arXiv: 15 August 2011

The WiggleZ Dark Energy Survey: measuring the cosmic expansion history using the Alcock-Paczynski test and distant supernovae

Chris Blake, Karl Glazebrook, Tamara Davis, Sarah Brough, Matthew Colless, Carlos Contreras, Warrick Couch, Scott Croom, Michael J. Drinkwater, Karl Forster, David Gilbank, Mike Gladders, Ben Jelliffe, Russell J. Jurek, I-hui Li, Barry Madore, Chris Martin, Kevin Pimbblet, Gregory Poole, Michael Pracy,Rob Sharp, Emily Wisnioski, David Woods, Ted Wyder, Howard Yee

arXiv:1108.2637v1

Astronomical observations suggest that today's Universe is dominated by a dark energy of unknown physical origin. One of the most notable consequences in many models is that dark energy should cause the expansion of the Universe to accelerate: but the expansion rate as a function of time has proven very difficult to measure directly. We present a new determination of the cosmic expansion history by combining distant supernovae observations with a geometrical analysis of large-scale galaxy clustering within the WiggleZ Dark Energy Survey, using the Alcock-Paczynski test to measure the distortion of standard spheres. Our result constitutes a robust and non-parametric measurement of the Hubble expansion rate as a function of time, which we measure with 10-15% precision in four bins within the redshift range 0.1 < z < 0.9. We demonstrate that the cosmic expansion is accelerating, in a manner independent of the parameterization of the cosmological model (although assuming cosmic homogeneity in our data analysis). Furthermore, we find that this expansion history is consistent with a cosmological-constant dark energy.

The WiggleZ Dark Energy Survey: mapping the distance-redshift relation with baryon acoustic oscillations

Chris Blake, Eyal Kazin, Florian Beutler, Tamara Davis, David Parkinson, Sarah Brough, Matthew Colless, Carlos Contreras, Warrick Couch, Scott Croom,Darren Croton, Michael J. Drinkwater, Karl Forster, David Gilbank, Mike Gladders, Karl Glazebrook, Ben Jelliffe, Russell J. Jurek, I-hui Li, Barry Madore,Chris Martin, Kevin Pimbblet, Gregory Poole, Michael Pracy, Rob Sharp, Emily Wisnioski, David Woods, Ted Wyder, Howard Yee

arXiv:1108.2635v1

We present measurements of the baryon acoustic peak at redshifts z = 0.44, 0.6 and 0.73 in the galaxy correlation function of the final dataset of the WiggleZ Dark Energy Survey. We combine our correlation function with lower-redshift measurements from the 6-degree Field Galaxy Survey and Sloan Digital Sky Survey, producing a stacked survey correlation function in which the statistical significance of the detection of the baryon acoustic peak is 4.9-sigma relative to a zero-baryon model with no peak. We fit cosmological models to this combined baryon acoustic oscillation (BAO) dataset comprising six distance-redshift data points, and compare the results to similar fits to the latest compilation of supernovae (SNe) and Cosmic Microwave Background (CMB) data. The BAO and SNe datasets produce consistent measurements of the equation-of-state w of dark energy, when separately combined with the CMB, providing a powerful check for systematic errors in either of these distance probes. Combining all datasets we determine w = -1.03 +/- 0.08 for a flat Universe, consistent with a cosmological constant model. Assuming dark energy is a cosmological constant and varying the spatial curvature, we find Omega_k = -0.004 +/- 0.006.

arXiv: 12 August 2011

A Method for Measuring (Slopes of) the Mass Profiles of Dwarf Spheroidal Galaxies

Matthew G. Walker, Jorge Peñarrubia

arXiv:1108.2404v2

We introduce a method for measuring the slopes of mass profiles within dwarf spheroidal (dSph) galaxies directly from stellar spectroscopic data and without adopting a dark matter halo model. Our method combines two recent results: 1) spherically symmetric, equilibrium Jeans models imply that the product of halflight radius and (squared) stellar velocity dispersion provides an estimate of the mass enclosed within the halflight radius of a dSph stellar component, and 2) some dSphs have chemo-dynamically distinct stellar \textit{sub}components that independently trace the same gravitational potential. We devise a statistical method that uses measurements of stellar positions, velocities and spectral indices to distinguish two dSph stellar subcomponents and to estimate their individual halflight radii and velocity dispersions. For a dSph with two detected stellar subcomponents, we obtain estimates of masses enclosed at two discrete points in the same mass profile, immediately defining a slope. Applied to published spectroscopic data, our method distinguishes stellar subcomponents in the Fornax and Sculptor dSphs, for which we measure slopes $\Gamma\equiv \Delta \log M / \Delta \log r=2.61_{-0.37}^{+0.43}$ and $\Gamma=2.95_{-0.39}^{+0.51}$, respectively. These values are consistent with 'cores' of constant density within the central few-hundred parsecs of each galaxy and rule out `cuspy' NFW profiles ($d\log M/d\log r \leq 2$ at all radii) with significance $\ga 96%$ and $\ga 99%$, respectively. Tests with synthetic data indicate that our method tends systematically to overestimate the mass of the inner stellar subcomponent to a greater degree than that of the outer stellar subcomponent, and therefore to underestimate the slope $\Gamma$ (implying that the stated NFW exclusion levels are conservative).

Halo models in modified gravity theories with self-accelerated expansion

Tatsuya Narikawa, Rampei Kimura, Tatsunosuke Yano, Kazuhiro Yamamoto

arXiv:1108.2346v1

We investigate the structure of halos in the sDGP (self-accelerating branch of the Dvali-Gavadadze-Porrati braneworld gravity) model and the galileon modified gravity model on the basis of the static and spherically symmetric solutions of the collisionless Boltzmann equation, which reduce to the singular isothermal sphere model and the King model in the limit of Newtonian gravity. The common feature of these halos is that the density of a halo in the outer region is larger (smaller) in the sDGP (galileon) model, respectively, in comparison with Newtonian gravity. This comes from the suppression (enhancement) of the effective gravity at large distance in the sDGP (galileon) model, respectively. However, the difference between these modified gravity models and Newtonian gravity only appears outside the halo due to the Vainshtein mechanism, which makes it difficult to distinguish between them. We also discuss the case in which the halo density profile is fixed independently of the gravity model for comparison between our results and previous work.

Does Baryonic Tully-Fisher Relation Conflict with LCDM?

Nickolay Y. Gnedin

arXiv:1108.2271v1

The "baryonic" Tully-Fisher relation (BTFR) is an important observational constraint on cosmological and galactic models. However, the observed BTFR only accounts for stars, molecular, and atomic gas, while the contribution of ionized gas is almost universally missed. Comparison of such observations to theoretical predictions is, therefore, highly non-trivial, and requires a proper modeling of radiative transfer of ionizing radiation, at least in an approximate form. An example of such modeling is presented in the form of a cosmological numerical simulation with radiative transfer. The observed BTFR of model galaxies is in reasonable agreement with the observational data, because the contribution of ionized gas progressively increases for smaller and smaller galaxies. Hence, the observed BTFR cannot be used as an argument against the LCDM cosmological model.

arXiv: 11 August 2011

Classical non-Gaussianity from non-linear evolution of curvature perturbations

Jinn-Ouk Gong, Jai-chan Hwang, Hyerim Noh

arXiv:1108.2267v1 

We study the non-linear evolution of the curvature perturbations during matter dominated era. We show that regardless of the origin of the primordial perturbation, the Bardeen potential receives sizable contributions from the classical non-linear evolution effects, and quantify them exactly. We divide these effects into two groups, being dominant on super- and sub-horizon scales. The former gives rise to squeezed peak of the bispectrum and contributes, in terms of the local non-linear parameter, -3/2 < f_{NL} < -2/5, depending on the configuration of momenta. The latter is highly scale dependent with equilateral shape, and can serve as a potential probe of general relativity.

Primordial Gravitational Wave Detectability with Deep Small-Sky CMB Experiments

Marzieh Farhang, J. Richard Bond, Olivier Doré, C. Barth Netterfield

arXiv:1108.2043v1 

We use Bayesian estimation on direct T-Q-U CMB polarization maps to forecast errors on the tensor-to- scalar power ratio r, and hence on primordial gravitational waves, as a function of sky coverage fsky. This method filters the quadratic pixel-pixel space into the optimal combinations needed for r detection for cut skies, providing enhanced information over a first-step linear separation into a combination of E, B and mixed modes, and ignoring the latter. With current computational power and for typical resolutions appropriate for r detection, the large matrix inversions required are accurate and fast. We explore two classes of experiments. One is motivated by a long duration balloon experiment like Spider, with pixel noise \propto \sqrt{fsky} for a specified observing period, but also applies to ground-based array experiments. We find that, ignoring systematic effects and foregrounds, an experiment with Spider-like noise over fsky ~0.02-0.2 could place a 2sigma_r ~0.014 (~95% CL) bound, rising to 0.02 with an L-dependent foreground residual. We contrast this with a Planck-like fixed instrumental noise as fsky varies, which gives a Galaxy-masked (fsky = 0.75) 2sigma_r ~0.015, rising to ~0.05 with the foreground residuals. Using for a figure of merit the (marginalized) 1D Shannon entropy of r, taken relative to the first 2003 WMAP1 CMB-only constraint, gives -1.7 (-1.9) bits from the 2010 WMAP7+ACT (2011 WMAP7+SPT) data, forecasts of -6 bits from Spider (plus Planck), of up to -11 bits for post-Planck satellites and -13 bits for a perfect cosmic variance limited experiment. We thus confirm the wisdom of the current strategy for r detection of deeply probed patches covering the fsky minimum-error trough with balloon and ground experiments.

A symmetry of the spatially flat Friedmann equations with barotropic fluid

Valerio Faraoni (Bishop's University)

arXiv:1108.2102v1

A string-inspired duality symmetry of the spatially flat Friedmann equations of general-relativistic cosmology is discussed and generalized, providing a map between exact solutions corresponding to different values of the barotropic index.

arXiv: 10 August 2011

A positive energy theorem for Einstein-aether and Hořava gravity

David Garfinkle, Ted Jacobson

arXiv:1108.1835v1 

Energy positivity is established for a class of solutions to Einstein-aether theory and the IR limit of Ho\v{r}ava gravity within a certain range of coupling parameters. The class consists of solutions where the aether 4-vector is divergence free on a spacelike surface to which it is orthogonal (which implies that the surface is maximal). In particular, this result holds for spherically symmetric solutions at a moment of time symmetry.

A universal bound on N-point correlations from inflation

Kendrick M. Smith, Marilena LoVerde, Matias Zaldarriaga

arXiv:1108.1805v1

Models of inflation in which non-Gaussianity is generated outside the horizon, such as curvaton models, generate distinctive higher-order correlation functions in the CMB and other cosmological observables. Testing for violation of the Suyama-Yamaguchi inequality tauNL >= (6/5 fNL)^2, where fNL and tauNL denote the amplitude of the three-point and four-point functions in certain limits, has been proposed as a way to distinguish qualitative classes of models. This inequality has been proved for a wide range of models, but only weaker versions have been proved in general. In this paper, we give a proof that the Suyama-Yamaguchi inequality is always satisfied. We discuss scenarios in which the inequality may appear to be violated in an experiment such as Planck, and how this apparent violation should be interpreted. We analyze a specific example, the "ungaussiton" model, in which leading-order scaling relations suggest that the Suyama-Yamaguchi inequality is eventually violated, and show that the inequality always holds.

Scale-Dependent Growth from a Transition in Dark Energy Dynamics

Mustafa A. Amin, Phillip Zukin, Edmund Bertschinger

arXiv:1108.1793v1 

We investigate the observational consequences of the quintessence field rolling to and oscillating near a minimum in its potential, "if" it happens close to the present epoch (z<0.2). We show that in a class of models, the oscillations lead to a rapid growth of the field fluctuations and the gravitational potential on subhorizon scales. The growth in the gravitational potential occurs on timescales << H^(1). This effect is present even when the quintessence parameters are chosen to reproduce an expansion history consistent with observations. For linearized fluctuations, we find that although the gravitational potential power spectrum is enhanced in a scale-dependent manner, the shape of the dark matter/galaxy power spectrum is not significantly affected. We find that the best constraints on such a transition in the quintessence field is provided via the integrated Sachs-Wolfe (ISW) effect in the CMB temperature power spectrum. Going beyond the linearized regime, the quintessence field can fragment into large, localized, long lived excitations (oscillons) with sizes comparable to galaxy clusters; this fragmentation could provide additional observational constraints. 
Two quoted "signatures" of modified gravity are a scale-dependent growth of the gravitational potential and a difference between the matter power spectrum inferred from measurements of lensing and galaxy clustering. Here, both effects are achieved by a minimally coupled scalar field in general relativity with a canonical kinetic term.

arXiv: 9 August 2011

The Relative Abundance of Isolated Clusters as a Probe of Dark Energy

Jounghun Lee (Seoul National Univ.)

arXiv:1108.1712v1

The galaxy clusters are regarded as isolated if they do not belong to the superclusters. The ratio of the abundance of the isolated clusters to that of the non-isolated clusters reflects how fast the structures grow on the largest scale in the Universe and thus may provide a useful probe of the dark energy equation of state. We find that the mass function of the isolated clusters can be separately evaluated in the frame of the recently developed Corasaniti-Achitouv mass function theory with drifting average coefficient ($\beta$) and diffusion coefficient ($D_{B}$) that are related to the average and variance of the stochastic collapse barrier, respectively. For the mass function of the isolated clusters, we set $D_{B}=0$ and then determine empirically the best value of $\beta$ with the help of the numerical results from the MICE simulations. The numerical mass functions of the isolated clusters at redshifts $z=0,\ 0.5$ and 1 are obtained by applying the friends-of-friends algorithm to the cluster catalogs from the MICE simulations and counting the number density of those clusters which have no neighbor clusters within the FoF linking length. It is found that the best-fit value of $\beta$ scales with the squares of the linear growth factor as $\beta\propto D^{2}(z)$ and the Corasaniti-Achitouv mass functions with $D_{B}=0$ and best-fit value of $\beta$ match the numerical results excellently at all redshifts. The relative abundance of the isolated clusters is calculated as the ratio of the cumulative mass function of the isolated clusters to that of the non-isolated clusters beyond a certain mass limit, and its evolution is found indeed to sensitively change with the dark energy equation of state, $w$.

Dark Matter Detection with Hard X-ray Telescopes

Tesla E. Jeltema, Stefano Profumo

arXiv:1108.1407v1

We analyze the impact of future hard X-ray observations on the search for indirect signatures of particle dark matter in large extragalactic systems such as nearby clusters or groups of galaxies. We argue that the hard X-ray energy band falls squarely at the peak of the inverse Compton emission from electrons and positrons produced by dark matter annihilation or decay for a large class of dark matter models. Specifically, the most promising are low-mass models with a hard electron-positron annihilation final state spectrum and intermediate-mass models with a soft electron-positron spectrum. We find that constraints on dark matter models similar to the current constraints from the Fermi Gamma-Ray Space Telescope will be close to the limit of the sensitivity of the near-term hard X-ray telescopes NuSTAR and ASTRO-H for relatively long observations, but an instrument similar to the Wide Field Imager proposed for the International X-ray Observatory would allow significant gains to be made. In the future, the ability to probe low to intermediate dark matter particle masses with hard X-ray observations may provide a good complement to next-generation gamma-ray instruments like the Cherenkov Telescope Array (CTA) which will be sensitive to high-energy gamma-rays and thus only to relatively large particle masses.

Extended LCDM: generalized non-minimal coupling for dark matter fluids

Dario Bettoni, Stefano Liberati, Lorenzo Sindoni

arXiv:1108.1728v1

In this paper we discuss a class of models that address the issue of explaining the gravitational dynamics at the galactic scale starting from a geometric point of view. Instead of claiming the existence of some hidden coupling between dark matter and baryons, or abandoning the existence of dark matter itself, we consider the possibility that dark matter and gravity have some non trivial interaction able to modify the dynamics at astrophysical scales. This interaction is implemented assuming that dark matter gets non--minimally coupled with gravity at suitably small scales and late times. After showing the predictions of the model in the Newtonian limit we also discuss the possible origin of it non-minimal coupling. This investigation seems to suggest that phenomenological mechanisms envisaged for the dark matter dynamics, such as the Bose--Einstein condensation of dark matter halos, could be connected to this class of models.

Spatially Covariant Theories of a Transverse, Traceless Graviton, Part I: Formalism

Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley

arXiv:1108.1397v1

General relativity is a covariant theory of two transverse, traceless graviton degrees of freedom. According to a theorem of Hojman, Kuchar, and Teitelboim, modifications of general relativity must either introduce new degrees of freedom or violate the principle of general covariance. In this paper, we explore modifications of general relativity that retain the same number of gravitational degrees of freedom, and therefore explicitly break general covariance. Motivated by cosmology, the modifications of interest maintain spatial covariance. Demanding consistency of the theory forces the physical Hamiltonian density to obey an analogue of the renormalization group equation. In this context, the equation encodes the invariance of the theory under flow through the space of conformally equivalent spatial metrics. This paper is dedicated to setting up the formalism of our approach and applying it to a realistic class of theories. Forthcoming work will apply the formalism more generally.

Solar-System Constraints on f(R) Chameleon Gravity

Je-An Gu, Wei-Ting Lin

arXiv:1108.1782v1

We investigate the solar-system constraint on the f(R) theory of modified gravity with chameleon mechanism, where f(R) represents the deviation from general relativity in the gravity action. We obtain a stringent bound to a general, non-constant deviation function f(R): -10^{-15} < df/dR < 0 when R ~ 3*10^5*H0^2, and a loose bound: 0 < R*d(df/dR)/dR < 2/5 when R > 3*10^5*H0^2, by requiring the thin-shell condition in the solar system, particularly in the atmosphere of the Earth. These bounds can be conveniently utilized to test the f(R) models with given functional forms of f(R) and to obtain the constraints on the parameters therein. For demonstration we apply these bounds to several widely considered f(R) models. (H0: Hubble constant)

21 cm observation of LSS at z~1 Instrument sensitivity and foreground subtraction

R. Ansari, J.E. Campagne, P.Colom, J.M. Le Goff, C. Magneville, J.M. Martin, M. Moniez, J.Rich, C.Yèche

arXiv:1108.1474v1

Large Scale Structures (LSS) in the universe can be traced using the neutral atomic hydrogen HI through its 21cm emission. Such a 3D matter distribution map can be used to test the Cosmological model and to constrain the Dark Energy properties or its equation of state. A novel approach, called intensity mapping can be used to map the HI distribution, using radio interferometers with large instantaneous field of view and waveband. In this paper, we study the sensitivity of different radio interferometer configurations, or multi-beam instruments for the observation of large scale structures and BAO oscillations in 21cm and we discuss the problem of foreground removal. For each configuration, we determine instrument response by computing the (u,v) or Fourier angular frequency plane coverage using visibilities. The (u,v) plane response is the noise power spectrum, hence the instrument sensitivity for LSS P(k) measurement. We describe also a simple foreground subtraction method to separate LSS 21 cm signal from the foreground due to the galactic synchrotron and radio sources emission. We have computed the noise power spectrum for different instrument configuration as well as the extracted LSS power spectrum, after separation of 21cm-LSS signal from the foregrounds. We have also obtained the uncertainties on the Dark Energy parameters for an optimized 21 cm BAO survey. We show that a radio instrument with few hundred simultaneous beams and a collecting area of ~10000 m^2 will be able to detect BAO signal at redshift z ~ 1 and will be competitive with optical surveys.

arXiv: 8 August 2011

Exact Extreme Value Statistics and the Halo Mass Function

Ian Harrison, Peter Coles

arXiv:1108.1358v1

Motivated by observations that suggest the presence of extremely massive clusters at uncomfortably high redshifts for the standard cosmological model to explain, we develop a theoretical framework for the study of the most massive haloes, e.g. the most massive cluster found in a given volume, based on Extreme Value Statistics (EVS). We proceed from the exact distribution of the extreme values drawn from a known underlying distribution, rather than relying on asymptotic theory (which is independent of the underlying form), arguing that the former is much more likely to furnish robust statistical results. We illustrate this argument with a discussion of the use of extreme value statistics as a probe of primordial non-Gaussianity.

Precision measurement of the Hubble constant using gravitational waves

Walter Del Pozzo

arXiv:1108.1317v1

The precise measurement of the Hubble constant $H_0$ is one of the foundations of the current cosmological paradigm. Due to correlations between $H_0$ and the remaining cosmological parameters, a precise measurement of $H_0$ is critical in view of future high redshift surveys. Second generation ground-based laser interferometers are expected to deliver a wealth of gravitational waves (GW) events from coalescing compact binaries up to a redshift of about 0.3. Being free of the systematics affecting electromagnetic measurements, GW offer the possibility of an independent measurement of $H_0$ with great accuracy. This \emph{Letter} presents a general method based on Bayesian inference aimed at estimating the value of the cosmological parameters for any GW event. In contrast to earlier work, this framework does not require the precise identification of the putative optical counterpart, but it considers all the potential galaxy hosts consistent with the recovered sky position and distance posterior distributions. When applied to the worldwide network of second generation interferometers, 50 GW events will yield a measurement of $H_0$ with an uncertainty of few percent.

arXiv: 5 August 2011

Reconstructing the Cosmic Velocity and Tidal Fields with Galaxy Groups Selected from the Sloan Digital Sky Survey

Huiyuan Wang, H.J. Mo, Xiaohu Yang, Frank C. van den Bosch

arXiv:1108.1008v1

[abridge]Cosmic velocity and tidal fields are important for the understanding of the cosmic web and the environments of galaxies, and can also be used to constrain cosmology. In this paper, we reconstruct these two fields in SDSS volume from dark matter halos represented by galaxy groups. Detailed mock catalogues are used to test the reliability of our method against uncertainties arising from redshift distortions, survey boundaries, and false identifications of groups by our group finder. We find that both the velocity and tidal fields, smoothed on a scale of ~2Mpc/h, can be reliably reconstructed in the inner region (~66%) of the survey volume. The reconstructed tidal field is used to split the cosmic web into clusters, filaments, sheets, and voids, depending on the sign of the eigenvalues of tidal tensor. The reconstructed velocity field nicely shows how the flows are diverging from the centers of voids, and converging onto clusters, while sheets and filaments have flows that are convergent along one and two directions, respectively. We use the reconstructed velocity field and the Zel'dovich approximation to predict the mass density field in the SDSS volume as function of redshift, and find that the mass distribution closely follows the galaxy distribution even on small scales. We find a large-scale bulk flow of about 117km/s in a very large volume, equivalent to a sphere with a radius of ~170Mpc/h, which seems to be produced by the massive structures associated with the SDSS Great Wall. Finally, we discuss potential applications of our reconstruction to study the environmental effects of galaxy formation, to generate initial conditions for simulations of the local Universe, and to constrain cosmological models. The velocity, tidal and density fields in the SDSS volume, specified on a Cartesian grid with a spatial resolution of ~700kpc/h, are available from the authors upon request.

Testing dark energy using pairs of galaxies in redshift space

Elise Jennings (ICC, IPPP, Durham), C. M. Baugh (ICC, Durham), S. Pascoli (IPPP, Durham)

arXiv:1108.0932v1

The distribution of angles subtended between pairs of galaxies, which is uniform in real space, is distorted by their peculiar motions, and has been proposed as a probe of cosmic expansion. We test this idea using N-body simulations of structure formation in a cold dark matter universe with a cosmological constant and in two variant cosmologies with different dark energy models. We find that the distortion of the distribution of angles is sensitive to the nature of dark energy. However, for the first time, our simulations also reveal dependences of the normalization of the distribution on both redshift and cosmology that have been neglected in previous work. This introduces systematics that severely limit the usefulness of the original method. Guided by our simulations, we devise a new, improved test of the nature of dark energy. We demonstrate that this test does not require prior knowledge of the background cosmology and that it can even distinguish between models that have the same baryonic acoustic oscillations and dark matter halo mass functions. Our technique could be applied to the completed BOSS galaxy redshift survey to constrain the expansion history of the Universe to better than 2%. The method will also produce different signals for dark energy and modified gravity cosmologies even when they have identical expansion histories, through the different peculiar velocities induced in these cases.

arXiv: 4 August 2011

Out of equilibrium: understanding cosmological evolution to lower-entropy states

Anthony Aguirre, Sean M. Carroll, Matthew C. Johnson

arXiv:1108.0417v1

Despite the importance of the Second Law of Thermodynamics, it is not absolute. Statistical mechanics implies that, given sufficient time, systems near equilibrium will spontaneously fluctuate into lower-entropy states, locally reversing the thermodynamic arrow of time. We study the time development of such fluctuations, especially the very large fluctuations relevant to cosmology. Under fairly general assumptions, the most likely history of a fluctuation out of equilibrium is simply the CPT conjugate of the most likely way a system relaxes back to equilibrium. We use this idea to elucidate the spacetime structure of various fluctuations in (stable and metastable) de Sitter space and thermal anti-de Sitter space.

Requirement of a Primordial Magnetic Field in Chameleon Models

David F. Mota, Camilla A.O. Schelpe

arXiv:1108.0892v1

We show that the presence of primordial magnetic fields (PMFs) are imperative in order to satisfy constraints placed on chameleon models from variation of particle masses at big bang nucleosynthesis. For initial field values after inflation of order the Planck mass, the combined magnetic field strength, B, and chameleon-photon coupling, 1/M_F, must satisfy (B/5nG)^2(M_F/10^{9}GeV)^{-1} > 2.4x10^{-6}. Combining these constraints with those derived from considering the degree of mixing between chameleons and CMB photons in a PMF, implies the existence of a primordial magnetic field with strength B > 0.1nG, for theories of Modified Gravity with a chameleon mechanism to be viable.

Loop Quantum Cosmology: A Status Report

Abhay Ashtekar, Parampreet Singh

arXiv:1108.0893v1

The goal of this article is to provide an overview of the current state of the art in loop quantum cosmology for three sets of audiences: young researchers interested in entering this area; the quantum gravity community in general; and, cosmologists who wish to apply loop quantum cosmology to probe modifications in the standard paradigm of the early universe. An effort has been made to streamline the material so that each of these communities can read only the sections they are most interested in, without a loss of continuity.

arXiv: 3 August 2011

Search for non-Gaussianities in the WMAP data with the Scaling Index Method

G. Rossmanith, H. Modest, C. Raeth, A. J. Banday, K. M. Gorski, G. Morfill

arXiv:1108.0596v1

In the recent years, non-Gaussianity and statistical isotropy of the Cosmic Microwave Background (CMB) was investigated with various statistical measures, first and foremost by means of the measurements of the WMAP satellite. In this Review, we focus on the analyses that were accomplished with a measure of local type, the so-called Scaling Index Method (SIM). The SIM is able to detect structural characteristics of a given data set, and has proven to be highly valuable in CMB analysis. It was used for comparing the data set with simulations as well as surrogates, which are full sky maps generated by randomisation of previously selected features of the original map. During these investigations, strong evidence for non-Gaussianities as well as asymmetries and local features could be detected. In combination with the surrogates approach, the SIM detected the highest significances for non-Gaussianity to date.

Gravitational Microlensing of a Reverberating Quasar Broad Line Region - I. Method and Qualitative Results

H. Garsden, N. F. Bate, G. F. Lewis

arXiv:1108.0504v1

The kinematics and morphology of the broad emission line region (BELR) of quasars are the subject of significant debate. The two leading methods for constraining BELR properties are microlensing and reverberation mapping. Here we combine these two methods with a study of the microlensing behaviour of the BELR in Q2237+0305, as a change in continuum emission (a "flare") passes through it. Beginning with some generic models of the BELR - sphere, bicones, disk - we slice in velocity and time to produce brightness profiles of the BELR over the duration of the flare. These are numerically microlensed to determine whether microlensing of reverberation mapping provides new information about the properties of BELRs. We describe our method and show images of the models as they are flaring, and the unlensed and lensed spectra that are produced. Qualitative results and a discussion of the spectra are given in this paper, highlighting some effects that could be observed. Our conclusion is that the influence of microlensing, while not strong, can produce significant observable effects that will help in differentiating the properties of BELRs.

arXiv: 2 August 2011

Production of dust by massive stars at high redshift

C. Gall, J. Hjorth, A. C. Andersen (Dark Cosmology Centre)

arXiv:1108.0403v1

The large amounts of dust detected in sub-millimeter galaxies and quasars at high redshift pose a challenge to galaxy formation models and theories of cosmic dust formation. At z > 6 only stars of relatively high mass (> 3 Msun) are sufficiently short-lived to be potential stellar sources of dust. This review is devoted to identifying and quantifying the most important stellar channels of rapid dust formation. We ascertain the dust production efficiency of stars in the mass range 3-40 Msun using both observed and theoretical dust yields of evolved massive stars and supernovae (SNe) and provide analytical expressions for the dust production efficiencies in various scenarios. We also address the strong sensitivity of the total dust productivity to the initial mass function. From simple considerations, we find that, in the early Universe, high-mass (> 3 Msun) asymptotic giant branch stars can only be dominant dust producers if SNe generate <~ 3 x 10^-3 Msun of dust whereas SNe prevail if they are more efficient. We address the challenges in inferring dust masses and star-formation rates from observations of high-redshift galaxies. We conclude that significant SN dust production at high redshift is likely required to reproduce current dust mass estimates, possibly coupled with rapid dust grain growth in the interstellar medium.

Expanding Universe: slowdown or speedup?

Yu.L. Bolotin, O.A. Lemets, D.A. Yerokhin

arXiv:1108.0203v1

The purpose of the review - to provide affordable to a wider audience a description of kinematics of the cosmological expansion, and dynamic interpretation of this process. The focus will be the accelerated expansion of the Universe. Will be considered by virtually all major opportunity to explain the accelerated expansion of the Universe, including an introduction to the energetic budget of the universe of so-called dark energy, a modification of Einstein's equations, and a new approach based on the holographic principle.

Models for Little Rip Dark Energy

Paul H. Frampton, Kevin J. Ludwick, Shin'ichi Nojiri, Sergei D. Odintsov, Robert J. Scherrer

arXiv:1108.0067v1

We examine in more detail specific models which yield a little rip cosmology, i.e., a universe in which the dark energy density increases without bound but the universe never reaches a finite-time singularity. We derive the conditions for the little rip in terms of the inertial force in the expanding universe and present two representative models to illustrate in more detail the difference between little rip models and those which are asymptotically de Sitter. We derive conditions on the equation of state parameter of the dark energy to distinguish between the two types of models. We show that coupling between dark matter and dark energy with a little rip equation of state can alter the evolution, changing the little rip into an asymptotic de Sitter expansion. We give conditions on minimally-coupled phantom scalar field models and on scalar-tensor models that indicate whether or not they correspond to a little rip expansion.

arXiv: 1 August 2011

The intrinsic dispersion of the Faber-Jackson relation for early-type galaxies as function of the mass and redshift

A. Nigoche-Netro, J. A. L. Aguerri, P. Lagos, A. Ruelas-Mayorga, L. J. Sánchez, C. Muñoz-Tuñón, A. Machado

arXiv:1107.6017v1

Aims. Recently it has been reported that the intrinsic dispersion at constant magnitude of the structural relations from early-type galaxies is a useful tool to study the universality of these structural relations, that is to say, to study whether the structural relations depend on luminosity, wavelength, redshift and/or environment. In this work we study the intrinsic dispersion of the Faber-Jackson relation as function of the luminosity, mass and redshift. 
Methods. We use a sample of approximately 90 000 early-type galaxies from the Sloan Digital Sky Survey (SDSS-DR7) spanning a magnitude range of 7 $mag$ in both $g$ and $r$ filters. We calculate the intrinsic dispersion of the Faber-Jackson relation at approximately constant magnitude and compare this at different luminosities, masses and redshifts. 
Results. The main results are the following: i) The intrinsic dispersion of the Faber-Jackson relation depends on the luminosity, mass and redshift. ii) The distribution for brighter and more massive galaxies has smaller intrinsic dispersion than that for fainter and less massive galaxies. iii) The distribution of bright and massive galaxies at higher redshift has smaller intrinsic dispersion than those similar galaxies at low redshift. 
Conclusions. Comparisons of the results found in this work with recent studies from the literature make us conclude that the intrinsic dispersion of the Faber-Jackson relation could depend on the history of galaxies, in other words, the intrinsic dispersion could depend on the number and nature of transformation events that have affected the galaxies along their life times, such as collapse, accretion, interaction and merging.

Testing homogeneity with the fossil record of galaxies

Alan F. Heavens, Raul Jimenez, Roy Maartens

arXiv:1107.5910v1

The standard Friedmann model of cosmology is based on the Copernican Principle, i.e. the assumption of a homogeneous background on which structure forms via perturbations. Homogeneity underpins both general relativistic and modified gravity models and is central to the way in which we interpret observations of the CMB and the galaxy distribution. It is therefore important to probe homogeneity via observations. We describe a test based on the fossil record of distant galaxies: if we can reconstruct key intrinsic properties of galaxies as functions of proper time along their worldlines, we can compare such properties at the same proper time for our galaxy and others. We achieve this by computing the lookback time using radial Baryon Acoustic Oscillations, and the time along galaxy world line using stellar physics, allowing us to probe homogeneity, in principle anywhere inside the past light cone. Agreement in the results would be an important consistency test -- although it would not in itself prove homogeneity. Any significant deviation in the results however would signal a breakdown of homogeneity.

Collapse thresholds for the formation of CDM haloes in the ellipsoidal collapse model

Aaron D. Ludlow, Cristiano Porciani (Argelander Institute for Astronomy, Bonn, Germany)

arXiv:1107.5808v1

In the excursion set approach to structure formation initially spherical regions of the linear density field collapse if the average density contrast within them exceeds some critical value, \delta_c. This model allows one to make predictions for several fundamental aspects of structure formation such as the mass function of dark matter halos, their merger histories and clustering properties. The value of \delta_c is often calculated from the spherical or ellipsoidal collapse model, which provide well-defined predictions given auxiliary properties of the linear tidal field at a given point. We use two cosmological simulations of structure growth in a LCDM cosmology to test a key assumption used in calculating \delta_c: that the shapes of the initial Lagrangian patches that eventually collapse (or proto-haloes) are spherical. Our results indicate that the vast majority of dark matter proto-haloes are non-spherical, and have minor-to-major axis ratios that vary from <a_3/a_1>~0.4 at the galaxy mass scale to ~0.65 for rich galaxy clusters. We show that this non-sphericity likely originates from the asymmetry of the linear tidal field which pushes material onto, or away from, local density maxima in the linear density field. We study the implications of these results for the collapse barriers for CDM halo formation inferred from the classic ellipsoidal collapse model. Our results indicate that the "standard" ellipsoidal collapse model commonly adopted in the literature does not provide a full account of the possible collapse thresholds for halo formation, since the model predictions depend sensitively on the assumed shape of the primordial perturbation. We show that an improved model, which accounts for the intrinsic shapes of proto-haloes, provides a much more accurate description of their measured minimum overdensities.