effect (EFE): A subsystem, such as a globular cluster or a dwarf galaxy, moving in the field of a mother galaxy, or a galaxy in a cluster, may be subject to an EFE that depends on the accelerations all along its orbit, not only on the instantaneous value. And, it is even possible to construct MI theories with practically no EFE. Other predictions that may differ are also discussed. Since we do not yet have a full fledged, modified-inertia formulation, simple, heuristic models have been used to demonstrate these points.

Structure formation in cosmologies with oscillating dark energy

Authors: F. Pace, C. Fedeli, L. Moscardini, M. Bartelmann

arXiv:1111.1556v1

{abridged} We study the imprints on the formation and evolution of cosmic structures of dynamical dark energy models, characterized by an oscillating equation of state. The redshift evolution of the equation of state parameter w(z) for dark energy is characterized by two parameters, describing the amplitude and the frequency of the oscillations. We consider six different oscillating dark energy models, each characterized by a different set of parameter values. Under the common assumption that dark energy is not clustering on the scales of interest, we study different aspects of cosmic structure formation. In particular, we self-consistently solve the spherical collapse problem. We then estimate the behavior of several cosmological observables, such as the linear growth factor, the Integrated Sachs-Wolfe (ISW) effect, the number counts of massive structures, and the matter and cosmic shear power spectra. We show that, independently of the amplitude and the frequency of the dark energy oscillations, none of the aforementioned observables show an oscillating behavior as a function of redshift. This is a consequence of the said observables' being integrals over some functions of the expansion rate over cosmic history. We also notice that deviations with respect to the expectations for a fiducial LambdaCDM cosmology are generically small, and in the majority of the cases distinguishing an oscillating dark energy model would be difficult. Exceptions to this conclusion are provided by the cosmic shear power spectrum, which for some of the models shows a difference at the level of \sim 10% over a wide range of angular scales, and the abundance of galaxy clusters, which is modified at the $\sim 10-20%$ level at $z \gtrsim 0.6$ for future wide weak lensing surveys.

A WDM model for the evolution of galactic halos

Authors: L. Acedo

arXiv:1111.1536v1

It is a well-known fact that the gravitational effect of dark matter in galaxies is only noticeable when the orbital accelerations drop below $a_0 \simeq 2\times 10^{-8}$ cm s$^{-1}$ (Milgrom's Law). This peculiarity of the dynamic behaviour of galaxies was initially ascribed to a modification of Newtonian dynamics (MOND theory) and, consequently, it was used as an argument to criticize the dark matter hypothesis. In our model, warm dark matter is composed by collisionless Vlasov particles with a primordial typical velocity $\simeq 330$ km s$^{-1}$ and, consequently, they evaporated from galactic cores and reorganized in halos with a cusp at a finite distance from the galactic center (in contrast with Cold Dark Matter simulations which predict a cusp at the center of galaxies). This is confirmed by mean-field N-body simulations of the self-gravitating Vlasov dark matter particles in the potential well of the baryonic core. The rest mass of these particles, $\mu$, is determined from a kinetic theory of the early universe with a cosmological constant. We find that $\mu$ is in the range of a few keV. This result makes sterile neutrinos the best suited candidates for the main component of dark matter.

Why Are AGN and Host Galaxies Misaligned?

Authors: Philip F. Hopkins (Berkeley), Lars Hernquist (Harvard), Christopher C. Hayward (Harvard), Desika Narayanan (Arizona)

arXiv:1111.1236v1

It is well-established observationally that the characteristic angular momentum axis on small scales around AGN, traced by radio jets and the putative torus, is not well-correlated with the large-scale angular momentum axis of the host galaxy. In this paper, we show that such misalignments arise naturally in high-resolution simulations in which we follow angular momentum transport and inflows from galaxy to sub-pc scales near AGN, triggered either during galaxy mergers or by instabilities in isolated disks. Sudden misalignments can sometimes be caused by single massive clumps falling into the center slightly off-axis, but more generally, they arise even when the gas inflows are smooth and trace only global gravitational instabilities. When several nested, self-gravitating modes are present, the inner ones can precess and tumble in the potential of the outer modes. Resonant angular momentum exchange can flip or re-align the spin of an inner mode on a short timescale, even without the presence of massive clumps. We therefore do not expect that AGN and their host galaxies will be preferentially aligned, nor should the relative alignment be an indicator of the AGN fueling mechanism. We discuss implications of this conclusion for AGN feedback and BH spin evolution. The misalignments imply that even BHs accreting from a smooth large-scale disk at near-Eddington will not be spun up to maximal rotation, but to moderate values of angular momentum |a|~0.3-0.9. This corresponds to a narrow range in radiative efficiencies epsilon_r~0.1, and relatively inefficient jet formation at high Eddington ratio. Even lower spins are possible if there is further, un-resolved clumpiness and misalignment and higher spins occur if the accretion is slower, from material that is not self-gravitating.

arXiv: 7 Novemeber 2011

2011-11-09T17:08:00.003-08:00

Testing Cosmology with Extreme Galaxy Clusters

Authors: Ian Harrison, Peter Coles

arXiv:1111.1184v1

Motivated by recent suggestions that a number of observed galaxy clusters have masses which are too high for their given redshift to occur naturally in a standard model cosmology, we use Extreme Value Statistics to construct confidence regions in the mass-redshift plane for the most extreme objects expected in the universe. We show how such a diagram not only provides a way of potentially ruling out the concordance cosmology, but also allows us to differentiate between alternative models of enhanced structure formation. We compare our theoretical prediction with observations, placing currently observed high and low redshift clusters on a mass-redshift diagram and find -- provided we consider the full sky to avoid a posteriori selection effects -- that none are in significant tension with concordance cosmology.

Dwarf spheroidal galaxy kinematics and spiral galaxy scaling laws

Authors: Paolo Salucci, Mark I. Wilkinson, Matthew G. Walker, Gerard F. Gilmore, Eva K. Grebel, Andreas Koch, Christiane Frigerio Martins, Rosemary F.G. Wyse

arXiv:1111.1165v1

Kinematic surveys of the dwarf spheroidal (dSph) satellites of the Milky Way are revealing tantalising hints about the structure of dark matter (DM) haloes at the low-mass end of the galaxy luminosity function. At the bright end, modelling of spiral galaxies has shown that their rotation curves are consistent with the hypothesis of a Universal Rotation Curve whose shape is supported by a cored dark matter halo. In this paper, we investigate whether the internal kinematics of the Milky Way dSphs are consistent with the particular cored DM distributions which reproduce the properties of spiral galaxies. Although the DM densities in dSphs are typically almost two orders of magnitude higher than those found in (larger) disk systems, we find consistency between dSph kinematics and Burkert DM haloes whose core radii r0 and central densities {\rho}0 lie on the extrapolation of the scaling law seen in spiral galaxies: log {\rho}0 \simeq {\alpha} log r0 + const with 0.9 < {\alpha} < 1.1. We similarly find that the dSph data are consistent with the relation between {\rho}0 and baryon scale length seen in spiral galaxies. While the origin of these scaling relations is unclear, the finding that a single DM halo profile is consistent with kinematic data in galaxies of widely varying size, luminosity and Hubble Type is important for our understanding of observed galaxies and must be accounted for in models of galaxy formation.

Constraints on Modified Gravity from Sunyaev-Zeldovich Cluster Surveys

Authors: Daisy S. Y. Mak, Elena Pierpaoli, Fabian Schmidt, Nicolo' Macellari

arXiv:1111.1004v1

We investigate the constraining power of current and future Sunyaev-Zeldovich cluster surveys on the f(R) gravity model. We use a Fisher matrix approach, adopt self-calibration for the mass- observable scaling relation, and evaluate constraints for the SPT, Planck, SPTPol and ACTPol surveys. The modified gravity effects on the mass function, halo bias, matter power spectrum, and mass-observable relation are taken into account. We show that, relying on number counts only, the Planck cluster catalog is expected to reduce current upper limits by about a factor of four, to {\sigma}fR0 = 3 {\times} 10-5 (68% confidence level). Adding the cluster power spectrum further improves the constraints to {\sigma}fR0 = 10-5 for SPT and Planck, and {\sigma}fR0 = 3 {\times} 10-6 for SPTPol, pushing cluster constraints significantly beyond the limit where number counts have no constraining power due to the chameleon screening mechanism. Further, the combination of both observables breaks degeneracies, especially with the expansion history (effective dark energy density and equation of state). The constraints are only mildly worsened by the use of self-calibration but depend strongly on the mass threshold of the cluster samples.

Probes of Lorentz Violation

Authors: John Ellis, Nick E. Mavromatos

arXiv:1111.1178v1

Lorentz invariance is such an important principle of fundamental physics that it should constantly be subjected to experimental scrutiny as well as theoretical questioning. Distant astrophysical sources of energetic photons with rapid time variations, such as active galactic nuclei (AGNs) and gamma-ray bursters (GRBs), provide ideal experimental opportunities for testing Lorentz invariance. The Cherenkov Telescope Array (CTA) is an excellent experimental tool for making such tests with sensitivities exceeding those possible using other detectors.

arXiv: 4 November 2011

2011-11-09T17:08:00.001-08:00

A measurement of secondary cosmic microwave background anisotropies with two years of South Pole Telescope observations

Authors: C. L. Reichardt, L. Shaw, O. Zahn, K. A. Aird, B. A. Benson, L. E. Bleem, J. E. Carlstrom, C. L. Chang, H. M. Cho, T. M. Crawford, A. T. Crites, T. de Haan, M. A. Dobbs, J. Dudley, E. M. George, N. W. Halverson, G. P. Holder, W. L. Holzapfel, S. Hoover, Z. Hou, J. D. Hrubes, M. Joy, R. Keisler, L. Knox, A. T. Lee, E. M. Leitch, M. Lueker, D. Luong-Van, J. J. McMahon, J. Mehl, S. S. Meyer, M. Millea, J. J. Mohr, T. E. Montroy, T. Natoli, S. Padin, T. Plagge, C. Pryke, J. E. Ruhl, K. K. Schaffer, E. Shirokoff, H. G. Spieler, Z. Staniszewski, A. A. Stark, K. Story, A. van Engelen, K. Vanderlinde, J. D. Vieira, R. Williamson

arXiv:1111.0932v1

We present the first three-frequency South Pole Telescope (SPT) cosmic microwave background (CMB) power spectra. The band powers presented here cover angular scales 2000 < ell < 9400 in frequency bands centered at 95, 150, and 220 GHz. At these frequencies and angular scales, a combination of the primary CMB anisotropy, thermal and kinetic Sunyaev-Zel'dovich (SZ) effects, radio galaxies, and cosmic infrared background (CIB) contributes to the signal. We combine Planck and SPT data at 220 GHz to constrain the amplitude and shape of the CIB power spectrum and find strong evidence for non-linear clustering. We explore the SZ results using a variety of cosmological models for the CMB and CIB anisotropies and find them to be robust with one exception: allowing for spatial correlations between the thermal SZ effect and CIB significantly degrades the SZ constraints. Neglecting this potential correlation, we find the thermal SZ power at 150 GHz and ell = 3000 to be 3.65 +/- 0.69 muK^2, and set an upper limit on the kinetic SZ power to be less than 2.8 muK^2 at 95% confidence. When a correlation between the thermal SZ and CIB is allowed, we constrain a linear combination of thermal and kinetic SZ power: D_{3000}^{tSZ} + 0.5 D_{3000}^{kSZ} = 4.60 +/- 0.63 muK^2, consistent with earlier measurements. We use the measured thermal SZ power and an analytic, thermal SZ model calibrated with simulations to determine sigma8 = 0.807 +/- 0.016. Modeling uncertainties involving the astrophysics of the intracluster medium rather than the statistical uncertainty in the measured band powers are the dominant source of uncertainty on sigma8 . We also place an upper limit on the kinetic SZ power produced by patchy reionization; a companion paper uses these limits to constrain the reionization history of the Universe.

Testing Multi-Field Inflation: A Geometric Approach

Authors: Courtney M. Peterson, Max Tegmark

arXiv:1111.0927v1

We develop an approach for testing inflation models with multiple scalar fields by linking geometric and kinematical features of their inflationary Lagrangians to observable quantities like the power spectra, bispectrum, and trispectrum. Our approach also provides geometric intuition for when a complicated multi-field model can be well-approximated by a model with one, two, or a handful of fields. To arrive at these results, we focus on the mode interactions, simplify them using a novel result, and then explore how these interactions depend on the geometry of the inflationary Lagrangian and on the kinematics of the associated field trajectory. In the process, we introduce a multi-field observable \beta_2 that can potentially distinguish two-field scenarios from scenarios involving three or more effective fields. We also present a multi-field consistency relation, which involves the primordial bispectrum parameter f_{NL}, trispectrum parameter \tau_{NL}, and other spectral observables. These combined results provide better intuition into how features in multi-field inflationary Lagrangians translate into cosmic observables.

The importance of the local density in shaping the galaxy stellar mass functions

Authors: Benedetta Vulcani (1,2), Bianca M. Poggianti (2), Giovanni Fasano (2), Vandana Desai (3), Alan Dressler (4), August Oemler Jr. (4), Rosa Calvi (1), Mauro D'Onofrio (1), Alessia Moretti (1,2) ((1) Astronomical Department, Padova University, Italy, (2) INAF-Astronomical Observatory of Padova, Italy, (3) Spitzer Science Center, California Institute of Technology, Pasadena, CA, USA, (4) Observatories of the Carnegie Institution of Science, Pasadena, CA, USA,)

arXiv:1111.0832v1

Exploiting the capabilities of four different surveys --- the Padova-Millennium Galaxy and Group Catalogue (PM2GC), the WIde-field Nearby Galaxy-cluster Survey (WINGS), the IMACS Cluster Building Survey (ICBS) and the ESO Distant Cluster Survey (EDisCS) --- we analyze the galaxy stellar mass distribution as a function of local density in mass-limited samples, in the field and in clusters from low (z>0.04) to high (z<0.8) redshift. We find that at all redshifts and in all environments, local density plays a role in shaping the mass distribution. In the field, it regulates the shape of the mass function at any mass above the mass limits. In clusters, it seems to be important only at low masses (log M_ast/M_sun <10.1 in WINGS and log M_ast/M_sun < 10.4 in EDisCS), otherwise it seems not to influence the mass distribution. Putting together our results with those of Calvi et al. and Vulcani et al. for the global environment, we argue that at least at $z\leq 0.8$ local density is more important than global environment in determining the galaxy stellar mass distribution, suggesting that galaxy properties are not much dependent of halo mass, but do depend on local scale processes.

Supersymmetry vis-à-vis Observation: Dark Matter Constraints, Global Fits and Statistical Issues

Authors: Yashar Akrami

arXiv:1111.0710v1

Weak-scale supersymmetry is one of the most favoured theories beyond the Standard Model of particle physics that elegantly solves various theoretical and observational problems in both particle physics and cosmology. In this thesis, I describe the theoretical foundations of supersymmetry, issues that it can address and concrete supersymmetric models that are widely used in phenomenological studies. I discuss how the predictions of supersymmetric models may be compared with observational data from both colliders and cosmology. I show why constraints on supersymmetric parameters by direct and indirect searches of particle dark matter are of particular interest in this respect. Gamma-ray observations of astrophysical sources, in particular dwarf spheroidal galaxies, by the Fermi satellite, and recording nuclear recoil events and energies by future ton-scale direct detection experiments are shown to provide powerful tools in searches for supersymmetric dark matter and estimating supersymmetric parameters. I discuss some major statistical issues in supersymmetric global fits to experimental data. In particular, I further demonstrate that existing advanced scanning techniques may fail in correctly mapping the statistical properties of the parameter spaces even for the simplest supersymmetric models. Complementary scanning methods based on Genetic Algorithms are proposed.

Cosmic flows in the nearby universe from Type Ia Supernovae

Authors: Stephen J. Turnbull, Michael J. Hudson, Hume A. Feldman, Malcolm Hicken, Robert P. Kirshner, Richard Watkins

arXiv:1111.0631v2

Peculiar velocities are one of the only probes of very large-scale mass density fluctuations in the nearby Universe. We present new "minimal variance" bulk flow measurements based upon the "First Amendment" compilation of 245 Type Ia supernovae (SNe) peculiar velocities and find a bulk flow of 249 +/- 76 km/s in the direction l= 319 +/- 18 deg, b = 7 +/- 14 deg. The SNe bulk flow is consistent with the expectations of \Lambda CDM. However, it is also marginally consistent with the bulk flow of a larger compilation of non-SNe peculiar velocities (Watkins, Feldman, & Hudson 2009). By comparing the SNe peculiar velocities to predictions of the IRAS Point Source Catalog Redshift survey (PSCz) galaxy density field, we find \Omega_{m}^{0.55} \sigma_{8,lin} = 0.40 +/- 0.07, which is in agreement with \Lambda CDM. However, we also show that the PSCz density field fails to account for 150 +/- 43 km/s of the SNe bulk motion.

arXiv: 3 November 2011

2011-11-09T17:07:00.001-08:00

Resolving astrophysical uncertainties in dark matter direct detection

Authors: Mads T. Frandsen, Felix Kahlhoefer, Christopher McCabe, Subir Sarkar, Kai Schmidt-Hoberg

arXiv:1111.0292v1

We study the impact of the assumed velocity distribution of galactic dark matter particles on the interpretation of results from nuclear recoil detectors. By converting experimental data to variables that make the astrophysical unknowns explicit, different experiments can be compared without implicit assumptions concerning the dark matter halo. We extend this framework to include the annual modulation signal, as well as multiple target elements. Recent results from DAMA, CoGeNT and CRESST-II can be brought into agreement if the velocity distribution is very anisotropic and thus allows a large modulation fraction. However constraints from CDMS and XENON cannot be evaded by appealing to such astrophysical uncertainties alone.

arXiv: 1 November 2011

2011-10-31T19:49:00.001-07:00

Extragalactic point source detection in WMAP 7-year data at 61 and 94 GHz

Authors: L. F. Lanz, D. Herranz, M. López-Caniego, J. González-Nuevo, G. de Zotti, M. Massardi, J. L. Sanz

arXiv:1110.6877v1

The detection of point sources in Cosmic Microwave Background maps is usually based on a single-frequency approach, whereby maps at each frequency are filtered separately and the spectral information on the sources is derived combining the results at the different frequencies. On the contrary, in the case of multi-frequency detection methods, source detection and spectral information are tightly interconnected in order to increase the source detection efficiency.
In this work we apply the \emph{matched multifilter} method to the detection of point sources in the WMAP 7yr data at 61 and 94 GHz. This linear filtering technique takes into account the spatial and the cross-power spectrum information at the same time using the spectral behaviour of the sources without making any a priori assumption about it. We follow a two-step approach. First, we do a blind detection of the sources. Second, we do a refined analysis at their positions to improve the signal-to-noise ratio. At 94 GHz we detect 157 $5\sigma$ objects at $|b|>5^\circ$ (excluding the Large Magellanic Cloud region); 111 of them lie outside the WMAP Point Source Catalog mask. 28 of our detections are known Galactic sources or lie in regions of intense Galactic emission. All the other 129 have counterparts in lower frequency radio catalogues and are presumably extragalactic; 18 of them are not included in the WMAP 7yr catalogue. Our results constitute a substantial improvement over the WMAP Five-Band Point Source Catalog, which contains 87 $5\sigma$ detections at 94 GHz.

Environment Dependence of Dark Matter Halos in Symmetron Modified Gravity

Authors: Hans A. Winther, David F. Mota, Baojiu Li

arXiv:1110.6438v1

We investigate the environment dependence of dark matter halos in the symmetron modified gravity scenario. The symmetron is one of three known mechanisms for screening a fifth-force and thereby recovering General Relativity in dense environments. The effectiveness of the screening depends on both the mass of the object and the environment it lies in. Using high-resolution N-body simulations we find a significant difference, which depends on the halos mass and environment, between the lensing and dynamical masses of dark matter halos similar to the f(R) modified gravity. The symmetron can however yield stronger signatures due to a freedom in the strength of the coupling to matter.

An Extended Excursion Set Approach to Structure Formation in Chameleon Models

Authors: Baojiu Li (Cambridge and Durham), George Efstathiou (Cambridge)

arXiv:1110.6440v1

In attempts to explain dark energy, a number of models have been proposed in which the formation of large-scale structure depends on the local environment. These models are highly non-linear and difficult to analyse analytically. N-body simulations have therefore been used to study their non-linear evolution. Here we extend excursion set theory to incorporate environmental effects on structure formation. We apply the method to a chameleon model and calculate observables such as the non-linear mass function at various redshifts. The method can be generalized to study other obervables and other models of environmentally dependent interactions. The analytic methods described here should prove useful in delineating which models deserve more detailed study with N-body simulations.

arXiv: 28 October 2011

2011-10-30T15:38:00.003-07:00

On the cosmological evolution of the black hole - host galaxy relation in quasars

Authors: L. Portinari, J. Kotilainen, R. Falomo, R. Decarli

arXiv:1110.6067v1

Quasars are useful tracers of the cosmological evolution of the black hole mass - galaxy relation. We compare the expectations of Semi-Analytical Models (SAM) of galaxy evolution, to the largest available datasets of quasar host galaxies out to z=3.
Observed quasar hosts are consistent with no evolution from the local M(BH) - L(host) relation, and suggest a significant increase of the mass ratio Gamma = M(BH)/M(host) from z=0 to z=3. Taken at face value, this is totally at odds with the predictions of SAM, where the intrinsic Gamma shows little evolution and quasar host galaxies at high redshift are systematically overluminous (and/or have undermassive BH). However, since quasars preferentially trace very massive black holes (10^9-10^10 Msun) at the steep end of the luminosity and mass function, the ensuing selection biases can reconcile the present SAM with the observations. A proper interpretation of quasar host data thus requires the global approach of SAM so as to account for statistical biases.

Running Spectral Index and Formation of Primordial Black Hole in Single Field Inflation Models

Authors: Manuel Drees, Encieh Erfani

arXiv:1110.6052v1

A broad range of single field models of inflation are analyzed in light of all relevant recent cosmological data, checking whether they can lead to the formation of long-lived Primordial Black Holes (PBHs). To that end we calculate the spectral index of the power spectrum of primordial perturbations as well as its first and second derivatives. PBH formation is possible only if the spectral index increases significantly at small scales, i.e. large wave number $k$. Since current data indicate that the first derivative $\alpha_S$ of the spectral index $n_S(k_0)$ is negative at the pivot scale $k_0$, PBH formation is only possible in the presence of a sizable and positive second derivative ("running of the running") $\beta_S$. Among the three small-field and five large-field models we analyze, only one small-field model, the "running mass" model, allows PBH formation, for a narrow range of parameters. We also note that none of the models we analyze can accord for a large and negative value of $\alpha_S$, which is weakly preferred by current data.

The central dark matter distribution of NGC 2976

Authors: Joshua J. Adams, Karl Gebhardt, Guillermo A. Blanc, Maximilian H. Fabricius, Gary J. Hill, Jeremy D. Murphy, Remco C. E. van den Bosch, Glenn van de Ven

arXiv:1110.5951v1

We study the mass distribution in the late-type dwarf galaxy NGC 2976 through stellar kinematics obtained with the VIRUS-P integral-field spectrograph and anisotropic Jeans models as a test of cosmological simulations and baryonic processes that putatively alter small-scale structure. Previous measurements of the H-alpha emission-line kinematics have determined that the dark matter halo of NGC 2976 is most consistent with a cored density profile. We find that the stellar kinematics are best fit with a cuspy halo. Cored dark matter halo fits are only consistent with the stellar kinematics if the stellar mass-to-light ratio is significantly larger than that derived from stellar population synthesis, while the best-fitting cuspy model has no such conflict. The inferred mass distribution from a harmonic decomposition of the gaseous kinematics is inconsistent with that of the stellar kinematics. This difference is likely due to the gas disk not meeting the assumptions that underlie the analysis such as no pressure support, a constant kinematic axis, and planar orbits. By relaxing some of these assumptions, in particular the form of the kinematic axis with radius, the gas-derived solution can be made consistent with the stellar kinematic models. A strong kinematic twist in the gas of NGC 2976's center suggests caution, and we advance the mass model based on the stellar kinematics as more reliable. The analysis of this first galaxy shows promising evidence that dark matter halos in late-type dwarfs may in fact be more consistent with cuspy dark matter distributions than earlier work has claimed.

Isocurvature Perturbations and Non-Gaussianity of Gravitationally Produced Nonthermal Dark Matter

Authors: Daniel J. H. Chung, Hojin Yoo

arXiv:1110.5931v1

Gravitational particle production naturally occurs during the transition from the inflationary phase to the non-inflationary phase. If the particles are stable and very weakly interacting, they are natural nonthermal dark matter candidates. We show that such nonthermal dark matter particles can produce local non-Gaussianities large enough to be observed by ongoing and near future experiments without being in conflict with the existing isocurvature bounds. Of particular interest is the fact that these particles can be observable through local non-Gaussianities even when they form a very small fraction of the total dark matter content.

arXiv: 26 October 2011

2011-10-30T15:38:00.001-07:00

Far-Infrared Properties of Lyman Break Galaxies from Cosmological Simulations

Authors: Renyue Cen (Princeton University Observatory)

arXiv:1110.5645v1

Utilizing state-of-the-art, adaptive mesh-refinement cosmological hydrodynamic simulations with ultra-high resolution (114h-1pc) and large sample size (>3300 galaxies of stellar mass >10^9Msun), we show how the stellar light of Lyman Break Galaxies at z=2 is distributed between optical/ultra-violet (UV) and far-infrared (FIR) bands. With a single scalar parameter for dust obscuration we can simultaneously reproduce the observed UV luminosity function for the entire range (3-100 Msun/yr) and extant FIR luminosity function at the bright end (>20Msun/yr). We quantify that galaxies more massive or having higher SFR tend to have larger amounts of dust obscuration mostly due to a trend in column density and in a minor part due to a mass (or SFR)-metallicity relation. It is predicted that the FIR luminosity function in the range SFR=1-100Msun/yr is a powerlaw with a slope about -1.7. We further predict that there is a "galaxy desert" at SFR(FIR) < 0.02 (SFR(UV)/10Msun/yr)^2.1 Msun/yr in the SFR(UV)-SFR(FIR) plane. Detailed distributions of SFR(FIR) at a fixed SFR(UV) are presented. Upcoming observations by ALMA should test this model. If confirmed, it validates the predictions of the standard cold dark matter model and has important implications on the intrinsic SFR function of galaxies at high redshift.

Conceptual Problems in Cosmology

Authors: F. J. Amaral Vieira

arXiv:1110.5634v1

In this essay a critical review of present conceptual problems in current cosmology is provided from a more philosophical point of view. In essence, a digression on how could philosophy help cosmologists in what is strictly their fundamental endeavor is presented. We start by recalling some examples of enduring confrontations among philosophers and physicists on what could be contributed by the formers to the day-time striving of the second ones. Then, a short review of the standard model Friedmann-Lema\^itre-Robertson-Walter (FLRW) of cosmology is given. It seems apparent that cosmology is living a golden age with the advent of observations of high precision. Nonetheless, a critical revisiting of the direction in which it should go on appears also needed, for misconcepts like "quantum backgrounds for cosmological classical settings" and "quantum gravity unification" have not been properly constructed up-to-date. Thus, knowledge-building in cosmology, more than in any other field, should begin with visions of the reality, then taking technical form whenever concepts and relations inbetween are translated into a mathematical structure. It is mandatory, therefore, that the meaning of such concepts be the same for all cosmologists, and that any relationship among all them be tested both logically as well as mathematically. In other words, the notorius feature of improbability of our universe, as is well-known, assures to cosmologists a priviledged degree of freedom for formulating interpretations and theories. However, at the same time, it demands for their formulations and conclusions to be considered in the light of data taken from astrophysical observations.

arXiv: 26 October 2011

2011-10-25T19:34:00.001-07:00

Constraints on dark energy from H II starburst galaxy apparent magnitude versus redshift data

Authors: Data Mania, Bharat Ratra

arXiv:1110.5626v1

In this paper we use H II starburst galaxy apparent magnitude versus redshift data from Siegel et al. (2005) to constrain dark energy cosmological model parameters. These constraints are generally consistent with those derived using other data sets, but are not as restrictive as the tightest currently available constraints.

Ram pressure drag - the effects of ram pressure on dark matter and stellar disk dynamics

Authors: R. Smith, M. Fellhauer, P. Assmann

arXiv:1110.5555v1

We investigate the effects of ram pressure stripping on gas-rich disk galaxies in the cluster environment. Ram pressure stripping principally effects the atomic gas in disk galaxies, stripping away outer disk gas to a truncation radius. We demonstrate that the drag force exerted on truncated gas disks is passed to the stellar disk, and surrounding dark matter through their mutual gravity. Using a toy model of ram pressure stripping, we show that this can drag a stellar disk and dark matter cusp off centre within it's dark matter halo by several kiloparsecs. We present a simple analytical description of this process that predicts the drag force strength and its dependency on ram pressures and disk galaxy properties to first order. The motion of the disk can result in temporary deformation of the stellar disk. However we demonstrate that the key source of stellar disk heating is the removal of the gas potential from within the disk. This can result in disk thickening by approximately a factor of two in gas-rich disks.

Probing Dark Energy with Alpha Shapes and Betti Numbers

Authors: Rien van de Weygaert, Pratyush Pranav, Bernard J.T. Jones, E.G.P. (Patrick)Bos, Gert Vegter, Herbert Edelsbrunner, Monique Teillaud, Wojciech A. Hellwing, Changbom Park, Johan Hidding, Mathijs Wintraecken

arXiv:1110.5528v1

We introduce a new descriptor of the weblike pattern in the distribution of galaxies and matter: the scale dependent Betti numbers which formalize the topological information content of the cosmic mass distribution. While the Betti numbers do not fully quantify topology, they extend the information beyond conventional cosmological studies of topology in terms of genus and Euler characteristic used in earlier analyses of cosmological models. The richer information content of Betti numbers goes along with the availability of fast algorithms to compute them. When measured as a function of scale they provide a "Betti signature" for a point distribution that is a sensitive yet robust discriminator of structure. The signature is highly effective in revealing differences in structure arising in different cosmological models, and is exploited towards distinguishing between different dark energy models and may likewise be used to trace primordial non-Gaussianities.
In this study we demonstrate the potential of Betti numbers by studying their behaviour in simulations of cosmologies differing in the nature of their dark energy.

arXiv: 25 October 2011

2011-10-25T19:33:00.000-07:00

Deformed Distance Duality Relations and Supernovae Dimming

Authors: J. A. S. Lima, J. V. Cunha, V. T. Zanchin

arXiv:1110.5065v1

The basic cosmological distances are linked by the Etherington cosmic distance duality relation, $\eta (z) = D_{L}(z)(1+z)^{-2}/D_{A}(z) \equiv 1$, where $D_{L}$ and $D_{A}$ are, respectively, the luminosity and angular diameter distances. In order to test its validity, some authors have proposed phenomenological expressions for $\eta(z)$ thereby deforming the original Etherington's relation and comparing the resulting expressions with the available and future cosmological data. The relevance of such studies is unquestionable since any violation of the cosmic distance duality relation could be the signal of new physics or non-negligible astrophysical effects in the usually assumed perfectly transparent Universe. In this letter, we show that under certain conditions such expressions can be derived from a more fundamental approach with the parameters appearing in the $\eta(z)$ expression defining the cosmic absorption parameter as recently discussed by Chen and Kantowski. Explicit examples involving four different parametrizations of the deformation function are given. Based on such an approach, it is also found that the latest Supernova data can also be explained in the framework of a pure cold dark matter model (Einstein-de Sitter). Two different scenarios with cosmic absorption are discussed. Only if the cosmic opacity is fully negligible, the description of an accelerating Universe powered by dark energy or some alternative gravity theory must be invoked.

TeVeS/MOND is in harmony with gravitational redshifts in galaxy clusters

Authors: Jacob D. Bekenstein, Robert H. Sanders

arXiv:1110.5048v1

Wojtak, Hansen and Hjorth have recently claimed to confirm general relativity and to rule out the tensor-vector-scalar (TeVeS) gravitational theory based on an analysis of the gravitational redshifts of galaxies in 7800 clusters. But their ubiquitous modeling of the sources of cluster gravitational fields with Navarro-Frenk-White mass profiles is neither empirically justified out to the necessary radii in clusters, nor germane in the case of TeVeS. Using MONDian isothermal sphere models consistently constructed within MOND (equivalent to TeVeS models), we can fit the determined redshifts no worse than does general relativity with dark halos. Wojtak, Hansen and Hjorth's work is further marred by confusion between the primitive mu-function of TeVeS and the MOND interpolation function.

The missing matter problem: from the dark matter search to alternative hypotheses

Authors: S. Capozziello, L. Consiglio, M. De Laurentis, G. De Rosa, C. Di Donato

arXiv:1110.5026v1

Dark matter is among the most important open problems in both astrophysics and particle physics. We review the status of art of dark matter search at theoretical and experimental level discussing also alternative hypotheses.

Towards singularity and ghost free theories of gravity

Authors: Tirthabir Biswas, Erik Gerwick, Tomi Koivisto, Anupam Mazumdar

arXiv:1110.5249v1

We present the most general ghost-free gravitational action in a Minkowski vacuum. Apart from the much studied f(R) models, this includes a large class of non-local actions with improved UV behavior, which nevertheless recover Einstein's general relativity in the IR.

24 October 2011

2011-10-23T19:51:00.001-07:00

The imprint of the relative velocity between baryons and dark matter on the 21-cm signal from reionization

Authors: Jonathan M. Bittner, Abraham Loeb

arXiv:1110.4659v1

The post-recombination streaming of baryons through dark matter keeps baryons out of low mass (<10^6 solar masses) halos coherently on scales of a few comoving Mpc. It has been argued that this will have a large impact on the 21-cm signal before and after reionization, as it raises the minimum mass required to form ionizing sources. Using a semi-numerical code, we show that the impact of the baryon streaming effect on the 21-cm signal during reionization (redshifts z approximately 7-20) depends strongly on the cooling scenario assumed for star formation, and the corresponding virial temperature or mass at which stars form. For the canonical case of atomic hydrogen cooling at 10^4 Kelvin, the minimum mass for star formation is well above the mass of halos that are affected by the baryon streaming and there are no major changes to existing predictions. For the case of molecular hydrogen cooling, we find that reionization is delayed by a change in redshift of approximately 2 and that more relative power is found in large modes at a given ionization fraction. However, the delay in reionization is degenerate with astrophysical assumptions, such as the production rate of UV photons by stars.

Horizon-preserving dualities and perturbations in non-canonical scalar field cosmologies

Authors: Ghazal Geshnizjani, William H. Kinney, Azadeh Moradinezhad Dizgah

arXiv:1110.4640v1

We generalize the cosmological duality between inflation and cyclic contraction under the interchange $a \leftrightarrow H$ to the case of non-canonical scalar field theories with varying speed of sound. The single duality in the canonical case generalizes to a family of three dualities constructed to leave the cosmological acoustic horizon invariant. We find three classes of models: (I) DBI inflation, (II) the non-canonical generalization of cyclic contraction, and (III) a new cosmological solution with rapidly decreasing speed of sound and relatively slowly growing scale factor, which we dub {\it stalled} cosmology. We construct dual analogs to the inflationary slow roll approximation, and solve for the curvature perturbation in all three cases. Both cyclic contraction and stalled cosmology predict a strongly blue spectrum for the curvature perturbations inconsistent with observations.

arXiv: 21 October 2011

2011-10-23T19:43:00.001-07:00

Prospects for determination of thermal history after inflation with future gravitational wave detectors

Authors: Sachiko Kuroyanagi, Kazunori Nakayama, Shun Saito

arXiv:1110.4169v1

Thermal history of the Universe between inflation and big-bang nucleosynthesis has not yet been revealed observationally. It will be probed by the detection of primordial gravitational waves generated during inflation, which contain information on the reheating temperature as well as the equation of state of the Universe after inflation. Based on Fisher information formalism, we examine how accurately the tensor-to-scalar ratio and reheating temperature after inflation can be simultaneously determined with space-based gravitational wave detectors such as the DECI-hertz Interferometer Gravitational-wave Observatory (DECIGO) and the Big-Bang Observer (BBO). We show that the reheating temperature is best determined if it is around 10^7 GeV for tensor-to-scalar ratio of around 0.1, and explore the detectable parameter space. We also find that equation of state of the early Universe can be also determined accurately enough to distinguish different equation-of-state parameters if the inflationary gravitational waves are successfully detected. Thus future gravitational wave detectors provide a unique and promising opportunity to reveal the thermal history of the Universe around 10^7 GeV.

arXiv: 20 October 2011

2011-10-23T19:40:00.001-07:00

Clustering of Primordial Black Holes. II. Evolution of Bound Systems

Authors: James R. Chisholm

arXiv:1110.4402v1

Primordial Black Holes (PBHs) that form from the collapse of density perturbations are more clustered than the underlying density field. In a previous paper, we showed the constraints that this has on the prospects of PBH dark matter. In this paper we examine another consequence of this clustering: the formation of bound systems of PBHs in the early universe. These would hypothetically be the earliest gravitationally collapsed structures, forming when the universe is still radiation dominated. Depending upon the size and occupation of the clusters, PBH merging occurs before they would have otherwise evaporated due to Hawking evaporation.

arXiv: 19 October 2011

2011-10-21T14:39:00.001-07:00

Ambiguous Tests of General Relativity on Cosmological Scales

Authors: Joe Zuntz, Tessa Baker, Pedro Ferreira, Constantinos Skordis

arXiv:1110.3830v1

There are a number of approaches to testing General Relativity (GR) on linear scales using Parameterized Post-Friedmann (PPF) methods. It is generally assumed that the details of any given parameterization are unimportant if one uses it as a diagnostic for deviations from GR. In this brief report we show that this is not so by taking two particular parameterizations and analyzing a subset of the current cosmological data. We argue that any PPF approach should always be accompanied by a characterization of the class of modified gravity models it is seeking to approximate.

arXiv: 18 October 2011

2011-10-21T14:37:00.001-07:00

Cepheid Period-Luminosity Relations in the Near-Infrared and the Distance to M31 from the Hubble Space Telescope Wide Field Camera 3

Authors: Adam G. Riess (JHU/STScI), Juergen Fliri (OBSPM), David Valls-Gabaud (OBSPM)

arXiv:1110.3769v1

We present measurements of 68 classical Cepheids with periods from 10 to 78 days observed in the near-infrared by the PHAT Program using the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). The combination of HST's resolution and the use of near-infrared measurements provides a dramatic reduction in the dispersion of the Period--Luminosity relation over the present optical, ground-based data. Even using random-phase magnitudes we measure a dispersion of just 0.17 mag, implying a dispersion of just 0.12 mag for mean magnitudes. The error in the mean for this relation is 1% in distance. Combined with similar observations of Cepheids in other hosts and independent distance determinations, we measure a distance to M31 of mu_0=24.42 +/- 0.05 (statistical) +/- 0.03 (systematic), 765 +/- 28 kpc, in good agreement with past measurements though with a better, 3% precision here. The result is also in good agreement with independent distance determinations from two detached eclipsing binaries allowing for an independent calibration of the Cepheid luminosities and a determination of the Hubble constant.

arXiv: 17 October 2011

2011-10-21T14:32:00.001-07:00

Non-Gaussianity from Step Features in the Inflationary Potential

Authors: Peter Adshead, Cora Dvorkin, Wayne Hu, Eugene A. Lim

arXiv:1110.3050v1

We provide analytic solutions for the power spectrum and bispectrum of curvature fluctuations produced by a step feature in the inflaton potential, valid in the limit that the step is short and sharp. In this limit, the bispectrum is strongly scale dependent and its effective non-linearity attains a large oscillatory amplitude. The perturbations to the curvature power spectrum, on the other hand, remain a small component on top of the usual spectrum of fluctuations generated by slow roll. We utilize our analytic solutions to assess the observability of the predicted non-Gaussian signatures and show that, if present, only very sharp steps on scales larger than ~ 2 Gpc are likely to be able to be detected by Planck. Such features are not only consistent with WMAP7 data, but can also improve its likelihood by 2 Delta ln L ~ 12 for two extra parameters, the step location and height. If this improvement were due to a slow roll violating step as considered here, a bispectrum or corresponding polarization power spectrum detection would provide definitive checks as to its primordial origin.

Clustering and redshift-space distortions in interacting dark energy cosmologies

Authors: Federico Marulli, Marco Baldi, Lauro Moscardini

arXiv:1110.3045v1

We investigate the spatial properties of the large scale structure (LSS) of the Universe in the framework of coupled dark energy (cDE) cosmologies. Using the public halo catalogues from the CoDECS simulations -- the largest set of N-body experiments to date for such cosmological scenarios -- we estimate the clustering and bias functions of cold dark matter (CDM) haloes, both in real- and redshift-space. Moreover, we investigate the effects of the dark energy (DE) coupling on the geometric and dynamic redshift-space distortions, quantifying the difference with respect to the concordance LambdaCDM model. At z~0, the spatial properties of CDM haloes in cDE models appear very similar to the LambdaCDM case, even if the cDE models are normalized at last scattering in order to be consistent with the latest Cosmic Microwave Background (CMB) data. At higher redshifts, we find that the DE coupling produces a significant scale-dependent suppression of the halo clustering and bias function. This effect, that strongly depends on the coupling strength, is not degenerate with sigma8 at scales r<5-10 Mpc/h. Moreover, we find that the coupled DE strongly affects both the linear distortion parameter, beta, and the pairwise peculiar velocity dispersion, sigma12. Although the models considered in this work are found to be all in agreement with presently available observational data, the next generation of galaxy surveys will be able to put strong constraints on the level of coupling between DE and CDM exploiting the shape of redshift-space clustering anisotropies.

A Generalized Theory of Varying Alpha

Authors: John D. Barrow, Sean Z.W. Lip

arXiv:1110.3120v1

In this paper, we formulate a generalization of the simple Bekenstein-Sandvik-Barrow-Magueijo (BSBM) theory of varying alpha by allowing the coupling constant, \omega, for the corresponding scalar field \psi\ to depend on \psi. We focus on the situation where \omega\ is exponential in \psi\ and find the late-time behaviours that occur in matter-dominated and dark-energy dominated cosmologies. We also consider the situation when the background expansion scale factor of the universe evolves in proportion to an arbitrary power of the cosmic time. We find the conditions under which the fine structure `constant' increases with time, as in the BSBM theory, and establish a cosmic no-hair behaviour for accelerating universes. We also find the conditions under which the fine structure `constant' can decrease with time and compare the whole family of models with astronomical data from quasar absorption spectra. Finally, we show that spatial variations on sub-horizon scales can dominate over the cosmological time evolution at late times, and we examine the effects on the external gravitational fields of spherical masses.

Counter-Orbiting Tidal Debris as the Origin of the MW DoS

Authors: Marcel S. Pawlowski

arXiv:1110.3043v1

The Milky Way satellite galaxies show a phase-space distribution that is not expected from the standard scenario of galaxy formation. This is a strong hint at them being of tidal origin, which would naturally explain their spacial distribution in a disc of satellites. It is shown that also their orbital directions can be reproduced with the debris of galaxy collisions. Both co- and counter-orbiting satellites are formed naturally in merger and fly-by interactions.

arXiv: 14 October 2011

2011-10-18T18:38:00.003-07:00

The inflating curvaton

Authors:Konstantinos Dimopoulos, Kazunori Kohri, David H. Lyth, Tomohiro Matsuda

arXiv:1110.2951v1

The primordial curvature perturbation \zeta may be generated by some curvaton field \sigma, which is negligible during inflation and has more or less negligible interactions until it decays. In the current version of this scenario, the curvaton starts to oscillate while its energy density \rho_\sigma is negligible. We propose a radically different version, in which \rho_\sigma drives a few e-folds of inflation before the oscillation begins. In the simplest case we find f_NL ~ -1 for the non-gaussianity parameter, which may eventually be observable.

A multiwavelength study of near- and mid-infrared selected galaxies at high redshift: ERGs, AGN-identification and the contribution from dust

Authors:Hugo Messias

arXiv:1110.2787v1

The main focus of this thesis is the IR spectral regime, which since the 70's and 80's has revolutionised our understanding of the Universe. A multi-wavelength analysis on Extremely Red Galaxy populations is first presented in one of the most intensively observed patch of the sky, the Chandra Deep Field South. By adopting a purely statistical methodology, we consider all the photometric and spectroscopic information available on large samples of Extremely Red Objects (EROs, 553 sources), IRAC EROs (IEROs, 259 sources), and Distant Red Galaxies (DRGs, 289 sources). We derive general properties: redshift distributions, AGN host fraction, star-formation rate densities, dust content, morphology, mass functions and mass densities. The results point to the fact that EROs, IEROs, and DRGs all belong to the same population, yet seen at different phases of galaxy evolution. The second part of this thesis is dedicated to the AGN selection in the IR, with particular relevance to the James Webb Space Telescope, to be launched in 2018. We develop an improved IR criterion (using K and IRAC bands) as an alternative to existing IR AGN criteria for the z<2.5 regime, and develop another IR criterion which reliably selects AGN hosts at 0<z<7 (using K, Spitzer-IRAC, and Spitzer-MIPS24um bands, KIM). The ability to track AGN activity since the end of reionization holds great advantages for the study of galaxy evolution. The thesis then focus on the importance of dust. Based on deep IR data on the Cosmological Survey, we derive rest-frame 1.6, 3.3, and 6.2um luminosity functions and their dependency on redshift. We estimate the dust contribution to those wavelengths and show that the hot dust luminosity density evolves since z=1-2 with a much steeper drop than the star-formation history of the Universe. Future prospects are finally discussed in the last chapter.

Light Loop Echoes and Blinking Black Holes

Authors:Latham Boyle (CITA, Perimeter), Matthew Russo (CITA)

arXiv:1110.2789v1

Radiation emitted near a black hole reaches the observer by multiple paths; and when this radiation varies in time, the time-delays between the various paths generate a "blinking" effect in the observed light curve L(t) or its auto-correlation function xi(T)= <L(t)L(t-T)>. For the particularly important "face-on" configuration (in which the hole is viewed roughly along its spin axis, while the emission comes roughly from its equatorial plane -- e.g. from the inner edge of its accretion disk, or from the violent flash of a nearby/infalling star) we calculate the blinking in detail by computing the time delay Delta t_{j}(r,a) and magnification mu_{j}(r,a) of the jth path (j=1,2,3,...), relative to the primary path (j=0), as a function of the emission radius r and black hole spin 0<a/M<1. The particular geometry and symmetry of the nearly-face-on configuration enhances and "protects" the blinking signal, making it more detectable and more independent of certain astrophysical and observational details. The effect can be surprisingly strong: e.g. for radiation from the innermost stable circular orbit ("ISCO") of a black hole of critical spin (a_{crit}/M = 0.853), the j=1,2,3 fluxes are, respectively, 27%, 2% and 0.1% of the j=0 flux.

arXiv: 13 October 2011

2011-10-18T18:38:00.001-07:00

Searches for Particle Dark Matter: An Introduction

Authors:Pat Scott

arXiv:1110.2757v1

The identity of dark matter is one of the key outstanding problems in both particle and astrophysics. In this thesis, I describe a number of complementary searches for particle dark matter. I discuss how the impact of dark matter on stars can constrain its interaction with nuclei, focussing on main sequence stars close to the Galactic Centre, and on the first stars as seen through the upcoming James Webb Space Telescope. The mass and annihilation cross-section of dark matter particles can be probed with searches for gamma rays produced in astronomical targets. Dwarf galaxies and ultracompact, primordially-produced dark matter minihalos turn out to be especially promising in this respect. I illustrate how the results of these searches can be combined with constraints from accelerators and cosmology to produce a single global fit to all available data. Global fits in supersymmetry turn out to be quite technically demanding, even with the simplest predictive models and the addition of complementary data from a bevy of astronomical and terrestrial experiments; I show how genetic algorithms can help in overcoming these challenges.

Dwarf Galaxies in the Coma Cluster: I. Velocity Dispersion Measurements

Authors:E. Kourkchi (1, 2), H. G. Khosroshahi (1), D. Carter (3), A.M. Karick (3), E. Mármol-Queraltó (4), K. Chiboucas (5, 6), R. B. Tully (5), B. Mobasher (7), R. Guzmán (8), A. Matković (9), N. Gruel (10) ((1) School of Astronomy (IPM), Tehran, Iran, (2) Department of Physics, Sharif University of Technology, Tehran, Iran, (3) Astrophysics Research Institute, Liverpool John Moores University, UK, (4) Departamento de Astrofśica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Madrid, Spain, (5) Institute for Astronomy, University of Hawaii, USA, (6) Gemini Observatory, Northern Operations Center, Hawaii, USA, (7) Department of Physics and Astronomy, University of California, USA, (8) Department of Astronomy, University of Florida, USA, (9) Department of Astronomy and Astrophysics, Pennsylvania State University, USA, (10) Centro de Estudios de Física del Cosmos de Aragón, Spain.)

arXiv:1110.2649v1

We present the study of a large sample of early-type dwarf galaxies in the Coma cluster observed with DEIMOS on the Keck II to determine their internal velocity dispersion. We focus on a subsample of 41 member dwarf elliptical galaxies for which the velocity dispersion can be reliably measured, 26 of which were studied for the first time. The magnitude range of our sample is $-21<M_R<-15$ mag.
This paper (paper I) focuses on the measurement of the velocity dispersion and their error estimates. The measurements were performed using {\it pPXF (penalised PiXel Fitting)} and using the Calcium triplet absorption lines. We use Monte Carlo bootstrapping to study various sources of uncertainty in our measurements, namely statistical uncertainty, template mismatch and other systematics. We find that the main source of uncertainty is the template mismatch effect which is reduced by using templates with a range of spectral types.
Combining our measurements with those from the literature, we study the Faber-Jackson relation ($L\propto\sigma^\alpha$) and find that the slope of the relation is $\alpha=1.99\pm0.14$ for galaxies brighter than $M_R\simeq-16$ mag. A comprehensive analysis of the results combined with the photometric properties of these galaxies is reported in paper II.

Dwarf Galaxies in the Coma Cluster: II. Spectroscopic and Photometric Fundamental Planes

Authors:E. Kourkchi (1 and 2), H. G. Khosroshahi (1), D. Carter (3), B. Mobasher (4) ((1) School of Astronomy (IPM) - Tehran - Iran., (2) Department of Physics - Sharif University of Technology - Tehran - Iran., (3) Astrophysics Research Institute - Liverpool John Moores University - UK., (4) Department of Physics and Astronomy - University of California - USA.)

arXiv:1110.2648v1

We present a study of the fundamental plane, FP, for a sample of 71 dwarf galaxies in the core of Coma cluster in magnitude range $-21 < M_I <-15$. Taking advantage of high resolution DEIMOS spectrograph on Keck II for measuring the internal velocity dispersion of galaxies and high resolution imaging of HST/ACS, which allows an accurate surface brightness modeling, we extend the fundamental plane (FP) of galaxies to $\sim$1 magnitude fainter luminosities than all the previous studies of the FP in Coma cluster. We find that, the scatter about the FP depends on the faint-end luminosity cutoff, such that the scatter increases for fainter galaxies. The residual from the FP correlates with the galaxy colour, with bluer galaxies showing larger residuals from FP.
We find $M/L \propto M^{-0.15\pm0.22}$ in F814W-band indicating that in faint dwarf ellipticals, the $M/L$ ratio is insensitive to the mass. We find that less massive dwarf ellipticals are bluer than their brighter counterparts, possibly indicating ongoing star formation activity. Although tidal encounters and harassment can play a part in removing stars and dark matter from the galaxy, we believe that the dominant effect will be the stellar wind associated with the star formation, which will remove material from the galaxy resulting in larger $M/L$ ratios. We attribute the deviation of a number of faint blue dwarfs from the FP of brighter ellipticals to this effect.
We also study other scaling relations involving galaxy photometric properties including the photometric plane. We show that, compared to the FP, the scatter about the photometric plane is smaller at the faint end.

arXiv: 12 October 2011

2011-10-11T19:10:00.001-07:00

Galaxy cluster number count data constraints on dark energy

Authors:L. Campanelli, G.L. Fogli, T. Kahniashvili, A. Marrone, Bharat Ratra

arXiv:1110.2310v1

We use data on massive galaxy clusters (M_cluster > 8 10^14 h^(-1) M_solar within a comoving radius of R_cluster = 1.5 h^(-1)Mpc) in the redshift range 0.05 < z < 0.83 to place constraints on the dark energy equation-of-state parameters (w_0,w_a), as well as on the nonrelativistic matter density parameter Omega_m, on the amplitude of mass fluctuations sigma_8, on the index n of the power-law spectrum of the density perturbations, and on the Hubble constant H_0. We show that present data weakly constrain (w_0,w_a) around the values corresponding to a cosmological constant, i.e. (w_0,w_a) = (-1,0). Cluster data alone prefer low values of the amplitude of mass fluctuations, sigma_8 < 0.69 (1 sigma C.L.), and large amounts of nonrelativistic matter, Omega_m > 0.38 (1 sigma C.L.), in slight tension with the LambdaCDM concordance cosmological model (the results are however compatible with LambdaCDM at 2sigma). We derive a sigma_8 normalization relation, sigma_8 Omega_m^(1/3) = 0.49 \pm 0.06 (2 sigma C.L.), and we find that the fit to data is almost independent of n and only very weakly dependent on H_0. Combining cluster data with baryon acoustic oscillation observations, cosmic microwave background data, Hubble constant measurements, Hubble parameter determination from passively-evolving red galaxies, and magnitude-redshift data of type Ia supernovae, we find sigma_8 = 0.73^(+0.03)_(-0.03), Omega_m = 0.28^(+0.03)_(-0.02), w_0 = -1.14^(+0.14)_(-0.16), w_a = 0.85^(+0.42)_(-0.60), H_0 = 69.1^(+1.3)_(-1.5) km/s/Mpc (all 1 sigma C.L. errors), in substantial agreement with the concordance cosmological model. Very similar results are found in the case of time-evolving dark energy with a constant equation-of-state parameter w = const (the XCDM parametrization). Finally, we show that the impact of bounds on (w_0,w_a) is to favor top-down phantom models of evolving dark energy.

Supersonic baryon-CDM velocities and CMB B-mode polarization

Authors:Simone Ferraro (1), Kendrick M. Smith (1), Cora Dvorkin (2) ((1) Princeton University, (2) Institute for Advanced Study, Princeton)

arXiv:1110.2182v1

It has recently been shown that supersonic relative velocities between dark matter and baryonic matter can have a significant effect on formation of the first structures in the universe. If this effect is still non-negligible during the epoch of hydrogen reionization, it generates large-scale anisotropy in the free electron density, which gives rise to a CMB B-mode. We compute the B-mode power spectrum and find a characteristic shape with acoustic peaks at l ~ 200, 400, ... The amplitude of this signal is a free parameter which is related to the dependence of the ionization fraction on the relative baryon-CDM velocity during the epoch of reionization. However, we find that the B-mode signal is undetectably small for currently favored reionization models in which hydrogen is reionized promptly at z ~ 10, although constraints on this signal by future experiments may help constrain models in which partial reionization occurs at higher redshift, e.g. by accretion onto primordial black holes.

Halo Scale Predictions of Symmetron Modified Gravity

Authors:Joseph Clampitt, Bhuvnesh Jain, Justin Khoury

arXiv:1110.2177v1

We offer predictions of symmetron modified gravity in the neighborhood of realistic dark matter halos. The predictions for the fifth force are obtained by solving the nonlinear symmetron equation of motion in the spherical NFW approximation. In addition, we compare the three major known screening mechanisms: Vainshtein, Chameleon, and Symmetron around such dark matter sources, emphasizing the significant differences between them and highlighting observational tests which exploit these differences. Finally, we demonstrate the host halo environmental screening effect ("blanket screening") on smaller satellite halos by solving for the modified forces around a density profile which is the sum of satellite and approximate host components.

arXiv: 11 October 2011s

2011-10-11T06:37:00.001-07:00

The key role of the Calan/Tololo project in the discovery of the accelerating Universe

Authors:Mario Hamuy

arXiv:1110.1958v1

The Nobel Prize in Physics 2011 has just been awarded to three astronomers: Saul Perlmutter, Brian Schmidt, and Adam Riess, for their amazing discovery of the accelerating expansion of the Universe. Without diminishing the achievement of our community's laureates, here I elaborate on the role of the C&T project in this discovery.

arXiv: 10 October 2011

2011-10-10T10:48:00.003-07:00

Testing Inflation with Dark Matter Halos

Authors:Marilena LoVerde, Simone Ferraro, Kendrick M. Smith

arXiv:1110.1594v1

Cosmic inflation provides a mechanism for generating the early density perturbations that seeded the large-scale structures we see today. Primordial non-Gaussianity is among the most promising of few observational tests of physics at this epoch. At present non-Gaussianity is best constrained by the cosmic microwave background, but in the near term large-scale structure data may be competitive so long as the effects of primordial non-Gaussianity can be modeled through the non-linear process of structure formation. We discuss recent work modeling effects of a few types of primordial non-Gaussianity on the large-scale halo clustering and the halo mass function. More specifically, we compare analytic and N-body results for two variants of the curvaton model of inflation: (i) a "tauNL" scenario in which the curvaton and inflaton contribute equally to the primordial curvature perturbation and (ii) a "gNL" model where the usual quadratic fNL term in the potential cancels, but a large cubic term remains.

Evolution of the Galaxy - Dark Matter Connection and the Assembly of Galaxies in Dark Matter Halos

Authors:Xiaohu Yang (SHAO), H.J. Mo (UMass), Frank C. van den Bosch (Yale), Youcai Zhang (SHAO), Jiaxin Han (SHAO)

arXiv:1110.1420v1

We present a new model to describe the galaxy-dark matter connection across cosmic time, which unlike the popular subhalo abundance matching technique is self-consistent in that it takes account of the facts that (i) subhalos are accreted at different times, and (ii) the properties of satellite galaxies may evolve after accretion. Using observations of galaxy stellar mass functions out to $z \sim 4$, the conditional stellar mass function at $z\sim 0.1$ obtained from SDSS galaxy group catalogues, and the two-point correlation function (2PCF) of galaxies at $z \sim 0.1$ as function of stellar mass, we constrain the relation between galaxies and dark matter halos over the entire cosmic history from $z \sim 4$ to the present. This relation is then used to predict the median assembly histories of different stellar mass components within dark matter halos (central galaxies, satellite galaxies, and halo stars). We also make predictions for the 2PCFs of high-$z$ galaxies as function of stellar mass. Our main findings are the following: (i) Our model reasonably fits all data within the observational uncertainties, indicating that the $\Lambda$CDM concordance cosmology is consistent with a wide variety of data regarding the galaxy population across cosmic time. (ii) ... [abridged]

Mapping the Universe: The 2010 Russell Lecture

Authors:Margaret J. Geller, Antonaldo Diaferio, Michael J. Kurtz

arXiv:1110.1380v1

Redshift surveys are a powerful tool of modern cosmology. We discuss two aspects of their power to map the distribution of mass and light in the universe: (1) measuring the mass distribution extending into the infall regions of rich clusters and (2) applying deep redshift surveys to the selection of clusters of galaxies and to the identification of very large structures (Great Walls). We preview the HectoMAP project, a redshift survey with median redshift z = 0.34 covering 50 square degrees to r= 21. We emphasize the importance and power of spectroscopy for exploring and understanding the nature and evolution of structure in the universe.

ECOSMOG: An Efficient Code for Simulating Modified Gravity

Authors:Baojiu Li (Durham and Cambridge), Gong-Bo Zhao (Portsmouth), Romain Teyssier (Zurich and CEA Saclay), Kazuya Koyama (Portsmouth)

arXiv:1110.1379v1

We introduce a new code, ECOSMOG, to run N-body simulations for a wide class of modified gravity and dynamical dark energy theories. These theories generally have one or more new dynamical degrees of freedom, the dynamics of which are governed by their (usually rather nonlinear) equations of motion. Solving these non-linear equations has been a great challenge in cosmology. Our code is based on the RAMSES code, which solves the Poisson equation on adaptively refined meshes to gain high resolutions in the high-density regions. We have added a solver for the extra degree(s) of freedom and performed numerous tests for the f(R) gravity model as an example to show its reliability. We find that much higher efficiency could be achieved compared with other existing mesh/grid-based codes thanks to two new features of the present code: (1) the efficient parallelisation and (2) the usage of the multigrid relaxation to solve the extra equation(s) on both the regular domain grid and refinements, giving much faster convergence even under much more stringent convergence criteria. This code is designed for performing high-accuracy, high-resolution and large-volume cosmological simulations for modified gravity and general dark energy theories, which can be utilised to test gravity and the dark energy hypothesis using the upcoming and future deep and high-resolution galaxy surveys.

arXiv: 7 October 2011

2011-10-10T10:48:00.001-07:00

Anti-de-Sitter spacetime and its uses

Authors:G.W. Gibbons

arXiv:1110.1206v1

This is a pedagogic account of some of the global properties of Anti-de-Sitter spacetime with a view to their application to the AdS/CFT correspondence. Particular care is taken over the distinction between Anti-de-Sitter and it's covering space.
Written version of lectures given at 2nd Samos Meeting held at at Pythagoreon, Samos, Greece, 31 August - 4 September 1998 and published as Anti-de-Sitter spacetime and its uses, in Mathematical and Quantum Aspects of Relativity and Cosmology. Proceedings of the 2nd Samos Meeting on Cosmology, Geometry and Relativity, S Cotsakis and G W Gibbons eds, {\it Lecture Notes in Physics}\, {\bf 537} (2000)}

On the Effects of Line-of-Sight Structures on Lensing Flux-ratio Anomalies in a LCDM Universe

Authors:D. D. Xu, Shude Mao, Andrew Cooper, Liang Gao, Carlos Frenk, Raul Angulo, John Helly

arXiv:1110.1185v1

The flux-ratio anomalies observed in multiply-lensed quasar images are most plausibly explained as the result of perturbing structures superposed on the underlying smooth matter distribution of the primary lens. The cold dark matter cosmological model predicts that a large number of substructures should survive inside larger halos but, surprisingly, this population alone has been shown to be insufficient to explain the observed distribution of the flux ratios of quasar's multiple images. Additional contributions from other halos (and their own subhalos) projected along the line of sight to the primary lens have been considered as a possible explanation, with inconclusive results so far. We use ray tracing through the Millennium II simulation to investigate the importanceof projection effects due to halos and subhalos of mass m>1E8 Msun/h and extend our analysis to lower masses, m>1E6 Msun/h, using Monte-Carlo halo distributions. We find that violations of the cusp-caustic relation caused by line-of-sight haloes are comparable to (or even larger than) those caused by intrinsic substructures. The magnitude of the violation depends strongly on the density profile and concentration of the intervening halos, but clustering plays only a minor role. For a typical lensing geometry (lens at redshift 0.6 and source at redshift 2), background haloes (behind the main lens) are more likely to cause a violation than foreground halos. The combined effect of substructures within the lens and along the line of sight in a LCDM universe results in a cusp-violation probability from lensing flux-ratio observations of ~20%. This is enough to reconcile the model with current data, but larger samples are required for a stronger test of the theory.

arXiv: 6 OCtober 2011

2011-10-06T06:42:00.001-07:00

GCG Parametrization for Growth Function and Current Constraints

Authors:Gaveshna Gupta, Somasri Sen, Anjan A Sen

arXiv:1110.0956v1

We study the linear growth function $f$ of the large scale structures in a cosmological scenario where Generalised Chaplygin Gas (GCG) serves as the dark energy candidate. %Along the line of Wang and Steinhardt \cite{ws:98}%, In doing so, we parametrize the growth index parameter as a function of redshift and do a comparative study between the theoretical growth rate and the proposed parametrization. Moreover, with this new parametrization, we demonstrate the growth rate of a wide range of dark energy models and compare their actual behaviour with our proposed parametrization. We show that our proposed parametrization accurately represents the growth rate for a number of different dark energy models. Finally we compile a data set consisting of 28 datapoints within redshift range (0.15,3.8) to constrain the growth rate. It includes direct growth data from various projects/surveys including the latest data from the Wiggle-Z measurements. It also includes data constraining growth indirectly through the rms mass fluctuation $\sigma_8(z)$ inferred from Ly-$\alpha$ measurements at various redshifts. By fitting our proposed parametrization for $f$ with these data, we show that the growth history of the large scale structure of the universe may prefer a transient late time acceleration.

Metric-Palatini gravity unifying local constraints and late-time cosmic acceleration

Authors:Tiberiu Harko, Tomi S. Koivisto, Francisco S.N. Lobo, Gonzalo J. Olmo

arXiv:1110.1049v1

We present a novel approach to modified theories of gravity that consists of adding to the Einstein-Hilbert Lagrangian an f(R) term constructed a la Palatini. Using the respective dynamically equivalent scalar-tensor representation, we show that the theory can pass the Solar System observational constraints even if the scalar field is very light. This implies the existence of a long-range scalar field, which is able to modify the cosmological and galactic dynamics, but leaves the Solar System unaffected. We also verify the absence of instabilities in perturbations and provide explicit models which are consistent with local tests and lead to the late-time cosmic acceleration.

arXiv: 5 October 2011

2011-10-05T07:08:00.001-07:00

Properties of Dark Compact Ultra Dense Objects

Authors:Christopher Dietl, Lance Labun, Johann Rafelski

arXiv:1110.0551v1

We consider compact astrophysical objects formed from dark matter fermions of mass 250 GeV to 100 TeV or from massless fermions hidden by vacuum structure of similar energy scale. These macroscopic objects have maximum stable masses of sub-planetary scale (asteroids) and radii of micron to centimeter scale. We describe the surface gravity and tidal forces near these compact ultra dense objects, as pertinent to signatures of their collisions with visible matter objects.

The imprint of cosmological non-Gaussianities on primordial structure formation

Authors:Umberto Maio, Sadegh Khochfar

arXiv:1110.0493v1

We study via numerical N-body/SPH chemistry simulations the effects of primordial non-Gaussianities on the formation of the first stars and galaxies, and investigate the impact of supernova feedback in cosmologies with different fnl. Density distributions are biased to higher values, so star formation and the consequent feedback processes take place earlier in high-fnl models and later in low-fnl ones. Mechanical feedback is responsible for shocking and evacuating the gas from star forming sites earlier in the highly non-Gaussian cases, because of the larger bias at high densities. Chemical feedback translates into high-redshift metal filling factors that are larger by some orders of magnitude for larger fnl, but that converge within one Gyr, for both population III and population II-I stellar regimes. The efficient enrichment process, though, leads to metallicities > 0.01 Zsun by redshift ~9, almost independently from fnl. The impact of non-Gaussianities on the formation of dark-matter haloes at high redshift is directly reflected in the properties of the gas in these haloes, as models with larger fnl show more concentrated gas profiles at early times. Non-Gaussian signatures in the gas behaviour are lost after the first feedback takes place and introduces a significant degree of turbulence and chaotic motions.

arXiv: 4 October 2011

2011-10-04T07:20:00.001-07:00

Precision cosmography with stacked voids

Authors:Guilhem Lavaux, Benjamin D. Wandelt

arXiv:1110.0345v1

We present a purely geometrical method for probing the expansion history of the Universe from the observation of the shape of stacked voids in spectroscopic re dshift surveys. Our method is an Alcock-Pasczinsky test based on the average sphericity of voids posited on the local isotropy of the Universe. It works by comparing the temporal extent of cosmic voids along the line of sight with their angular, spatial extent. We describe the algorithm that we use to detect and stack voids in redshift shells on the light cone and test it on mock light cones produced from N-body simulations. We establish a robust statistical model for estimating the average stretching of voids in redshift space and quantify the contamination by peculiar velocities. Finally, we assess the capability of this approach to constrain dark energy parameters in terms of the figure of merit (FoM) of the dark energy task force and in particular of the proposed Euclid mission which is particularly suited for this technique since it is a spectroscopic survey. The FoM due to stacked voids from the Euclid wide survey is double that of all other dark energy probes derived from Euclid data alone (combined with Planck priors). In particular, voids outperform Baryon Acoustic Oscillations by an order of magnitude. This result is consistent with simple estimates based on mode-counting. The Alcock-Paczinsky test based on stacked voids is therefore a potentially significant addition to the portfolio of major dark energy probes and the systematics that affect it should be studied in detail.

Astrophysical Constraints on Dark Matter

Authors:Charling Tao

arXiv:1110.0298v1

Astrophysics gives evidence for the existence of Dark Matter and puts constraints on its nature. The Cold Dark Matter model has become "standard" cosmology combined with a cosmological constant. There are indications that "Cold" Dark Matter could be "warmer" than initially discussed. This paper reviews the main information on the Cold/Warm nature of Dark Matter

Modeling the evolution of infrared galaxies : clustering of galaxies in the Cosmic Infrared Background

Authors:Aurélie Pénin, Olivier Doré, Guilaine Lagache, Matthieu Béthermin

arXiv:1110.0395v1

Star-forming galaxies are a highly biased tracer of the underlying dark matter density field. Their clustering can be studied through the cosmic infrared background anisotropies. These anisotropies have been measured from 100 \mum to 2 mm in the last few years. In this paper, we present a fully parametric model allowing a joint analysis of these recent observations. In order to develop a coherent model at various wavelengths, we rely on two building blocks. The first one is a parametric model that describes the redshift evolution of the luminosity function of star-forming galaxies. It compares favorably to measured differential number counts and luminosity functions. The second one is a halo model based description of the clustering of galaxies. Starting from a fiducial model, we investigate parameter degeneracies using a Fisher analysis. We then discuss how halo of different mass and redshift, how LIRGs and ULIRGs, contribute to the CIB angular power spectra.
From the Fisher analysis, we conclude that we cannot constrain the parameters of the model of evolution of galaxies using clustering data only. The use of combined data of C\ell, counts and luminosity functions improves slightly the constraints but does not remove any degeneracies. On the contrary, the measurement of the anisotropies allows us to set interesting constraints on the halo model parameters, even if some strong degeneracies remain. Using our fiducial model, we establish that the 1-halo and 2-halo terms are not sensitive to the same mass regime. We also illustrate how the 1-halo term can be misinterpreted with the Poisson noise term. Conclusions. We present a new model of the clustering of infrared galaxies. However such a model has a few limitations, as the parameters of the halo occupation suffer from strong degeneracies.

arXiv: 3 October 2011

2011-10-03T15:39:00.001-07:00

Variation of fundamental parameters and dark energy. A principal component approach

Authors:L. Amendola (ITP, U. Heidelberg), A.C.O. Leite (U. Porto), C.J.A.P. Martins (CAUP, U. Porto), N.J. Nunes (ITP, U. Heidelberg), P.O.J. Pedrosa (U. Porto), A. Seganti (U. Roma "La Sapienza")

arXiv:1109.6793v1

We discuss methods based on Principal Component Analysis for reconstructing the dark energy equation of state and constraining its evolution, using a combination of Type Ia supernovae at low redshift and spectroscopic measurements of varying fundamental couplings at higher redshifts. We discuss the performance of this method when future better-quality datasets are available, focusing on two forthcoming ESO spectrographs -- ESPRESSO for the VLT and CODEX for the E-ELT -- which include these measurements as a key part of their science cases. These can realize the prospect of a detailed characterization of dark energy properties all the way up to redshift 4.

Segue 1: the best dark matter candidate dwarf galaxy surveyed by MAGIC

Authors:J. Aleksić, M. Doro, S. Lombardi, D. Nieto, for the MAGIC Collaboration, M. Fornasa

arXiv:1109.6781v1

Despite the interest in Dark Matter (DM) searches is currently more focused on underground experiments, a signature of DM annihilation/decay in gamma-rays from space would constitute a smoking gun for its identification. In this contribution, we start with a brief review of the efforts of the ground-based MAGIC Cherenkov telescopes system to detect DM signatures from dwarf satellite galaxies orbiting the Milky Way halo. We then present the recent survey of Segue 1, considered by many as possibly the most DM dominated satellite galaxy known in our galaxy. No significant gamma-ray emission was found above the background in around 30 hours of observation. This is the largest survey ever made on a single dwarf by Cherenkov telescopes. We present a novel analysis that fully takes into account the spectral features of the gamma-ray spectrum of specific DM models in a Supersymmetric scenario. We also discuss the prospects of detection after the Fermi observation of similar objects at lower energies.

arXiv: 30 September 2011

2011-09-30T09:00:00.001-07:00

Gravitational redshift of galaxies in clusters as predicted by general relativity

Radoslaw Wojtak, Steen H. Hansen, Jens Hjorth

arXiv:1109.6571v1

The theoretical framework of cosmology is mainly defined by gravity, of which general relativity is the current model. Recent tests of general relativity within the \Lambda Cold Dark Matter (CDM) model have found a concordance between predictions and the observations of the growth rate and clustering of the cosmic web. General relativity has not hitherto been tested on cosmological scales independent of the assumptions of the \Lambda CDM model. Here we report observation of the gravitational redshift of light coming from galaxies in clusters at the 99 per cent confidence level, based upon archival data. The measurement agrees with the predictions of general relativity and its modification created to explain cosmic acceleration without the need for dark energy (f(R) theory), but is inconsistent with alternative models designed to avoid the presence of dark matter.

arXiv: 29 September 2011

2011-09-29T15:41:00.001-07:00

Interacting Dark Energy -- constraints and degeneracies

Timothy Clemson, Kazuya Koyama, Gong-Bo Zhao, Roy Maartens, Jussi Väliviita

arXiv:1109.6234v1

In standard cosmologies, dark energy interacts only gravitationally with dark matter. There could be a non-gravitational interaction in the dark sector, leading to changes in the effective DE equation of state, in the redshift dependence of the DM density and in structure formation. We use CMB, BAO and SNIa data to constrain a model where the energy transfer in the dark sector is proportional to the DE density. There are two subclasses, defined by the vanishing of momentum transfer either in the DM or the DE frame. We conduct a Markov-Chain Monte-Carlo analysis to obtain best-fit parameters. The background evolution allows large interaction strengths, and the constraints from CMB anisotropies are weak. The growth of DM density perturbations is much more sensitive to the interaction, and can deviate strongly from the standard case. However, the deviations are degenerate with galaxy bias and thus more difficult to constrain. Interestingly, the ISW signature is suppressed since the non-standard background evolution can compensate for high growth rates. We also discuss the partial degeneracy between interacting DE and modified gravity, and how this can be broken.

The Halo Model of Large Scale Structure for Warm Dark Matter

Robyn M. Dunstan, Kevork N. Abazajian, Emil Polisensky, Massimo Ricotti

arXiv:1109.6291v1

We present a comprehensive analysis of the halo model of cosmological large to small-scale structure statistics in the case of warm dark matter (WDM) structure formation scenarios. We include the effects of WDM on the linear matter power spectrum, halo density profile, halo concentration relation, halo mass function, subhalo density profile, subhalo mass function and biasing of the smooth dark matter component. As expected, we find large differences at the smallest physical scales in the nonlinear matter power spectrum predicted in the halo model between WDM and cold dark matter even for reasonably high-scale WDM particle masses. We find that significant effects are contributed from the alteration of the halo density profile and concentration, as well as the halo mass function. We further find that the effects of WDM on the subhalo population are important but sub-dominant. Clustering effects of the biasing of the smooth component in WDM is not largely significant.

arXiv: 28 September 2011

2011-09-29T15:37:00.001-07:00

Modified Gravity and the CMB

Philippe Brax, Anne-Christine Davis

arXiv:1109.5862v1

We consider the effect of modified gravity on the peak structure of the Cosmic Microwave Background (CMB) spectrum. We focus on simple models of modified gravity mediated by a massive scalar field coupled to both baryons and cold dark matter. This captures the features of chameleon, symmetron, dilaton and $f(R)$ models. We find that the CMB peaks can be affected in three independent ways provided the Compton radius of the massive scalar is not far-off the sound horizon at last scattering. When the coupling of the massive scalar to Cold Dark Matter (CDM) is large, the anomalous growth of the CDM perturbation inside the Compton radius induces a change in the peak amplitudes. When the coupling to baryons is moderately large, the speed of sound is modified and the peaks shifted to higher momenta. Finally when both couplings are non-vanishing, a new contribution proportional to the Newton potential appears in the Sachs-Wolfe temperature and increases the peak amplitudes. We also show how, given any temporal evolution of the scalar field mass, one can engineer a corresponding modified gravity model of the chameleon type. This opens up the possibility of having independent constraints on modified gravity from the CMB peaks and large scale structures at low redshifts.

arXiv: 27 September 2011

2011-09-29T15:26:00.001-07:00

Dark Energy from the log-transformed convergence field

Hee-Jong Seo, Masanori Sato, Masahiro Takada, Scott Dodelson

arXiv:1109.5639v1

A logarithmic transform of the convergence field improves `the information content', ie., the overall precision associated with the measurement of the amplitude of the convergence power spectrum by improving the covariance matrix properties. The translation of this improvement in the information content to that in cosmological parameters, such as those associated with dark energy, requires knowing the sensitivity of the log-transformed field to those cosmological parameters. In this paper we use N-body simulations with ray tracing to generate convergence fields at multiple source redshifts as a function of cosmology. The gain in information associated with the log-transformed field does lead to tighter constraints on dark energy parameters, but only if shape noise is neglected. The presence of shape noise quickly diminishes the advantage of the log mapping, more quickly than we would expect based on the information content. With or without shape noise, using a larger pixel size allows for a more efficient log-transformation.

Hybrid Inflation: Multi-field Dynamics and Cosmological Constraints

Sébastien Clesse

arXiv:1109.5575v1

The dynamics of hybrid models is usually approximated by the evolution of a scalar field slowly rolling along a nearly flat valley. Inflation ends with a waterfall phase, due to a tachyonic instability. This final phase is usually assumed to be nearly instantaneous. In this thesis, we go beyond these approximations and analyze the exact 2-field dynamics of hybrid models. Several effects are put in evidence: 1) the possible slow-roll violations along the valley induce the non existence of inflation at small field values. Provided super-planckian fields, the scalar spectrum of the original model is red, in agreement with observations. 2) The initial field values are not fine-tuned along the valley but also occupy a considerable part of the field space exterior to it. They form a structure with fractal boundaries. Using bayesian methods, their distribution in the whole parameter space is studied. Natural bounds on the potential parameters are derived. 3) For the original model, inflation is found to continue for more than 60 e-folds along waterfall trajectories in some part of the parameter space. The scalar power spectrum of adiabatic perturbations is modified and is generically red, possibly in agreement with CMB observations. Topological defects are conveniently stretched outside the observable Universe. 4) The analysis of the initial conditions is extended to the case of a closed Universe, in which the initial singularity is replaced by a classical bounce. In the third part of the thesis, we study how the present CMB constraints on the cosmological parameters could be ameliorated with the observation of the 21cm cosmic background, by future giant radio-telescopes. Forecasts are determined for a characteristic Fast Fourier Transform Telescope, by using both Fisher matrix and MCMC methods.

arXiv: 26 September 2011

2011-09-29T14:52:00.001-07:00

Microlensing of Kepler Stars as a Method of Detecting Primordial Black Hole Dark Matter

Kim Griest, Matthew J. Lehner, Agnieszka M. Cieplak,Bhuvnesh Jain

arXiv:1109.4975v1

If the Dark Matter consists of primordial black holes (PBHs), we show that gravitational lensing of stars being monitored by NASA's Kepler search for extra-solar planets can cause significant numbers of detectable microlensing events. A search through the roughly 150,000 lightcurves would result in large numbers of detectable events for PBHs in the mass range $5 \ten{-10}\msun$ to $\aten{-4}\msun$. Non-detection of these events would close almost two orders of magnitude of the mass window for PBH dark matter. The microlensing rate is higher than previously noticed due to a combination of the exceptional photometric precision of the Kepler mission and the increase in cross section due to the large angular sizes of the relatively nearby Kepler field stars. We also present a new formalism for calculating optical depth and microlensing rates in the presence of large finite-source effects.

Cosmic String constraints from WMAP and SPT

Cora Dvorkin (1,2), Mark Wyman (1), Wayne Hu (1) ((1) KICP, University of Chicago, (2) Institute for Advanced Study, Princeton)

arXiv:1109.4947v1

The predictions of the inflationary LCDM paradigm match today's high-precision measurements of the cosmic microwave background anisotropy with remarkable precision. The same data put tight limits on other sources of anisotropy. Cosmic strings are a particularly interesting alternate source to constrain. Strings are topological defects, remnants of inflationary-era physics that persist after the big bang. They are formed in a variety of models of inflation, including popular string theory models such as brane inflation. In this paper, we show that measurements of temperature anisotropy by the South Pole Telescope break a parameter degeneracy in the WMAP data, permitting us to place a strong upper limit on the possible string contribution to the CMB anisotropy: the power sourced by zero-width strings must be <1.75% (95% CL) of the total. In the model we use, this translates to an upper limit on the string tension of Gmu < 1.7x10^{-7}. These limits imply that the best hope for detecting strings in the CMB will come from B-mode polarization measurements at arcminute scales rather than the degree scales pursued for gravitational wave detection.

The Mildly Non-Linear Regime of Structure Formation

Svetlin Tassev, Matias Zaldarriaga

arXiv:1109.4939v1

We present a simple physically motivated picture for the mildly non-linear regime of structure formation, which captures the effects of the bulk flows. We apply this picture to develop a method to significantly reduce the sample variance in cosmological N-body simulations at the scales relevant to the Baryon Acoustic Oscillations (BAO). The results presented in this paper will allow for a speed-up of an order of magnitude (or more) in the scanning of the cosmological parameter space using N-body simulations for studies which require a good handle of the mildly non-linear regime, such as those targeting the BAO. Using this physical picture we develop a simple model, which allows for the rapid calculation of the mildly non-linear matter power spectrum to percent level accuracy, and for robust estimation of the BAO scale.

arXiv: 23 September 2011

2011-09-29T14:43:00.001-07:00

Self-Calibration Technique for 3-point Intrinsic Alignment Correlations in Weak Lensing Surveys

M. A. Troxel, Mustapha Ishak (The University of Texas at Dallas)

arXiv:1109.4896v1

The intrinsic alignment (IA) of galaxies has been shown to be a significant barrier to precision cosmic shear measurements. Zhang [P. Zhang, Astrophys. J. 720, 1090 (2010)] proposed a self-calibration technique for the power spectrum to calculate the induced gravitational shear-galaxy intrinsic ellipticity correlation (GI) in weak lensing surveys with photo-z measurements which is expected to reduce the IA contamination by at least a factor of 10 for currently proposed surveys. We confirm this using an independent analysis and propose an expansion to the self-calibration technique for the bispectrum in order to calculate the dominant IA gravitational shear-gravitational shear-intrinsic ellipticity correlation (GGI) contamination. We first establish an estimator to extract the galaxy density-density-intrinsic ellipticity (ggI) correlation from the galaxy ellipticity-density-density measurement for a photo-z galaxy sample. We then develop a relation between the GGI and ggI bispectra, which allows for the estimation and removal of the GGI correlation from the cosmic shear signal. We explore the performance of these two methods, compare to other possible sources of error, and show that the GGI self-calibration technique can potentially reduce the IA contamination by up to a factor of 5-10 for all but a few bin choices, thus reducing the contamination to the percent level. The self-calibration is less accurate for adjacent bins, but still allows for a factor of three reduction in the IA contamination. The self-calibration thus promises to be an efficient technique to isolate both the 2-point and 3-point intrinsic alignment signals from weak lensing measurements.

Future constraints on the Hu-Sawicki modified gravity scenario

Matteo Martinelli, Alessandro Melchiorri, Olga Mena, Valentina Salvatelli, Zahara Girones

We present current and future constraints on the Hu and Sawicki modified gravity scenario. This model can reproduce a late time accelerated universe and evade solar system constraints. While current cosmological data still allows for distinctive deviations from the cosmological constant picture, future measurements of the growth of structure combined with Supernova Ia luminosity distance data will greatly improve present constraints.

Cosmic Acceleration from Causal Backreaction in a Smoothly Inhomogeneous Universe

Brett Bochner

arXiv:1109.4686v1

A phenomenological formalism is presented in which the apparent acceleration of the universe is generated by large-scale structure formation, thus eliminating the coincidence and magnitude fine-tuning problems of the Cosmological Constant in the Concordance Model, as well as potential instability issues with dynamical Dark Energy. The observed acceleration results from the combined effect of innumerable local perturbations, due to individually virialized systems, overlapping together in a smoothly-inhomogeneous adjustment of the FRW metric, in a process governed by the causal flow of inhomogeneity information outward from each clumped system. We discuss several arguments from the literature claiming to place sharp limits upon the strength of backreaction-related effects, and show why such arguments are not applicable in a physically realistic cosmological analysis. A selection of simply-parameterized models are presented, including several which are capable of fitting the luminosity distance data from Type Ia supernovae essentially as well as the best-fit flat \Lambda CDM model, without resort to Dark Energy, any modification to gravity, or a local void. Simultaneously, these models can reproduce measured cosmological parameters such as the age of the universe, the matter density required for spatial flatness, the present-day deceleration parameter, and the angular scale of the Cosmic Microwave Background to within a reasonable proximity of their Concordance values. We conclude by considering potential observational signatures for distinguishing this cosmological formalism from \Lambda CDM or Dark Energy, as well as the possible long-term fate of such a universe with ever-spreading spheres of influence for its increasingly superposed perturbations.

Moving mesh cosmology: characteristics of galaxies and haloes

Dusan Keres (1), Mark Vogelsberger (2), Debora Sijacki (2), Volker Springel (3), Lars Hernquist (2) ((1) UC Berkeley, (2) Harvard/CfA, (3) HITS)

arXiv:1109.4638v1

We discuss cosmological hydrodynamic simulations of galaxy formation performed with the new moving-mesh code AREPO, which promises higher accuracy compared with the traditional SPH technique that has been widely employed for this problem. We use an identical set of physics in corresponding simulations carried out with the well-tested SPH code GADGET, adopting also the same high-resolution gravity solver. We are thus able to compare both simulation sets on an object-by-object basis, allowing us to cleanly isolate the impact of different hydrodynamical methods on galaxy and halo properties. In accompanying papers, we focus on an analysis of the global baryonic statistics predicted by the simulation codes (Vogelsberger et al. (2011)), and complementary idealized simulations that highlight the differences between the hydrodynamical schemes (Sijacki et al. (2011)). Here we investigate their influence on the baryonic properties of simulated galaxies and their surrounding haloes. We find that AREPO leads to significantly higher star formation rates for galaxies in massive haloes and to more extended gaseous disks in galaxies, which also feature a thinner and smoother morphology than their GADGET counterparts. Consequently, galaxies formed in AREPO have larger sizes and higher specific angular momentum than their SPH correspondents. We show that these differences persist as a function of numerical resolution. While both codes agree to acceptable accuracy on a number of baryonic properties of cosmic structures, our results thus clearly demonstrate that galaxy formation simulations greatly benefit from the use of more accurate hydrodynamical techniques such as AREPO and call into question the reliability of galaxy formation studies in a cosmological context using traditional formulations of SPH. [Abridged]

arXiv: 21 September 2011

2011-09-20T23:02:00.005-07:00

Dark matter halo occupation: environment and clustering

Rupert Croft (CMU), Tiziana Di Matteo (CMU), Nishikanta Khandai (CMU), Volker Springel (HITS), Anirban Jana (PSC), Jeffrey Gardner (UW)

arXiv:1109.4169v1

We use a large dark matter simulation of a LambdaCDM model to investigate the clustering and environmental dependence of the number of substructures in a halo. Focusing on redshift z=1, we find that the halo occupation distribution is sensitive at the tens of percent level to the surrounding density and to a lesser extent to asymmetry of the surrounding density distribution. We compute the autocorrelation function of halos as a function of occupation, building on the finding of Wechsler et al. (2006) and Gao and White (2007) that halos (at fixed mass) with more substructure are more clustered. We compute the relative bias as a function of occupation number at fixed mass, finding a strong relationship. At fixed mass, halos in the top 5% of occupation can have an autocorrelation function ~ 1.5-2 times higher than the mean. We also compute the bias as a function of halo mass, for fixed halo occupation. We find that for group and cluster sized halos, when the number of subhalos is held fixed, there is a strong anticorrelation between bias and halo mass. Such a relationship represents an additional challenge to the halo model.

Galileons on Cosmological Backgrounds

Garrett Goon, Kurt Hinterbichler, Mark Trodden

arXiv:1109.3450v1

We construct four-dimensional effective field theories of a generalized DBI galileon field, the dynamics of which naturally take place on a Friedmann-Robertson-Walker spacetime. The theories are invariant under non-linear symmetry transformations, which can be thought of as being inherited from five-dimensional bulk Killing symmetries via the probe brane technique through which they are constructed. The resulting model provides a framework in which to explore the cosmological role that galileons may play as the universe evolves.

arXiv: 20 September 2011

2011-09-20T23:02:00.003-07:00

The integrated Sachs-Wolfe imprints of cosmic superstructures: a problem for $Λ$CDM

Seshadri Nadathur, Shaun Hotchkiss, Subir Sarkar

arXiv:1109.4126v1

A crucial diagnostic of the $\Lambda$CDM cosmological model is the integrated Sachs-Wolfe (ISW) effect of large-scale structure on the cosmic microwave background (CMB). The ISW imprint of superstructures of size $\sim100\;h^{-1}$Mpc at redshift $z\sim0.5$ has been detected with $>4\sigma$ significance, however it has been noted that the signal is much larger than expected. We revisit the calculation using linear theory predictions in $\Lambda$CDM cosmology for the number density of superstructures and their radial density profile, and take possible selection effects into account. While our expected signal is larger than previous estimates, it is still inconsistent by $>3\sigma$ with the observation. If the observed signal is indeed due to the ISW effect then huge, extremely underdense voids are far more common in the observed universe than predicted by $\Lambda$CDM.

The Matter Power Spectrum of Dark Energy Models and the Harrison-Zel'dovich Prescription

Iván Durán (Barcelona, Spain), Fernando Atrio-Barandela (Salamanca, Spain), Diego Pavón (Barcelona, Spain)

arXiv:1109.4038v1

The Harrison-Zel'dovich ansatz assumes that the amplitude of matter density perturbations at horizon crossing is the same at all scales. Using this prescription, we show how to construct the matter power spectrum of generic dark energy models that evolve similarly to the standard $\Lambda$CDM cosmology prior to matter radiation equality. Our approach allows to test models using data on the normalization of matter and radiation power spectra without fully solving the dynamical equations that describe the evolution of energy density perturbations.

Some aspects of field equations in generalised theories of gravity

T. Padmanabhan

arXiv:1109.3846v1

A class of theories of gravity based on a Lagrangian which depends on the curvature and metric - but not on the derivatives of the curvature tensor - is of interest in several contexts including in the development of the paradigm that treats gravity as an emergent phenomenon. This class of models contains, as an important subset, all Lanczos-Lovelock models of gravity. I derive several identities and properties which are useful in the study of these models and clarify some of the issues that seem to have received insufficient attention in the past literature.

arXiv: 19 September 2011

2011-09-20T23:02:00.001-07:00

Observing the Multiverse with Cosmic Wakes

Matthew Kleban (NYU), Thomas S. Levi (UBC), Kris Sigurdson (UBC)

arXiv:1109.3473v1

Current theories of the origin of the Universe, including string theory, predict the existence of a multiverse containing many bubble universes. These bubble universes will generically collide, and collisions with ours produce cosmic wakes that enter our Hubble volume, appear as unusually symmetric disks in the cosmic microwave background (CMB) and disturb large scale structure (LSS). There is preliminary observational evidence consistent with one or more of these disturbances on our sky. However, other sources can produce similar features in the CMB temperature map and so additional signals are needed to verify their extra-universal origin. Here we find, for the first time, the detailed three-dimensional shape and CMB temperature and polarization signals of the cosmic wake of a bubble collision in the early universe consistent with current observations. The predicted polarization pattern has distinctive features that when correlated with the corresponding temperature pattern are a unique and striking signal of a bubble collision. These features represent the first verifiable prediction of the multiverse paradigm and might be detected by current experiments such as Planck and future CMB polarization missions. A detection of a bubble collision would confirm the existence of the Multiverse, provide compelling evidence for the string theory landscape, and sharpen our picture of the Universe and its origins.

arXiv: 16 September 2011

2011-09-18T01:30:00.001-07:00

Properties of Galaxies and Groups: Nature versus Nurture

Sami-Matias Niemi

PHD theisi

arXiv:1109.3426v1

Due to the inherently nonlinear nature of gravity cosmological N-body simulations have become an invaluable tool when the growth of structure is being studied and modelled closer to the present epoch. Large simulations with high dynamical range have made it possible to model the formation and growth of cosmic structure with unprecedented accuracy. Moreover, galaxies, the basic building blocks of the Universe, can also be modelled in cosmological context. However, despite all the simulations and successes in recent decades, there are still many unanswered questions in the field of galaxy formation and evolution. One of the longest standing issue being the significance of the formation place and thus initial conditions to a galaxy's evolution in respect to environment, often formulated simply as "nature versus nurture" like in human development and psychology. Unfortunately, our understanding of galaxy evolution in different environments is still limited, albeit, for example, the morphology-density relation has shown that the density of the galaxy's local environment can affect its properties. Consequently, the environment should play a role in galaxy evolution, however despite the efforts, the exact role of the galaxy's local environment to its evolution remains open. This thesis introduction discusses briefly the background cosmology, cosmological N-body simulations and semi-analytical models. The second part is reserved for groups of galaxies, whether they are gravitationally bound, and what this may imply for galaxy evolution. The third part of the thesis concentrates on describing results of a case study of isolated field elliptical galaxies. The final chapter discusses another case study of luminous infra-red galaxies.

Second-order weak lensing from modified gravity

R. Ali Vanderveld, Robert R. Caldwell, Jason Rhodes

arXiv:1109.3189v1

We explore the sensitivity of weak gravitational lensing to second-order corrections to the spacetime metric within a cosmological adaptation of the parameterized post-Newtonian framework. Whereas one might expect nonlinearities of the gravitational field to introduce non-Gaussianity into the statistics of the lensing convergence field, we show that such corrections are actually always small within a broad class of scalar-tensor theories of gravity. We show this by first computing the weak lensing convergence within our parameterized framework to second order in the gravitational potential, and then computing the relevant post-Newtonian parameters for scalar-tensor gravity theories. In doing so we show that this potential systematic factor is generically negligible, thus clearing the way for weak lensing to provide a direct tracer of mass on cosmological scales despite uncertainties in possible departures from general relativity.

Cosmic acceleration from modified gravity with Palatini formalism

A. Borowiec, M. Kamionka, A. Kurek, M. Szydlowski

arXiv:1109.3420v1

We study new FRW type cosmological models of modified gravity treated on the background of Palatini approach. These models are generalization of Einstein gravity by the presence of a scalar field non-minimally coupled to the curvature. The models employ Starobinsky's term in the Lagrangian and dust matter. Therefore, as a by-product, an exhausted cosmological analysis of quadratic gravity is presented. We investigate dynamics of our models, confront them with the currently available astrophysical data as well as against $\Lambda$CDM model. We have used the dynamical system methods to investigate dynamics of the models. It reveals the presence of final sudden singularity. Fitting free parameters we have demonstrated that this class of models is in very good agreement with the data (including CMB measurements) in a sense that their Hubble diagrams are practically indistinguishable from the standard $\Lambda$CDM model. Therefore Bayesian methods of model selection have been employed in order to indicate preferred model. Only in the light of CMB data the concordance model remains invincible.

arXiv: 15 September 2011

2011-09-18T00:56:00.001-07:00

Feeding your Inflaton: Non-Gaussian Signatures of Interaction Structure

Neil Barnaby, Sarah Shandera

arXiv:1109.2985v1

Primordial non-Gaussianity is generated by interactions of the inflaton field, either self-interactions or couplings to other sectors. These two physically different mechanisms can lead to nearly indistinguishable bispectra of the equilateral type, but generate distinct patterns in the relative scaling of higher order moments. We illustrate these classes in a simple effective field theory framework where the flatness of the inflaton potential is protected by a softly broken shift symmetry. Since the distinctive difference between the two classes of interactions is the scaling of the moments, we investigate the implications for observables that depend on the series of moments. We obtain analytic expressions for the Minkowski functionals and the halo mass function for an arbitrary structure of moments, and use these to demonstrate how different classes of interactions might be distinguished observationally. Our analysis casts light on a number of theoretical issues, in particular we clarify the difference between the physics that keeps the distribution of fluctuations nearly Gaussian, and the physics that keeps the calculation under control.

The Sagittarius impact as an architect of spirality and outer rings in the Milky Way

Chris W. Purcell, James S. Bullock, Erik Tollerud, Miguel Rocha, Sukanya Chakrabarti

arXiv:1109.2918v1

Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar, although our knowledge of its structure and origin is incomplete. Traditional attempts to understand the Galaxy's morphology assume that it has been unperturbed by major external forces. Here we report simulations of the response of the Milky Way to the infall of the Sagittarius dwarf galaxy (Sgr), which results in the formation of spiral arms, influences the central bar and produces a flared outer disk. Two ring-like wrappings emerge towards the Galactic anti-Center in our model that are reminiscent of the low- latitude arcs observed in the same area of the Milky Way. Previous models have focused on Sgr itself to reproduce the dwarf's orbital history and place associated constraints on the shape of the Milky Way gravitational potential, treating the Sgr impact event as a trivial influence on the Galactic disk. Our results show that the Milky Way's morphology is not purely secular in origin and that low-mass minor mergers predicted to be common throughout the Universe probably have a similarly important role in shaping galactic structure.

arXiv: 14 September 2011

2011-09-18T00:47:00.001-07:00

Testing the cosmic distance duality with X-ray gas mass fraction and supernovae data

R. S. Goncalves, R. F. L. Holanda, J. S. Alcaniz

arXiv:1109.2790v1

In this paper we discuss a new cosmological model-independent test for the cosmic distance duality relation (CDDR), $\eta = D_{L}(L)(1+z)^{-2}/D_{A}(z)=1$, where $D_{A}(z)$ and $D_{L}(z)$ are the angular and luminosity distances, respectively. Using the general expression for X-ray gas mass fraction ($f_{gas}$) of galaxy clusters, $f_{gas} \propto D_L{D_A}^{1/2}$, we show that $f_{gas}$ observations jointly with type Ia supernovae (SNe Ia) data furnish a validity test for the CDDR. To perform our analysis we use 38 $f_{gas}$ measurements recently studied by two groups considering different assumptions to describe the clusters (La Roque {\it{et al.}} 2006 and Ettori {\it{et al.}} 2009) and two subsamples of SNe Ia distance lumonosity extracted from the Union2 compilation. In our test we consider the $\eta$ parameter as a function of the redshift parameterized by two different functional forms. It is found that the La Roque {\it{et al.}} (2006) sample is in perfect agreement with the duality relation ($\eta = 1$) whereas the Ettori {\it{et al.}} (2009) sample presents a significant conflict.

The Dynamical State of Dark Matter Haloes in Cosmological Simulations I: Correlations with Mass Assembly History

Chris Power, Alexander Knebe, Steffen R. Knollmann

arXiv:1109.2671v1

Using a statistical sample of dark matter haloes drawn from a suite of cosmological N-body simulations of the Cold Dark Matter (CDM) model, we quantify the impact of a simulated halo's mass accretion and merging history on two commonly used measures of its dynamical state, the virial ratio eta and the centre of mass offset Delta r. Quantifying this relationship is important because the degree to which a halo is dynamically equilibrated will influence the reliability with which we can measure characteristic equilibrium properties of the structure and kinematics of a population of haloes. We begin by verifying that a halo's formation redshift zform correlates with its virial mass Mvir and we show that the fraction of its recently accreted mass and the likelihood of it having experienced a recent major merger increases with increasing Mvir and decreasing zform. We then show that both eta and Delta r increase with increasing Mvir and decreasing zform, which implies that massive recently formed haloes are more likely to be dynamically unrelaxed than their less massive and older counterparts. Our analysis shows that both eta and Delta r are good indicators of a halo's dynamical state, showing strong positive correlations with recent mass accretion and merging activity, but we argue that Delta r provides a more robust and better defined measure of dynamical state for use in cosmological N-body simulations at z~0. We find that Delta r < 0.04 is sufficient to pick out dynamically relaxed haloes at z=0. Finally, we assess our results in the context of previous studies, and consider their observational implications.

What determines the fraction of elliptical galaxies in clusters?

Gabriella De Lucia, Fabio Fontanot, Dave Wilman

arXiv:1109.2599v1

We study the correlation between the morphological mix of cluster galaxies and the assembly history of the parent cluster by taking advantage of two independently developed semi-analytic models for galaxy formation and evolution. In our models, both the number of cluster members and that of elliptical members increase as a function of cluster mass, in such a way that the resulting elliptical fractions are approximately independent of cluster mass. The population of cluster ellipticals exhibit a marked bimodal distribution as a function of galaxy stellar mass, with a dip at masses $\sim 10^{10}\,{\rm M}_{\odot}$. In the framework of our models, this bimodality originates from the combination of a strongly decreasing number of galaxies with increasing stellar mass, and a correspondingly increasing probability of experiencing major mergers. We show that the correlation between the measured elliptical fraction and the assembly history of the parent cluster is weak, and that it becomes stronger in models that adopt longer galaxy merger times. We argue that this results from the combined effect of a decreasing bulge production due to a reduced number of mergers, and an increasing survival probability of pre-existing ellipticals, with the latter process being more important than the former.

Lensing Time Delays and Cosmological Complementarity

Eric V. Linder

arXiv:1109.2592v1

Time delays in strong gravitational lensing systems possess significant complementarity with distance measurements to determine the dark energy equation of state, as well as the matter density and Hubble constant. Time delays are most useful when observations permit detailed lens modeling and variability studies, requiring high resolution imaging, long time monitoring, and rapid cadence. We quantify the constraints possible between a sample of 150 such time delay lenses and a near term supernova program, such as might become available from an Antarctic telescope such as KDUST and the Dark Energy Survey. Adding time delay data to supernovae plus cosmic microwave background information can improve the dark energy figure of merit by almost a factor 5 and determine the matter density \Omega_m to 0.004, Hubble constant h to 0.7%, and dark energy equation of state time variation w_a to 0.26, systematics permitting.