arXiv: 1 July 2010

An asymmetric explosion as the origin of spectral evolution diversity in type Ia supernovae

Authors: K. Maeda, S. Benetti, M. Stritzinger, F.K. Roepke, G. Folatelli, J. Sollerman, S. Taubenberger, K. Nomoto, G. Leloudas, M. Hamuy, M. Tanaka, P.A. Mazzali, N. Elias-Rosa

http://arxiv.org/abs/1006.5888v1

Type Ia Supernovae (SNe Ia) form an observationally uniform class of stellar explosions, in that more luminous objects have smaller decline-rates. This one-parameter behavior allows SNe Ia to be calibrated as cosmological `standard candles', and led to the discovery of an accelerating Universe. Recent investigations, however, have revealed that the true nature of SNe Ia is more complicated. Theoretically, it has been suggested that the initial thermonuclear sparks are ignited at an offset from the centre of the white-dwarf (WD) progenitor, possibly as a result of convection before the explosion. Observationally, the diversity seen in the spectral evolution of SNe Ia beyond the luminosity decline-rate relation is an unresolved issue. Here we report that the spectral diversity is a consequence of random directions from which an asymmetric explosion is viewed. Our findings suggest that the spectral evolution diversity is no longer a concern in using SNe Ia as cosmological standard candles. Furthermore, this indicates that ignition at an offset from the centre of is a generic feature of SNe Ia.

 

N-body simulations with generic non-Gaussian initial conditions I: Power Spectrum and halo mass function

Authors: Christian Wagner, Licia Verde, Lotfi Boubekeur

http://arxiv.org/abs/1006.5793v1

We address the issue of setting up generic non-Gaussian initial conditions for N-body simulations. We consider inflationary-motivated primordial non-Gaussianity where the perturbations in the Bardeen potential are given by a dominant Gaussian part plus a non-Gaussian part specified by its bispectrum. The approach we explore here is suitable for any bispectrum, i.e. it does not have to be of the so-called separable or factorizable form. The procedure of generating a non-Gaussian field with a given bispectrum (and a given power spectrum for the Gaussian component) is not univocal, and care must be taken so that higher-order corrections do not leave a too large signature on the power spectrum. This is so far a limiting factor of our approach. We then run N-body simulations for the most popular inflationary-motivated non-Gaussian shapes. The halo mass function and the non-linear power spectrum agree with theoretical analytical approximations proposed in the literature, even if they were so far developed and tested only for a particular shape (the local one). We plan to make the simulations outputs available to the community via the non-Gaussian simulations comparison project web site this http URL

 

A robust approach to f(R) gravity

Authors: Luisa G. Jaime, Leonardo Patino, Marcelo Salgado

http://arxiv.org/abs/1006.5747v1

We consider metric f(R) theories of gravity without mapping them to their scalar-tensor counterpart, but using the Ricci scalar itself as an "extra" degree of freedom. This approach avoids then the introduction of a scalar-field potential that might be ill defined (not single valued). In order to explicitly show the usefulness of this method we focus on static and spherically symmetric spacetimes and deal with the recent controversy about the existence of extended relativistic objects in certain class of f(R) models.

New stability results for Einstein scalar gravity

Authors: Thomas Faulkner, Gary T. Horowitz, Matthew M. Roberts

http://arxiv.org/abs/1006.2387v1

We consider asymptotically anti de Sitter gravity coupled to a scalar field with mass slightly above the Breitenlohner-Freedman bound. This theory admits a large class of consistent boundary conditions characterized by an arbitrary function $W$. An important open question is to determine which $W$ admit stable ground states. It has previously been shown that the total energy is bounded from below if $W$ is bounded from below and the bulk scalar potential $V(\phi)$ admits a suitable superpotential. We extend this result and show that the energy remains bounded even in some cases where $W$ can become arbitrarily negative. As one application, this leads to the possibility that in gauge/gravity duality, one can add a double trace operator with negative coefficient to the dual field theory and still have a stable vacuum.

 

The High Time Resolution Universe Pulsar Survey I: System configuration and initial discoveries

Authors: M. J. Keith, A. Jameson, W. van Straten, M. Bailes, S. Johnston, M. Kramer, A. Possenti, S. D. Bates, N. D. R. Bhat, M. Burgay, S. Burke-Spolaor, N. D'Amico, L. Levin, S. Milia, B. W. Stappers

http://arxiv.org/abs/1006.5744v1

We have embarked on a survey for pulsars and fast transients using the 13-beam Multibeam receiver on the Parkes radio telescope. Installation of a digital backend allows us to record 400 MHz of bandwidth for each beam, split into 1024 channels and sampled every 64 us. Limits of the receiver package restrict us to a 340 MHz observing band centred at 1352 MHz. The factor of eight improvement in frequency resolution over previous multibeam surveys allows us to probe deeper into the Galactic plane for short duration signals such as the pulses from millisecond pulsars. We plan to survey the entire southern sky in 42641 pointings, split into low, mid and high Galactic latitude regions, with integration times of 4200, 540 and 270 s respectively. Simulations suggest that we will discover 400 pulsars, of which 75 will be millisecond pulsars. With ~30% of the mid-latitude survey complete, we have re-detected 223 previously known pulsars and discovered 27 pulsars, 5 of which are millisecond pulsars. The newly discovered millisecond pulsars tend to have larger dispersion measures than those discovered in previous surveys, as expected from the improved time and frequency resolution of our instrument.

 

Gravitational and mass distribution effects on stationary superwinds II. Extended dark matter haloes

Authors: G.A. Añorve-Zeferino, M.G. Corona-Galindo

http://arxiv.org/abs/1006.5750v1

In this second part, we generalize the results of the previous paper. We present an analytic superwind solution considering extended gravitationally-interacting dark-matter and baryonic haloes. The incorporation of the latter is critical, since they can have a substantial effect on the hydrodynamics of superwinds generated by massive galaxies. Although the presence of extended and massive haloes does not change the limit for the closed-box enrichment of galaxies established in the first paper, they can trigger an earlier activation of the open-box enrichment scenario, since their gravitational potentials can contribute to the inhibition of the free superwind. Moreover, the incorporation of the extended haloes will also enhance the physical setting behind the superwind model, as we consider mass distributions with properties that emulate the results of recent simulations of $\Lambda$CDM haloes.

arXiv: 30 June 2010

Dark Energy in Practice

Authors: Domenico Sapone

http://arxiv.org/abs/1006.5694v1

In this paper we review a part of the approaches that have been considered to explain the extraordinary discovery of the late time acceleration of the Universe. We discuss the arguments that have led physicists and astronomers to accept dark energy as the current preferable candidate to explain the acceleration. We highlight the problems and the attempts to overcome the difficulties related to such a component. We also consider alternative theories capable of explaining the acceleration of the Universe, such as modification of gravity. We compare the two approaches and point out the observational consequences, reaching the sad but foresightful conclusion that we will not be able to distinguish between a Universe filled by dark energy or a Universe where gravity is different from General Relativity. We review the present observations and discuss the future experiments that will help us to learn more about our Universe. This is not intended to be a complete list of all the dark energy models but this paper should be seen as a review on the phenomena responsible for the acceleration.
Moreover, in a landscape of hardly compelling theories, it is an important task to build simple measurable parameters useful for future experiments that will help us to understand more about the evolution of the Universe.

 

Direct detection of WIMP dark matter:Implications of a self-consistent truncated isothermal model of Milky Way's dark halo in the light of recent determination of Milky Way's extended rotation curve out to 60 kpc

Authors: Soumini Chaudhury, Pijushpani Bhattacharjee, Ramanath Cowsik

http://arxiv.org/abs/1006.5588v1

Direct detection of Weakly Interacting Massive Particle (WIMP) candidates of Dark Matter (DM) is studied within the context of a self-consistent truncated isothermal model of the finite-size dark halo of Milky Way in the light of the recent determination of the Galaxy's circular rotation curve out to 60 kpc. This new rotation curve declines with Galactocentric distance ($R$) for $R\gsim10\kpc$. Our Galaxy halo model takes into account the modifications of the phase-space structure of the dark matter halo due to the gravitational influence of the observed visible matter in a self-consistent manner. In contrast to the standard halo model (SHM) routinely used in analyzing the results of WIMP direct detection experiments, the velocity distribution of the WIMPs in our model is distinctly non-Maxwellian with a cut-off at a maximum velocity that is self-consistently determined by the model itself. For our halo model that provides the best fit to the new rotation curve data, the most stringent 90\% C.L. upper limit on the WIMP-nucleon spin-independent cross section from the recent results of the CDMS-II experiment, for example, is $\sim 5.3\times10^{-8}\pb$ for a WIMP mass of $\sim$ 71 GeV. We also find that there exists a range of small WIMP masses, typically $\sim$ 2 -- 10 GeV, within which DAMA collaboration's claimed annual modulation signal purportedly due to WIMPs is consistent with the null results of other experiments. For spin-dependent interaction the above upper WIMP mass range extends to about 16 GeV. Our results, based as they are on a self-consistent model of the dark matter halo of the Galaxy, the parameters of which are determined by a fit to the rotation curve of the Galaxy, strengthen the possibility of low-mass ($\lsim 10\gev$) WIMPs as a candidate for dark matter.

 

The Parameterised Post-Newtonian Limit of Bimetric Theories of Gravity

Authors: Timothy Clifton, Máximo Bañados, Constantinos Skordis

http://arxiv.org/abs/1006.5619v1

We consider the parameterised post-Newtonian limit of a general class of bimetric theories of gravity, in which both metrics are dynamical. It is found that these theories can evade solar system tests of post-Newtonian gravity remarkably well. We show that the perturbations about Minkowski space in these theories contain both massless and massive degrees of freedom, and that in general there are two different types of massive mode, each with a different mass. If both of these masses are sufficiently large then the predictions of the most general class of theories we consider are indistinguishable from those of general relativity, up to post-Newtonian order in a weak field, low velocity expansion. In the limit that the massive modes become massless, we find that these general theories do not exhibit a van Dam-Veltmam-Zakharov-like discontinuity in their gamma parameter, although there are discontinuities in other post-Newtonian parameters as the massless limit is approached. This smooth behaviour in gamma is due to the discontinuities from each of the two different massive modes cancelling each other out. Such cancellations cannot occur in special cases with only one mass parameter, such as the Isham-Salam-Strathdee theory.

 

arXiv: 29 June 2010

Large non-Gaussianities in the Effective Field Theory Approach to Single-Field Inflation: the Trispectrum

Authors: Nicola Bartolo, Matteo Fasiello, Sabino Matarrese, Antonio Riotto

http://arxiv.org/abs/1006.5411v1

We perform the analysis of the trispectrum of curvature perturbations generated by the interactions characterizing a general theory of single-field inflation obtained by effective field theory methods. We find that curvature-generated interaction terms, which can in general give an important contribution to the amplitude of the four-point function, show some new distinctive features in the form of their trispectrum shape-function. These interesting interactions are invariant under some recently proposed symmetries of the general theory and, as shown explicitly, do allow for a large value of the trispectrum.

 

Galaxy Formation Theory

Authors: Andrew J. Benson (1) ((1) California Institute of Technology)

http://arxiv.org/abs/1006.5394v1

We review the current theory of how galaxies form within the cosmological framework provided by the cold dark matter paradigm for structure formation. Beginning with the pre-galactic evolution of baryonic material we describe the analytical and numerical understanding of how baryons condense into galaxies, what determines the structure of those galaxies and how internal and external processes (including star formation, merging, active galactic nuclei etc.) determine their gross properties and evolution. Throughout, we highlight successes and failings of current galaxy formation theory. We include a review of computational implementations of galaxy formation theory and assess their ability to provide reliable modeling of this complex phenomenon. We finish with a discussion of several "hot topics" in contemporary galaxy formation theory and assess future directions for this field.

 

Component separation in CMB observations

Authors: Jacques Delabrouille

http://arxiv.org/abs/1006.5344v1

In these proceedings, we discuss the extraction, in WMAP 5 year data, of a clean CMB map, of foreground emission (dominated by emission of the interstellar medium of our galaxy), and of the tiny signal from Sunyaev Zel'dovich effect in the direction of known galaxy clusters. The implementation of an Internal Linear Combination method locally in both pixel and harmonic space, with the use of a decomposition of WMAP maps onto a frame of spherical needlets, permits to extract a full sky CMB map, with good accuracy even in regions close to the galactic plane. Proper subtraction of this estimated CMB from WMAP original observations provides us with CMB-free foreground maps, which can be used for the analysis of the emission of the galactic interstellar medium and for detecting and measuring emissions from compact sources. Finally, while the Sunyaev-Zel'dovich cannot be detected for individual clusters in WMAP data, due to lack of spatial resolution and sensitivity, a stacking analysis of tentative detections towards a number of known ROSAT clusters permits to detect the SZ effect in WMAP data and measure how the SZ flux scales with cluster mass and X-ray luminosity.

 

Radio emission from dark matter annihilation in the Large Magellanic Cloud

Authors: Beatriz B. Siffert, Angelo Limone, Enrico Borriello, Giuseppe Longo, Gennaro Miele

http://arxiv.org/abs/1006.5325v1

The Large Magellanic Cloud, at only 50 kpc away from us and known to be dark matter dominated, is clearly an interesting place where to search for dark matter annihilation signals. In this paper, we estimate the synchrotron emission due to WIMP annihilation in the halo of the LMC at two radio frequencies, 1.4 and 4.8 GHz, and compare it to the observed emission, in order to impose constraints in the WIMP mass vs. annihilation cross section plane. We use available Faraday rotation data from background sources to estimate the magnitude of the magnetic field in different regions of the LMC's disc, where we calculate the radio signal due to dark matter annihilation. We account for the e+ e- energy losses due to synchrotron, Inverse Compton Scattering and bremsstrahlung, using the observed hydrogen and dust temperature distribution on the LMC to estimate their efficiency. The extensive use of observations, allied with conservative choices adopted in all the steps of the calculation, allow us to obtain very realistic constraints.

 

Constraining Inflationary Scenarios with Braneworld Models and Second Order Cosmological Perturbations

Authors: Ian Huston

http://arxiv.org/abs/1006.5321v1

Inflationary cosmology is the leading explanation of the very early universe. Many different models of inflation have been constructed which fit current observational data. In this work theoretical and numerical methods for constraining the parameter space of a wide class of such models are described. First, string-theoretic models with large non-Gaussian signatures are investigated. An upper bound is placed on the amplitude of primordial gravitational waves produced by ultra-violet Dirac-Born-Infeld inflation. In all but the most finely tuned cases, this bound is incompatible with a lower bound derived for inflationary models which exhibit a red spectrum and detectable non-Gaussianity. By analysing general non-canonical actions, a class of models is found which can evade the upper bound when the phase speed of perturbations is small. The multi-coincident brane scenario with a finite number of branes is one such model. For models with a potentially observable gravitational wave spectrum the number of coincident branes is shown to take only small values. The second method of constraining inflationary models is the numerical calculation of second order perturbations for a general class of single field models. The Klein-Gordon equation at second order, written in terms of scalar field variations only, is numerically solved. The slow roll version of the second order source term is used and the method is shown to be extendable to the full equation. This procedure allows the evolution of second order perturbations in general and the calculation of the non-Gaussianity parameter in cases where there is no analytical solution available.

 

Switched magnetospheric regulation of pulsar spin-down

Authors: Andrew Lyne (1), George Hobbs (2), Michael Kramer (1,3), Ingrid Stairs (4), Ben Stappers (1) (1-Jodrell Bank Centre for Astrophysics, University of Manchester, 2 - CSIRO, ATNF, 3 - MPI fuer Radioastronomie, 4 - University of British Columbia)

http://arxiv.org/abs/1006.5184v1

Pulsars are famed for their rotational clock-like stability and their highly-repeatable pulse shapes. However, it has long been known that there are unexplained deviations (often termed "timing noise") from the rate at which we predict these clocks should run. We show that timing behaviour often results from typically two different spin-down rates. Pulsars switch abruptly between these states, often quasi-periodically, leading to the observed spin-down patterns. We show that for six pulsars the timing noise is correlated with changes in the pulse shape. Many pulsar phenomena including mode-changing, nulling, intermittency, pulse shape variability and timing noise are therefore linked and caused by changes in the pulsar's magnetosphere. We consider the possibility that high-precision monitoring of pulse profiles could lead to the formation of highly-stable pulsar clocks.

 

Dark energy from the gas of wormholes

Authors: A.A. Kirillov, E.P. Savelova

http://arxiv.org/abs/1006.5181v1

The observed dark energy phenomenon is attributed to the presence of the zero-point fluctuations of matter fields. We show that due to the presence of the gas of virtual wormholes the zero-point energy is finite and forms the finite (of the Planckian order) value. The observed value of the cosmological constant is somewhat reduced due to the two effects. First is the renormalization of the gravitational constant which presumably forms the initial (local) value $H^2_{inf}$ predicted by the inflationary scenario. And second is an additional reduction due to the presence of the gas of actual wormholes required by the dark matter phenomenon. We show that the Starobinsky model of inflation explains both, the inflationary stage in the past and the present day acceleration of the Universe. It also represents the so-called model of the eternal Universe.

 

Towards the unification of late-time acceleration and inflation by k-essence model

Authors: Shin'ichi Nojiri

http://arxiv.org/abs/1006.5201v1

Based on the formulation of the reconstruction for the k-essence model, which was recently proposed in arXiv:1001.0220 [hep-th], we explicitly construct cosmological model to unifying the late-time acceleration and the inflation in the early universe.

 

A noncommutative approach to the cosmological constant problem

Authors: Remo Garattini, Piero Nicolini

http://arxiv.org/abs/1006.5418v1

In this paper we study the cosmological constant emerging from the Wheeler-DeWitt equation, as an eigenvalue of the related Sturm-Liouville problem. We employ Gaussian trial functionals and we perform a mode decomposition to extract the transverse traceless component, namely the graviton contribution, at one loop. We implement a Noncommutative Geometry induced minimal length to calculate the number of graviton modes. As a result we find regular graviton fluctuation energies for the Schwarzschild, deSitter and Anti-deSitter backgrounds. No renormalization scheme is necessary to remove infinities in contrast to what happens in conventional approaches.

 

Cosmological Density Perturbations with a Scale-Dependent Newton's G

Authors: Herbert W. Hamber, Reiko Toriumi

http://arxiv.org/abs/1006.5214v1

We explore possible cosmological consequences of a running Newton's constant $ G ( \Box ) $, as suggested by the non-trivial ultraviolet fixed point scenario in the quantum field-theoretic treatment of Einstein gravity with a cosmological constant term. In particular we focus here on what possible effects the scale-dependent coupling might have on large scale cosmological density perturbations. Starting from a set of manifestly covariant effective field equations derived earlier, we systematically develop the linear theory of density perturbations for a non-relativistic, pressure-less fluid. The result is a modified equation for the matter density contrast, which can be solved and thus provides an estimate for the growth index parameter $\gamma$ in the presence of a running $G$. We complete our analysis by comparing the fully relativistic treatment with the corresponding results for the non-relativistic (Newtonian) case, the latter also with a weakly scale dependent $G$.

 

 

Gravitational Waves Astronomy: a cornerstone for gravitational theories

Authors: Christian Corda

http://arxiv.org/abs/1006.5355v1

Realizing a gravitational wave (GW) astronomy in next years is a great challenge for the scientific community. By giving a significant amount of new information, GWs will be a cornerstone for a better understanding of gravitational physics. In this paper we re-discuss that the GW astronomy will permit to solve a captivating issue of gravitation. In fact, it will be the definitive test for Einstein's general relativity (GR), or, alternatively, a strong endorsement for extended theories of gravity (ETG).

 

 

 

arXiv: 28 June 2010

A Theory of a Spot

Authors: Niayesh Afshordi (Perimeter/Waterloo), Anže Slosar (Brookhaven), Yi Wang (McGill)

http://arxiv.org/abs/1006.5021v1

We present a simple model of inflation that can produce arbitrarily large spherical underdense or overdense regions embedded in a standard Lambda cold dark matter paradigm, which we refer to as bubbles. We analyze the effect such bubbles would have on the Cosmic Microwave Background (CMB). For super-horizon sized bubble in the vicinity of the last scattering surface, a signal is imprinted onto CMB via a combination of Sach-Wolfe and an early integrated Sach-Wolfe (ISW) effects. Smaller, sub-horizon sized bubbles at lower redshifts (during matter domination and later) can imprint secondary anisotropies on the CMB via Rees-Sciama, late-time ISW and Ostriker-Vishniac effects. Our model, and arguably most similar inflationary models, produce bubbles which are over/underdense in potential: in density such bubbles are characterized by having a distinct wall with the interior staying at the cosmic mean density. We show that such models can, with only moderate fine tuning, explain the \emph{cold spot}, a non-Gaussian feature identified in the Wilkinson Microwave Anisotropy Probe (WMAP) data by several authors.

 

Gamma-rays from ultracompact minihalos: potential constraints on the primordial curvature perturbation

Authors: Amandeep S. Josan, Anne M. Green

http://arxiv.org/abs/1006.4970v1

Ultracompact minihalos (UCMHs) are dense dark matter structures which can form from large density perturbations shortly after matter-radiation equality. If dark matter is in the form of Weakly Interacting Massive Particles (WIMPs), then UCMHs may be detected via their gamma-ray emission. We investigate how the {\em{Fermi}} satellite could constrain the abundance of UCMHs and place limits on the power spectrum of the primordial curvature perturbation. Detection by {\em Fermi} would put a lower limit on the UCMH halo fraction. The smallest detectable halo fraction, $f_{\rm UCMH} \gtrsim 10^{-7}$, is for $M_{\rm UCMH} \sim 10^{3} M_{\odot}$. If gamma-ray emission from UCMHs is not detected, an upper limit can be placed on the halo fraction. The bound is tightest, $f_{\rm UCMH} \lesssim 10^{-5}$, for $M_{\rm UCMH} \sim 10^{5} M_{\odot}$. The resulting upper limit on the power spectrum of the primordial curvature perturbation in the event of non-detection is in the range $\mathcal{P_R} \lesssim 10^{-6.5}- 10^{-6}$ on scales $k \sim 10^{1}-10^{6} \, {\rm Mpc}^{-1}$. This is substantially tighter than the existing constraints from primordial black hole formation on these scales, however it assumes that dark matter is in the form of WIMPs and UCMHs are not disrupted during the formation of the Milky Way halo.

 

Testing DM halos using rotation curves and lensing: A warning on the determination of the halo mass

Authors: Dario Nunez, Alma X. Gonzalez Morales, Jorge L. Cervantes-Cota, Tonatiuh Matos

http://arxiv.org/abs/1006.4875v1

There are two observations of galaxies that can offer some insight into the nature of the dark matter (DM), namely the rotation curves and the gravitational lensing. While the first one can be studied using the Newtonian limit, the second one is completely relativistic. Each one separately can not determine the nature of DM, but both together give us key information about this open problem. In this work we use a static and spherically symmetric metric to model the DM halo in a galaxy or in a galaxy cluster. The metric contains two free functions, one associated with the distribution of mass and the other one with the gravitational potential. We use galactic, typical rotation curves to univocally determine the kinematics of the halos. We compute separately the mass functions for a perfect fluid and a scalar field, and demonstrate that both models can be fitted to the observations, though with different masses. We then employ lensing to discriminate between these models. This procedure represents a test of models using two measurements: rotation curves and lensing.

 

 

arXiv: 25 June 2010

Neutrino mass constraint with SDSS LRG power spectrum and perturbation theory

Authors: Shun Saito (1,2), Masahiro Takada (3), Atsushi Taruya (3,4) ((1) U. Tokyo (2) UC Berkeley (3) IPMU (4) RESCEU )

http://arxiv.org/abs/1006.4845v1

We compare the model power spectrum, computed based on the perturbation theory (PT) of structure formation, with the power spectrum of luminous red galaxies (LRG) measured from the Sloan Digital Sky Survey Data Release 7 catalog, assuming a flat, cold dark matter-dominated cosmology. The model includes the effects of massive neutrinos, nonlinear matter clustering and nonlinear, scale-dependent galaxy bias in a self-consistent manner. Combining with the recent results from Wilkinson Microwave Background Anisotropy Probe (WMAP), we found that the PT model well matches the LRG power spectrum down to k=0.1 h/Mpc. We then derive a upper limit on the sum of neutrino masses, sigma(m_nu,tot) < 0.81 eV (95% C.L.), marginalized over other parameters including nonlinear bias parameters and dark energy equation of state parameter. The neutrino mass limit is improved by a factor of 1.85 compared to the limit from the WMAP5 alone.

 

 Primordial non-Gaussianity in the large scale structure of the Universe

Authors: Vincent Desjacques, Uros Seljak

http://arxiv.org/abs/1006.4763v1

Primordial non-Gaussianity is a potentially powerful discriminant of the physical mechanisms that generated the cosmological fluctuations observed today. Any detection of significant non-Gaussianity would thus have profound implications for our understanding of cosmic structure formation. The large scale mass distribution in the Universe is a sensitive probe of the nature of initial conditions. Recent theoretical progress together with rapid developments in observational techniques will enable us to critically confront predictions of inflationary scenarios and set constraints as competitive as those from the Cosmic Microwave Background. In this paper, we review past and current efforts in the search for primordial non-Gaussianity in the large scale structure of the Universe.

 

Probing Hot Gas in Galaxy Groups through the Sunyaev-Zeldovich Effect

Authors: Ran Li, H. J. Mo, Zuhui Fan, Frank C. van den Bosch, Xiaohu Yang

http://arxiv.org/find/astro-ph/1/au:+Romano_A/0/1/0/all/0/1

We investigate the potential of exploiting the Sunyaev-Zeldovich effect (SZE) to study the properties of hot gas in galaxy groups. It is shown that, with upcoming SZE surveys, one can stack SZE maps around galaxy groups of similar halo masses selected from large galaxy redshift surveys to study the hot gas in halos represented by galaxy groups. We use various models for the hot halo gas to study how the expected SZE signals are affected by gas fraction, equation of state, halo concentration, and cosmology. Comparing the model predictions with the sensitivities expected from the SPT, ACT and Planck surveys shows that a SPT-like survey can provide stringent constraints on the hot gas properties for halos with masses M ~> 10^{13} h^{-1}Msun. We also explore the idea of using the cross correlation between hot gas and galaxies of different luminosity to probe the hot gas in dark matter halos without identifying galaxy groups to represent dark halos. Our results show that, with a galaxy survey as large as the Sloan Digital Sky Survey and with the help of the conditional luminosity function (CLF) model, one can obtain stringent constraints on the hot gas properties in halos with masses down to 10^{13} h^{-1}Msun. Thus, the upcoming SZE surveys should provide a very promising avenue to probe the hot gas in relatively low-mass halos where the majority of L*-galaxies reside.

 

Effects of inhomogeneities on apparent cosmological observables: "fake'' evolving dark energy

Authors: Antonio Enea Romano, Misao Sasaki, Alexei A. Starobinsky

http://arxiv.org/abs/1006.4735v1

Using the exact Lemaitre-Bondi-Tolman solution with a non-vanishing cosmological constant $\Lambda$, we investigate how the presence of a local spherically-symmetric inhomogeneity can affect apparent cosmological observables, such as the deceleration parameter or the effective equation of state of dark energy (DE), derived from the luminosity distance under the assumption that the real space-time is exactly homogeneous and isotropic. The presence of a local underdensity is found to produce apparent phantom behavior of DE, while a locally overdense region leads to apparent quintessence behavior. Our study shows how observations in an inhomogeneous $\Lambda$CDM universe with initial conditions compatible with the inflationary beginning, if interpreted under the wrong assumption of homogeneity, can lead to the wrong conclusion about the presence of ``fake'' evolving dark energy instead of $\Lambda$.

 

Fundamental Uncertainty in the BAO Scale from Isocurvature Modes

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

http://arxiv.org/abs/1006.4687v1

Small fractions of isocurvature perturbations correlated with the dominant adiabatic mode are shown to be a significant primordial systematic for future Baryon Acoustic Oscillation (BAO) surveys, distorting the standard ruler distance by broadening and shifting the peak in the galaxy correlation function. Untreated this systematic leads to biases that can exceed $10\sigma$ in the dark energy parameters even for Planck-level isocurvature constraints. Accounting for the isocurvature modes corrects for this bias but degrades the dark energy figure of merit by at least 50\%. The BAO data in turn provides extremely powerful new constraints on the nature of the primordial perturbations. Future large galaxy surveys will thus be powerful probes of the earliest phase of the universe in addition to helping pin down the nature of dark energy.

 

Constraining dark energy fluctuations with supernova correlations

Authors: Michael Blomqvist, Jonas Enander, Edvard Mortsell

http://arxiv.org/abs/1006.4638v1

We investigate constraints on dark energy fluctuations using type Ia supernovae. If dark energy is not in the form of a cosmological constant, that is if the equation of state is not equal to -1, we expect not only temporal, but also spatial variations in the energy density. Such fluctuations would cause local variations in the universal expansion rate and directional dependences in the redshift-distance relation. We present a scheme for relating a power spectrum of dark energy fluctuations to an angular covariance function of standard candle magnitude fluctuations. The predictions for a phenomenological model of dark energy fluctuations are compared to observational data in the form of the measured angular covariance of Hubble diagram magnitude residuals for type Ia supernovae in the Union2 compilation. The observational result is consistent with zero dark energy fluctuations. However, due to the limitations in statistics, current data still allow for quite general dark energy fluctuations as long as they are in the linear regime. 

 

Uniqueness of Current Cosmic Acceleration

Authors: Eric V. Linder

http://arxiv.org/abs/1006.4632v1

One of the strongest arguments against the cosmological constant as an explanation of the current epoch of accelerated cosmic expansion is the existence of an earlier, dynamical acceleration, i.e. inflation. We examine the likelihood that acceleration is an occasional phenomenon, putting stringent limits on the length of any accelerating epoch between recombination and the recent acceleration; such an epoch must last less than 0.05 e-fold (at z>2) or the matter power spectrum is modified by more than 20%. 

 

Measuring Coherent Motions in the Universe

Authors: Yong-Seon Song (ICG, Portsmouth), Cristiano G. Sabiu, Issha Kayo, Robert C. Nichol

http://arxiv.org/abs/1006.4630v1

We present new measurements of the coherent motion of galaxies based on observations of the large-scale redshift--space distortions seen in the two--dimensional two--point correlation function of Luminous Red Galaxies in Data Release Seven of the Sloan Digital Sky Survey. We have developed a new methodology for estimating these coherent motions, which is less dependent on the details of galaxy bias and of the cosmological model to explain the late--time acceleration of the expansion of the Universe. We measure a one--dimensional velocity dispersion of galaxies on large--scales of sigma_v=3.01^{+0.45}_{-0.46} Mpc/h and sigma_v=3.69^{+0.47}_{-0.47} Mpc/h at a mean redshift of z=0.25 and 0.38 respectively. These values are fully consistent with predictions for a WMAP7--normalised LCDM Universe and inconsistent (at >5 sigma) with a Dvali-Gabadadze-Porrati (DGP) model for the Universe. We can convert the units of these sigma_v measurements to 270^{+40}_{-41} km/s and 320^{+41}_{-41} km/s respectively (assuming a LCDM universe), which are much lower than that expected based on recent low redshift (z<0.2) measurements of the peculiar velocity field (or ``bulk flows"), i.e., we would have predicted motions of ~ 600 km/s over our redshift range (0.16<z<0.47) to be consistent with these local measurements. One possible explanation for such a large discrepancy is that our Galaxy is located in unusually over, or under, dense region of the Universe.

 

Primordial Density Perturbations and Reheating from Gravity

Authors: N. C. Tsamis (University of Crete), R. P. Woodard (University of Florida)

http://arxiv.org/abs/1006.4834v1

We consider the presence and evolution of primordial density perturbations in a cosmological model based on a simple ansatz which captures -- by providing a set of effective gravitational field equations -- the strength of the enhanced quantum loop effects that can arise during inflation. After deriving the general equations that perturbations obey, we concentrate on scalar perturbations and show that their evolution is quite different than that of conventional inflationary models but still phenomenologically acceptable. The main reason for this novel evolution is the presence of an oscillating regime after the end of inflation which makes all super-horizon scalar modes oscillate. The same reason allows for a natural and very fast reheating mechanism for the universe.

 

 

arXiv: 24 June 2010

Space number density of bright quasars in the halo model of galaxy formation

Authors: Yu. Kulinich, B. Novosyadlyj

http://arxiv.org/abs/1006.4466v1

We analyse the redshift dependence of space number density of quasars assuming that they are the short-lived active stages of the massive galaxies and arise immediately after the collapse of homogeneous central part of protogalaxy clouds. Obtained dependence fits the observational data ChaMP+CDF+ROSAT (Silverman et al. 2005) very well for protogalaxy clouds of mass $M\approx 8\cdot 10^{11}$ $h^{-1}M_{\odot}$ and ellipticity $e<0.4$. The lifetime of bright X-ray AGNs or QSOs with $L_X>10^{44.5}$ erg$\cdot s^{-1}$ in the range of energies $0.3-8$ keV is $\tau_{QSO}\sim 6\cdot10^6$ years when the mass of supermassive black hole is $M_{SMBH}\sim 10^{9}$ $M_{\odot}$ and the values of other quasar parameters are reasonable. The analysis and all calculations were carried out in the framework of $\Lambda$CDM-model with parameters determined from 5-years WMAP, SNIa and large scale structure data \cite{Komatsu09}. It is concluded, that the halo model of galaxy formation in the $\Lambda$CDM cosmological model matches well observational data on AGNs and QSOs number density coming from current optical and X-ray surveys.

 

What PhD students really want

Authors: Minnie Mao

http://arxiv.org/abs/1006.4421v1

The road to becoming an astronomer is exciting, but often fraught with danger and conflicting messages. A PhD student is inundated with catch-phrases such as "publish or perish" and "it's not about the quantity, but the quality of work". How do we know which advice to follow? How can we publish copious amounts of quality work in only three years so as to maximize our success in the future? How do we even know what "good quality" really is? With only a short time to prepare ourselves for the big wide world of Astronomy, what is the best way for a PhD student to maximize their research and ultimately maximize their success as a real astronomer? The PhD students of today are the astronomers of tomorrow, but their journey depends on a positive work environment in which they can thrive and improve. Here I present the results of a survey of current PhD students on how they believe they can maximize their success in science. I find that PhD students in Australia expect to write more papers during their PhD than is expected by their supervisors, but that they are generally happy with the quality of their supervision. Above all, students love telescopes, and hands-on observations are an important part of acquiring the knowledge and culture necessary to becoming a real astronomer.

 

Inevitable ghost and the degrees of freedom in f(R,G) gravity

Authors: Antonio De Felice, Takahiro Tanaka

http://arxiv.org/abs/1006.4399v1

The study of linear perturbation theory for general functions of the Ricci and Gauss-Bonnet scalars is done over an empty anisotropic universe, i.e. the Kasner-type background, in order to show that an anisotropic background in general has ghost degrees of freedom, which are absent on Friedmann-Lemaitre-Robertson-Walker (FLRW) backgrounds. The study of the scalar perturbation reveals that on this background the number of independent propagating degrees of freedom is four and reduces to three on FLRW backgrounds, as one mode becomes highly massive to decouple from the physical spectrum. When this mode remains physical, there is inevitably a ghost mode.

 

Dark energy from Neutrinos and Standard Model Higgs potential

Authors: Gaetano Lambiase, Hiranmaya Mishra, Subhendra Mohanty

http://arxiv.org/abs/1006.4461v1

If neutrino mass is a function of the Higgs potential then minimum of the total thermodynamic potential $\Omega$ (which is the Higgs potential minus the neutrino pressure) can shift from the standard electro-weak vev $v=246.2$ GeV by a small amount which depends on the neutrino pressure. If the neutrino mass is a very steep function of the Higgs field then the equilibrium thermodynamic potential can act like the dark energy with $\omega \simeq -1$. Choosing the neutrino mass as logarithmic function of the Higgs field and a heavy mass scale, we find that the correct magnitude of the cosmological density of the present universe $\rho_\lambda \simeq (0.002 eV)^4$ is obtained by choosing the heavy mass at the GUT scale.

arXiv: 23 June 2010

Constraining Primordial Magnetism

Authors: J. Richard Shaw, Antony Lewis

http://arxiv.org/abs/1006.4242v1

Primordial magnetic fields could provide an explanation for the galactic magnetic fields observed today, in which case they may also leave interesting signals in the CMB and the small-scale matter power spectrum. We discuss how to approximately calculate the important non-linear magnetic effects within the guise of linear perturbation theory, and calculate the matter and CMB power spectra including the SZ contribution. We then use various cosmological datasets to constrain the form of the magnetic field power spectrum. Using solely large-scale CMB data (WMAP7, QUaD and ACBAR) we find a 95% CL on the variance of the magnetic field at 1 Mpc of B_\lambda < 6.4 nG. When we include SPT data to constrain the SZ effect, we find a revised limit of B_\lambda < 4.1 nG. The addition of SDSS Lyman-alpha data lowers this limit even further, roughly constraining the magnetic field to B_\lambda < 1.3 nG.

Constraining H0 in General Dark Energy Models from Sunyaev-Zeldovich/X-ray Technique and Complementary Probes

Authors: R. F. L. Holanda, J. V. Cunha, L. Marassi, J. A. S. Lima

http://arxiv.org/abs/1006.4200v1

In accelerating dark energy models, the estimates of H0 from Sunyaev-Zel'dovich effect (SZE) and X-ray surface brightness of galaxy clusters may depend on the matter content (Omega_M), the curvature (Omega_K) and the equation of state parameter (w). In this article, by using a sample of 25 angular diameter distances from galaxy clusters obtained through SZE/X-ray technique, we constrain H_0 in the framework of a general LCDM models (free curvature) and a flat XCDM model with equation of state parameter, w=p_x/\rho_x (w=constant). In order to broke the degeneracy on the cosmological parameters, we apply a joint analysis involving the baryon acoustic oscillations (BAO) and the CMB Shift Parameter signature. By neglecting systematic uncertainties, for nonflat LCDM cosmologies we obtain $H_0=73.2^{+4.3}_{-3.7}$ km s$^{-1}$ Mpc$^{-1}$ (1sigma) whereas for a flat universe with constant equation of state parameter we find $H_0=71.4^{+4.4}_{-3.4}$ km s$^{-1}$ Mpc$^{-1}$ (1$\sigma$). Such results are also in good agreement with independent studies from the Hubble Space Telescope key project and recent estimates based on Wilkinson Microwave Anisotropy Probe, thereby suggesting that the combination of these three independent phenomena provides an interesting method to constrain the Hubble constant. In particular, comparing these results with a recent determination for a flat LCDM model using only the SZE technique and BAO [Cunha et al. MNRAS 379, L1 2007], we see that the geometry has a very weak influence on H0 estimates for this combination of data.

 

Fitting cosmological data to the function $q(z)$ from GR Theory: Modified Chaplygin Gas

Authors: Alan M. Velásquez-Toribio, Maria Luíza Bedran

http://arxiv.org/abs/1006.4198v1

In the Friedmann cosmology the deceleration of the expansion $q$ plays a fundamental role. We derive the deceleration as a function of redshift $q(z)$ in two scenarios: $\Lambda$CDM model and modified Chaplygin gas ($MCG$) model. The function for the $MCG$ model is then fitted to the cosmological data in order to obtain the cosmological parameters that minimize $\chi^2$. We use the Fisher matrix to construct the covariance matrix of our parameters and reconstruct the q(z) function. We use Supernovae Ia, WMAP5 and BAO measurements to obtain the observational constraints. We determined the present acceleration as $q_0=-0.60 \pm 0.12$ for the $MCG$ model using the Constitution dataset of SNeIa and BAO, and $q_0=-0.63 \pm 0.17$ for the Union dataset and BAO. The transition redshift from deceleration to acceleration was found to be around $0.6$ for both datasets. We have also determined the dark energy parameter for the $MCG$ model: $\Omega_{X0}=0.834 \pm 0.028$ for the Constitution dataset and $\Omega_{X0}=0.854 \pm 0.036$ using the Union dataset.

arXiv: 22 June 2010

Pulsars as gravitational wave detectors

Authors: George Hobbs

http://arxiv.org/abs/1006.3969v1

Pulsar timing array projects are carrying out high precision observations of millisecond pulsars with the aim of detecting ultra-low frequency (~ 10^{-9} to 10^{-8} Hz) gravitational waves. We show how unambiguous detections of such waves can be obtained by identifying a signal that is correlated between the timing of different pulsars. Here we describe the ongoing observing projects, the expected sources of gravitational waves, the processing of the data and the implications of current results.

 

A novel exact cosmological solution of Einstein equations

Authors: David H. Oaknin

http://arxiv.org/abs/1006.3950v1

We present a novel exact solution to Einstein's General Relativity equations for an ideal fluid and explore the possibility that it could describe the patch of the universe observable to us. We consider a Lorentzian manifold whose metric can be parameterized as $g_{\mu \nu} = diag\left(1, -a^2(t) r_0^2/r^2, -a^2(t) r_0^2/r^2, -a^2(t) r_0^2/r^2\right)$ in a cartesian comoving grid $(t,x,y,z)$ in which the fluid is assumed to be at rest $u^{\mu}=(1,0,0,0)$, $t$ denotes cosmological time, $r=\sqrt{x^2+y^2+z^2}$ is a radial comoving coordinate and $r_0$ is a characteristic $length$ scale. This novel class of solutions fits well with the observational pillars upon which relies the standard FLRW cosmology and, furthermore, it naturally solves some of its most outstanding problems.

 

Sunyaev-Zel'dovich galaxy clusters number counts : consequences of cluster scaling laws evolution

Authors: Pierre Delsart, Domingos Barbosa, Alain Blanchard

http://arxiv.org/abs/1006.3859v1

Galaxy cluster surveys based on the Sunyaev-Zeldovich effect (SZE) mapping are expected from ongoing experiments. Such surveys are anticipated to provide a significant amount of information relevant to cosmology from the number counts redshift distribution. We carry out an estimation of predicted SZE counts and their redshift distribution taking into account the current cosmological constraints and the X-ray cluster temperature distribution functions. Comparison between local and distant cluster temperature distribution functions provides evidence for an evolution in the abundance of X-ray clusters that is not consistent with the use of standard scaling relations of cluster properties in the framework of the current concordance model. The hypothesis of some evolution of the scaling law driven by non-gravitational processes is a natural solution to this problem. We perform a MCMC statistical study using COSMOMC, combining current CMB observations from WMAP, the SNIa Hubble diagram, the galaxy power spectrum data from SDSS and X-ray clusters temperature distributions to predict SZE cluster number counts. Models reproducing well the X-ray cluster temperature distribution function evolution lead to a significantly lower SZE clusters number counts with a distinctive redshift distribution. Ongoing microwave SZE surveys will therefore shed new light on intracluster gas physics and greatly help to identify the role of possible non-gravitational physics in the history of the hot gas component of x-ray clusters.

 

Isocurvature modes and Baryon Acoustic Oscillations

Authors: Anna Mangilli, Licia Verde, Maria Beltran

http://arxiv.org/abs/1006.3806v1

e measurement of Baryonic Acoustic Oscillations from galaxy surveys is well known to be a robust and powerful tool to constrain dark energy. This method relies on the knowledge of the size of the acoustic horizon at radiation drag derived from Cosmic Microwave Background Anisotropy measurements. In this paper we quantify the effect of non-standard initial conditions in the form of an isocurvature component on the determination of dark energy parameters from future BAO surveys. In particular, if there is an isocurvature component (at a level still allowed by present data) but it is ignored in the CMB analysis, the sound horizon and cosmological parameters determination is biased, and, as a consequence, future surveys may incorrectly suggest deviations from a cosmological constant. In order to recover an unbiased determination of the sound horizon and dark energy parameters, a component of isocurvature perturbations must be included in the model when analyzing CMB data. Fortunately, doing so does not increase parameter errors significantly.

 

Matter-gravity couplings and Lorentz violation

Authors: Alan Kostelecky, Jay Tasson

http://arxiv.org/abs/1006.4106v1

The gravitational couplings of matter are studied in the presence of Lorentz and CPT violation. At leading order in the coefficients for Lorentz violation, the relativistic quantum hamiltonian is derived from the gravitationally coupled minimal Standard-Model Extension. For spin-independent effects, the nonrelativistic quantum hamiltonian and the classical dynamics for test and source bodies are obtained. A systematic perturbative method is developed to treat small metric and coefficient fluctuations about a Lorentz-violating and Minkowski background. The post-newtonian metric and the trajectory of a test body freely falling under gravity in the presence of Lorentz violation are established. An illustrative example is presented for a bumblebee model. The general methodology is used to identify observable signals of Lorentz and CPT violation in a variety of gravitational experiments and observations, including gravimeter measurements, laboratory and satellite tests of the weak equivalence principle, antimatter studies, solar-system observations, and investigations of the gravitational properties of light. Numerous sensitivities to coefficients for Lorentz violation can be achieved in existing or near-future experiments at the level of parts in 10^3 down to parts in 10^{15}. Certain coefficients are uniquely detectable in gravitational searches and remain unmeasured to date.

 

Dark matter effects in vacuum spacetime

Authors: Luca Rizzi, Sergio Cacciatori, Vittorio Gorini, Alexander Kamenshchik, Oliver F. Piattella

http://arxiv.org/abs/1006.4059v1

We analyse a toy model describing an empty spacetime in which the motion of a test mass (and the trajectories of photons) evidence the presence of a continuous and homogeneous distribution of matter, though, since the energy-momentum tensor vanishes, no real matter or energy distribution is present at all. Thus, an hypothetical observer will conclude that he is immersed in some sort of dark matter, even though he has no chance to directly detect it. This suggests yet another possibility of explaining the elusive dark matter as a purely dynamical effect due to the curvature of spacetime.

 

 

arXiv: 21 June 2010

High-z massive clusters as a test for dynamical coupled dark energy

Authors: Marco Baldi, Valeria Pettorino

http://arxiv4.library.cornell.edu/abs/1006.3761v1

The recent detection (Jee etal 2009) of the massive cluster XMMU J2235.3-2557 at a redshift z = 1.4, with an estimated mass M = 6.4 +- 1.2 X 10^14 M_sol, has been claimed to be a possible challenge to the standard LCDM cosmological model. More specifically, the probability to detect such a cluster has been estimated to be 0.005 if a LCDM model with gaussian initial conditions is assumed, resulting in a 3 sigma discrepancy from the standard cosmological model. In this paper we propose to use high redshift clusters as the one detected in Jee etal 2009 to compare the cosmological constant scenario with interacting dark energy models. We show that coupled dark energy models, where an interaction is present between dark energy and cold dark matter, can significantly enhance the probability to observe very massive clusters at high redshift.

 

 

arXiv: 18 Jun 2010

Designing a space-based galaxy redshift survey to probe dark energy

Authors: Yun Wang, Will Percival, Andrea Cimatti, Pia Mukherjee, Luigi Guzzo, Carlton M. Baugh, Carmelita Carbone, Paolo Franzetti, Bianca Garilli, James E. Geach, Cedric G. Lacey, Elisabetta Majerotto, Alvaro Orsi, Piero Rosati, Lado Samushia, Giovanni Zamorani

http://arxiv.org/abs/1006.3517v1

A space-based galaxy redshift survey would have enormous power in constraining dark energy and testing general relativity, provided that its parameters are suitably optimized. We study viable space-based galaxy redshift surveys, exploring the dependence of the Dark Energy Task Force (DETF) figure-of-merit (FoM) on redshift accuracy, redshift range, survey area, target selection, and forecast method. Fitting formulae are provided for convenience. We also consider the dependence on the information used: the full galaxy power spectrum P(k), P(k) marginalized over its shape, or just the Baryon Acoustic Oscillations (BAO). We find that the inclusion of growth rate information (extracted using redshift space distortion and galaxy clustering amplitude measurements) leads to a factor of ~ 3 improvement in the FoM, assuming general relativity is not modified. This inclusion partially compensates for the loss of information when only the BAO are used to give geometrical constraints, rather than using the full P(k) as a standard ruler. We find that a space-based galaxy redshift survey covering ~20,000 sq deg over 0.5 < z < 2 with \sigma_z/(1+z)<= 0.001 exploits a redshift range that is only easily accessible from space, extends to sufficiently low redshifts to allow both a vast 3-D map of the universe using a single tracer population, and overlaps with ground-based surveys to enable robust modeling of systematic effects. We argue that these parameters are close to their optimal values given current instrumental and practical constraints.

 

Mass discrepancy in galaxy clusters as a result of the offset between dark matter and baryon distributions

Authors: HuanYuan Shan, Bo Qin, HongSheng Zhao

http://arxiv.org/abs/1006.3484v1

Recent studies of lensing clusters reveal that it might be fairly common for a galaxy cluster that the X-ray center has an obvious offset from its gravitational center which is measured by strong lensing. We argue that if these offsets exist, then X-rays and lensing are indeed measuring different regions of a cluster, and may thus naturally result in a discrepancy in the measured gravitational masses by the two different methods. Here we investigate theoretically the dynamical effects of such lensing-X-ray offsets, and compare with observational data. We find that for typical values, the offset alone can give rise to a factor of two difference between the lensing and X-ray determined masses for the core regions of a cluster, suggesting that such "offset effect" may play an important role and should not be ignored in our dynamical measurements of clusters.

 

Confronting Dark Energy Models using Galaxy Cluster Number Counts

Authors: S. Basilakos, M. Plionis, J. A. S. Lima

http://arxiv.org/abs/1006.3418v1

The mass function of galaxy clusters and their redshift distribution are computed for 12 distinct accelerating cosmological scenarios and confronted to the predictions of the conventional flat $\Lambda$CDM model. The comparison with $\Lambda$CDM is a two-step process. Firstly, we determine the free parameters of all models through a joint analysis involving the latest cosmological data from SNe type Ia, CMB shift parameter and BAO. Apart from a brane world inspired cosmology, it is found that the derived Hubble relation of the remaining models reproduce the $\Lambda$CDM results approximately with the same degree of statistical confidence. Secondly, in order to distinguish the different models from the expectations of $\Lambda$CDM, we discuss the predicted cluster redshift distribution on the basis of two future cluster surveys: (i)an X-ray survey based on the {\tt eROSITA} satellite, and (ii) a Sunayev-Zeldovich survey based on the Southern Polar Telescope. As a result, we find that the predictions of 6 out of 12 dark energy models can be clearly distinguished from the $\Lambda$CDM cosmology, only 1 of them can probably be distinguished, while 5 models are statistically equivalent to $\Lambda$CDM cosmology, as long as the expected cluster mass function and redshift distribution are concerned. The present analysis suggest that such a technique appears to be very competitive to independent tests probing the late time evolution of the Universe and the associated dark energy effects.

 

 

Gamma-ray Signal from Earth-mass Dark Matter Microhalos

Authors: Tomoaki Ishiyama, Junichiro Makino, Toshikazu Ebisuzaki

http://arxiv.org/abs/1006.3392v1

Earth-mass dark matter microhalos with size of $\sim$ 100 AUs are the first structures formed in the universe, if we consider neutralino as the dark matter candidate. Early studies suggested that a noticeable fraction of microhalos born in early universe have survived up to present time and they might be observed as the dominant sources of the annihilation signal. On the other hand, others claimed that small-scale structure have a negligible impact on dark matter detectability. Here, we report the results of ultra-high-resolution simulation of the formation and evolution of these microhalos. We found that microhalos have the central density cusp of the form $\rho \propto r^{-1.5}$, much steeper than the cusp of larger dark halos. The very central regions of these microhalos survive the encounters with stars down to the radius of a few kpcs from the galactic center. The nearest microhalos at distance of $\sim$ 0.1 pc, might be visible as point sources (radius less than 1'), with proper motion of $\sim 0.2$ degree per year. Subhalos are also observable by boosts due to microhalos. Also, we might be able to use the millisecond pulsar timing measurements by PPTA to detect microhalos.

 

On the Possibility of Anisotropic Curvature in Cosmology

Authors: Tomi S. Koivisto, David F. Mota, Miguel Quartin, Tom G. Zlosnik

http://arxiv.org/abs/1006.3321v1

In addition to shear and vorticity a homogeneous background may also exhibit anisotropic curvature. Here a class of spacetimes is shown to exist where the anisotropy is solely of the latter type, and the shear-free condition is supported by a canonical, massless 2-form field. A distortion of the luminosity distances is derived and used to test the model against supernovae and the CMB.

 

Conceptual problems in detecting the evolution of dark energy when using distance measurements

Authors: Krzysztof Bolejko

http://arxiv.org/abs/1006.3348v1

Dark energy is now one of the most important and topical problems in cosmology. The first step to reveal its nature is to detect the evolution of dark energy or to prove beyond doubt that the cosmological constant is indeed constant. However, in the standard approach to cosmology, the Universe is described by the homogeneous and isotropic Friedmann models and nearly all sets of cosmological observations are analyzed within the framework of the homogeneous models. This paper shows that in the perturbed universe (even if perturbations vanish if averaged over sufficiently large scale) the distance relation is not the same as in the unperturbed universe. This has a serious consequence when studying the nature of dark energy, and as shown here can impair the analysis and studies of dark energy. An example of the Swiss-Cheese model is presented and it is shown that perturbations (even if <\delta\rho> =0) do affect observations and that the perturbed distance relation does not oscillate around the unperturbed value. Therefore, if future observations are analyzed only within the homogeneous framework then the impact of inhomogeneities (such as voids and superclusters) can be mistaken for evolving dark energy.

arXiv: 17 June 2010

Bimetric structure formation: non-Gaussian predictions

Authors: Joao Magueijo, Johannes Noller, Federico Piazza

http://arxiv.org/abs/1006.3216v1

The minimal bimetric theory employing a disformal transformation between matter and gravity metrics is known to produce exactly scale-invariant fluctuations. It has a purely equilateral non-Gaussian signal, with an amplitude smaller than that of DBI inflation (with opposite sign) but larger than standard inflation. We consider non-minimal bimetric models, where the coupling $B$ appearing in the disformal transformation ${\hat g}_{\mn}= g_{\mn} -B\partial_\mu\phi\partial_\nu\phi$ can run with $\phi$. For power-law $B(\phi)$ these models predict tilted spectra. For each value of the spectral index, a distinctive distortion to the equilateral property can be found. The constraint between this distortion and the spectral index can be seen as a "consistency relation" for non-minimal bimetric models.

 

ASOHF: a new adaptive spherical overdensity halo finder

Authors: Susana Planelles, Vicent Quilis

We present and test a new halo finder based on the spherical overdensity (SO) method. This new adaptive spherical overdensity halo finder (ASOHF) is able to identify dark matter haloes and their substructures (subhaloes) down to the scales allowed by the analysed simulations. The code has been especially designed for the adaptive mesh refinement cosmological codes, although it can be used as a stand-alone halo finder for N-body codes. It has been optimised for the purpose of building the merger tree of the haloes. In order to verify the viability of this new tool, we have developed a set of bed tests that allows us to estimate the performance of the finder. Finally, we apply the halo finder to a cosmological simulation and compare the results obtained to those given by other well known publicly available halo finders.

 

 

arXiv: 16 June 2010

The statistics of the subhalo abundance of dark matter haloes

Authors: L. Gao, C. S. Frenk, M. Boylan-Kolchin, A. Jenkins, V. Springel, S. D. M. White

We study the population statistics of the surviving subhaloes of LCDM dark matter haloes using a set of very high resolution N-body simulations. These include both simulations of representative regions of the Universe and ultra-high resolution resimulations of individual dark matter haloes. We find that more massive haloes tend to have a larger mass fraction in subhaloes. For example, cluster size haloes typically have 7.5 percent of their mass in substructures of fractional mass larger than 1e-5, which is 25 percent higher than galactic haloes. There is, however, a large variance in the subhalo mass fraction from halo to halo, whereas the subhalo abundance shows much higher regularity. For dark matter haloes of fixed mass, the subhalo abundance decreases by 30 percent between redshift 2 and 0. The subhalo abundance function correlates with the host halo concentration parameter and formation redshift. However, the intrinsic scatter is not significantly reduced for narrow ranges of concentration parameter or formation redshift, showing that they are not the dominant parameters that determine the subhalo abundance in a halo.

 

Templates for the Sunyaev-Zel'dovich Angular Power Spectrum

Authors: Hy Trac, Paul Bode, Jeremiah P. Ostriker

http://arxiv.org/abs/1006.2828v1

We present templates for the Sunyaev-Zel'dovich (SZ) angular power spectrum based on four models for the nonlinear gas distribution. The frequency-dependent SZ temperature fluctuations, with thermal (TSZ) and kinetic (KSZ) contributions, are calculated by tracing through a dark matter simulation, processed to include gas in dark matter halos and in the filamentary intergalactic medium. Different halo gas models are compared to study how star formation, energetic feedback, and nonthermal pressure support influence the angular power spectrum. The standard model has been calibrated to reproduce the stellar and gas fractions and X-ray scaling relations measured from low redshift clusters and groups. The other models illustrate the current theoretical and empirical uncertainties relating to properties of the intracluster medium. Relative to the standard model, their angular power spectra differ by approximately 50% (TSZ), 20% (KSZ), and 40% (SZ at 148 GHz) for l=3000, sigma_8=0.8, and homogeneous reionization at z=10. The angular power spectrum decreases in amplitude as gas mass and binding energy is removed through star formation, and as gas is pushed out to larger radii by energetic feedback. With nonthermal pressure support, less pressure is required to maintain hydrostatic equilibrium, thus reducing the thermal contribution to the SZ power. We also calculate the SZ templates as a function of sigma_8 and quantify this dependence. Assuming C_l is proprotional to (sigma_8/0.8)^alpha, the effective scaling index ranges from 7<alpha_tsz<9, 4.5<alpha_ksz<5.5, and 6.5<alpha_sz(148 GHz)<8 at l=3000 for 0.6<sigma_8<1. The template spectra are publicly available and can be used when fitting for the SZ contribution to the cosmic microwave background on arcminute scales.

arXiv: 15 June 2010

Chameleons with Field Dependent Couplings

Authors: Philippe Brax, Carsten van de Bruck, David F. Mota, Nelson J. Nunes, Hans A. Winther

http://arxiv.org/abs/1006.2796v1

Certain scalar-tensor theories exhibit the so-called chameleon mechanism, whereby observational signatures of scalar fields are hidden by a combination of self-interactions and interactions with ambient matter. Not all scalar-tensor theories exhibit such a chameleon mechanism, which has been originally found in models with inverse power run-away potentials and field independent couplings to matter. In this paper we investigate field-theories with field-dependent couplings and a power-law potential for the scalar field. We show that the theory indeed is a chameleon field theory. We find the thin-shell solution for a spherical body and investigate the consequences for E\"ot-Wash experiments, fifth-force searches and Casimir force experiments. Requiring that the scalar-field evades gravitational tests, we find that the coupling is sensitive to a mass-scale which is of order of the Hubble scale today.

 

Simulations and Observations of the Microwave Universe

Authors: Michael Peel

http://arxiv.org/abs/1006.2760v1

Simulations and observations of the microwave sky are of great importance for understanding the Universe that we reside in. Specifically, knowledge of the CMB and its foregrounds - including the SZ effect from clusters of galaxies and radio point sources - tell us about the Universe on its very largest scales, and also what the Universe is made of.
We describe the creation of software to carry out large numbers of virtual sky simulations. The simulations include the CMB, SZ effect and point sources, and are designed to examine the effects of point sources and the SZ effect on present and recent observations of the CMB. Utilizing sets of 1,000 simulations, we find that the power spectrum resulting from the SZ effect is expected to have a larger standard deviation by a factor of 3 than would be expected from purely Gaussian realizations, and is significantly skewed towards increased values for the power spectrum. The effects of the clustering of galaxy clusters, residual point sources and uncertainties in the gas physics are also investigated, as are the implications for the excess power measured in the CMB power spectrum by the CBI and BIMA.
We carry out end-to-end simulations for OCRA-p observations of point sources. The introduction of simulated 1/ f noise significantly reduces the predicted ability of the instruments to observe weak sources by measuring the sources for long periods of time.
The OCRA-p receiver has been used to observe point sources in the VSA fields so that they can be subtracted from observations of the CMB power spectrum. We find that these point sources are split between steep and flat spectrum sources. We have also observed 550 CRATES flat spectrum radio sources, which will be useful for comparison to Planck satellite observations. Finally, the assembly and commissioning of the OCRA-F receiver is outlined.

 

The 21 cm Signature of Cosmic String Wakes

Authors: Robert H. Brandenberger, Rebecca J. Danos, Oscar F Hernandez, Gilbert P. Holder (McGill University)

http://arxiv.org/abs/1006.2514v1

We discuss the signature of a cosmic string wake in 21cm redshift surveys. Since 21cm surveys probe higher redshifts than optical large-scale structure surveys, the signatures of cosmic strings are more manifest in 21cm maps than they are in optical galaxy surveys. We find that, provided the tension of the cosmic string exceeds a critical value (which depends on both the redshift when the string wake is created and the redshift of observation), a cosmic string wake will generate an emission signal with a brightness temperature which approaches a limiting value which at a redshift of $z + 1 = 30$ is close to 400 mK in the limit of large string tension. The signal will have a specific signature in position space: the excess 21cm radiation will be confined to a wedge-shaped region whose tip corresponds to the position of the string, whose planar dimensions are set by the planar dimensions of the string wake, and whose thickness (in redshift direction) depends on the string tension. For wakes created at $z_i + 1 = 10^3$, then at a redshift of $z + 1 = 30$ the critical value of the string tension $\mu$ is $G \mu = 6 \times 10^{-7}$, and it decreases linearly with redshift (for wakes created at the time of equal matter and radiation, the critical value is a factor of two lower at the same redshift). For smaller tensions, cosmic strings lead to an observable absorption signal with the same wedge geometry.

 

Constraining Dark Matter in Galactic Substructure

Authors: Eric J. Baxter, Scott Dodelson, Savvas M. Koushiappas, Louis E. Strigari

http://arxiv.org/abs/1006.2399v1

Detecting the dark matter annihilation signal from Galactic substructure, or subhalos, is an important challenge for high-energy gamma-ray experiments. In this paper we discuss detection prospects by combining two different aspects of the gamma-ray signal: the angular distribution and the photon counts probability distribution function (PDF). The true PDF from subhalos has been shown recently (by Lee et al.) to deviate from Poisson; we extend this analysis and derive the signal PDF from a detailed Lambda-CDM-based model for the properties of subhalos. We combine our PDF with a model for Galactic and extra-Galactic diffuse gamma-ray emission to obtain an estimator and projected error on dark matter particle properties (mass and annihilation cross section) using the Fermi Gamma-Ray Space Telescope. We compare the estimator obtained from the true PDF to that obtained from the simpler Poisson analysis. We find that, although both estimators are unbaised in the presence of backgrounds, the error on dark matter properties derived from the true PDF is ~45% smaller than when utilizing the Poisson-based analysis.

 

The distribution of annihilation luminosities in dark matter substructure

Authors: Savvas M. Koushiappas (Brown U.), Andrew R. Zentner (U. Pittsburgh), Andrey V. Kravtsov (U. Chicago/KICP)

http://arxiv.org/abs/1006.2391v1

We calculate the probability distribution function (PDF) of the expected annihilation luminosities of dark matter subhalos as a function of subhalo mass and distance from the Galactic center using a semi-analytical model of halo evolution. The luminosity PDF allows for simple construction of mock samples of gamma-ray luminous subhalos and assessment of the variance in among predicted gamma-ray signals from dark matter annihilation. Other applications include quantifying the variance of the expected luminosities of dwarf spheroidal galaxies, assessing the level at which dark matter annihilation can be a contaminant in the expected gamma-ray signal from other astrophysical sources, as well as estimating the level at which nearby subhalos can contribute to the antimatter flux.

 

Dark Matter: A Primer

Authors: Katherine Garrett, Gintaras Duda

http://arxiv.org/abs/1006.2483v1

Dark matter is one of the greatest unsolved mysteries in cosmology at the present time. About 80% of the universe's gravitating matter is non-luminous, and its nature and distribution are for the most part unknown. In this paper, we will outline the history, astrophysical evidence, candidates, and detection methods of dark matter, with the goal to give the reader an accessible but rigorous introduction to the puzzle of dark matter. This review targets advanced students and researchers new to the field of dark matter, and includes an extensive list of references for further study.