arXiv: 29 July 2011

Primordial Non-Gaussianity and Extreme-Value Statistics of Galaxy Clusters

Sirichai Chongchitnan, Joseph Silk (Oxford)

arXiv:1107.5617v1

What is the size of the most massive object one expects to find in a survey of a given volume? In this paper, we present a solution to this problem using Extreme-Value Statistics. We calculate the probability density function (pdf) of extreme-mass clusters in a survey volume, and show how primordial non-Gaussianity shifts the peak of this pdf. We compare the extreme-value pdfs using various mass functions and find significant differences at high redshift. Applying our formalism to the recently reported massive high-redshift cluster XMMUJ0044.0-2-33, we find that fNL\approx360 is required to explain its existence as the most massive cluster observed in the redshift range 1.6<z<2.2. Finally, we argue that the probability distribution of extreme-mass clusters belongs to the so-called Fr\'echet family of distribution, regardless of the presence of non-Gaussianity.

arXiv: 28 July 2011

Large-scale clustering of galaxies in general relativity

Donghui Jeong, Fabian Schmidt, Christopher M. Hirata

arXiv:1107.5427v1 

Several recent studies have shown how to properly calculate the observed clustering of galaxies in a relativistic context, and uncovered corrections to the Newtonian calculation that become significant on scales near the horizon. Here, we retrace these calculations and show that, on scales approaching the horizon, the observed galaxy power spectrum depends strongly on which gauge is assumed to relate the intrinsic fluctuations in galaxy density to matter perturbations through a linear bias relation. Starting from simple physical assumptions, we derive a gauge-invariant expression relating galaxy density perturbations to matter density perturbations on large scales, and show that it reduces to a linear bias relation in synchronous-comoving gauge, corroborating an assumption made in several recent papers. We evaluate the resulting observed galaxy power spectrum, and show that it leads to corrections similar to an effective non-Gaussian bias corresponding to a local (effective) fNL < 0.5. This number can serve as a guideline as to which surveys need to take into account relativistic effects. We also discuss the scale-dependent bias induced by primordial non-Gaussianity in the relativistic context, which again is simplest in synchronous-comoving gauge.

arXiv: 27 July 2011

Can 21-cm observations discriminate between high-mass and low-mass galaxies as reionization sources?

Ilian T. Iliev (University of Sussex), Garrelt Mellema (Stockholm University), Paul R. Shapiro (University of Texas), Ue-Li Pen (CITA), Yi Mao, Jun Koda(University of Texas), Kyungjin Ahn (Chosun University)

arXiv:1107.4772v1

The prospect of detecting the first galaxies by observing their impact on the intergalactic medium as they reionized it during the first billion years leads us to ask whether such indirect observations are capable of diagnosing which types of galaxies were most responsible for reionization. We attempt to answer this by considering a set of large-scale radiative transfer simulations of reionization in sufficiently large volumes to make statistically meaningful predictions of observable signatures, while also directly resolving all atomically-cooling halos down to 10^8 M_solar. We focus here on predictions of the 21-cm background, to see if upcoming observations are capable of distinguishing a universe ionized primarily by high-mass halos from one in which both high-mass and low-mass halos are responsible, and to see how these results depend upon the uncertain source efficiencies. We find that 21-cm fluctuation power spectra observed by the first generation EoR/21-cm radio interferometer arrays should be able to distinguish the case of reionization by high-mass halos alone from that by both high- and low-mass halos, together. Some reionization scenarios yield very similar power spectra and rms evolution and thus can only be discriminated by their different mean reionization history and 21-cm PDF distributions. We find that the skewness of the 21-cm PDF distribution smoothed over LOFAR-like window shows a clear feature correlated with the rise of the rms due to patchiness. Measurements of the mean photoionization rates are sensitive to the average density of the regions being studied and therefore could be strongly skewed in certain cases. (abridged)

Pairwise Velocities of Dark Matter Halos: a Test for the Lambda Cold Dark Matter Model using the Bullet Cluster

Robert Thompson, Kentaro Nagamine (UNLV)

arXiv:1107.4645v1

The existence of a bullet cluster (such as 1E0657-56) poses a challenge to the concordance Lambda cold dark matter model. Here we investigate the velocity distribution of dark matter halo pairs in large N-body simulations with differing box sizes (250 Mpc/h - 2 Gpc/h) and resolutions. We examine various basic statistics such as the halo masses, pairwise halo velocities (v_{12}), collisional angles, and pair separation distances. We then compare the results to the observational properties of 1E0657-56. We find that the high velocity tail of the v_{12} distribution extends to greater velocities as we increase the simulation box size. We also find that the number of high-v_{12} pairs increases as we increase the particle count and resolution with a fixed box size, however, this increase is mostly due to lower mass halos which do not match the characteristics of 1E0657-56. We find that the redshift evolution effect is not very strong for the v_{12} distribution function between z=0.0 and z~0.5. We identify some pairs that resemble the properties of 1E0657-56, however, even the best candidates have either wrong halo mass ratios, or too large separations. Our simulations suggest that it is very difficult to produce a halo pair similar to 1E0657-56 at z=0.0, 0.296, & 0.489 in comoving volumes as large as (2 Gpc/h)^3. Based on the extrapolation of our cumulative v_{12} function, we find that one needs a simulation with a comoving box size of (4.48 Gpc/h)^3 and 2240^3 DM particles in order to produce at least one pair of halos that resemble the observed parameters of 1E0657-56. We find that the probability of finding a halo pair with v_{12} >= 3000 km/s and masses >= 10^{14} Msun to be 2.76x10^{-8} at z=0.489. We conclude that either a system like 1E0657-56 is very rare in the concordance LambdaCDM universe, or the observational estimates of 1E0657-56 properties need to be revised.

arXiv: 25 July 2011

Redshift and distances in a ΛCDM cosmology with non-linear inhomogeneities

Nikolai Meures, Marco Bruni (ICG, Portsmouth)

arXiv:1107.4433v1

In this paper we study the effects of inhomogeneities on light propagation in a flat {\Lambda}CDM background. To this end we use exact solutions of Einstein's equations (Meures & Bruni 2011) where, starting from small fluctuations, inhomogeneities arise from a standard growing mode and become non-linear. While the matter distribution in these models is necessarily idealised, there is still enough freedom to assume an arbitrary initial density profile along the line of sight. We can therefore mimic over- densities and voids of various sizes and distributions, e.g. single harmonic sinusoidal modes, coupled modes, and more general distributions in a {\Lambda}CDM background. Our models allow for an exact treatment of the light propagation problem, so that the results are unaffected by approximations and unambiguous. We look at the redshift, luminosity distance and angular diameter distance in these models, and find that if the spatial extent and amplitude of the inhomogeneities is large enough, this could lead to misinterpretations of observations and wrong parameter estimation: even if the Cosmological Principle is valid, the identification of the true {\Lambda}CDM background in an inhomogeneous Universe maybe more difficult than usually assumed.

arXiv: 22 July 2011

An Estimator for statistical anisotropy from the CMB bispectrum

N. Bartolo (Univ. of Padova and INFN Padova), E. Dimastrogiovanni (Univ. of Padova and INFN Padova), M. Liguori (Institut d' Astrophysique de Paris), S. Matarrese (Univ. of Padova and INFN Padova), A. Riotto (CERN and INFN Padova)

arXiv:1107.4304v1 

Various data analyses of the Cosmic Microwave Background (CMB) provide observational hints of statistical isotropy breaking. Some of these features can be studied within the framework of primordial vector fields in inflationary theories which generally display some level of statistical anisotropy both in the power spectrum and in higher-order correlation functions. Motivated by these observations and the recent theoretical developments in the study of primordial vector fields, we develop the formalism necessary to extract statistical anisotropy information from the three-point function of the CMB temperature anisotropy. We employ a simplified vector field model and parametrize the bispectrum of curvature fluctuations in such a way that all the information about statistical anisotropy is encoded in some parameters lambda_{LM}. For such a template bispectrum, we compute an optimal estimator for lambda_{LM} and the expected signal-to-noise ratio. We estimate that an experiment like Planck can be sensitive to a bispectrum anisotropic amplitude as small as 10%. Our results are complementary to the information coming from a power spectrum analysis and particularly relevant for those models where statistical anisotropy turns out to be suppressed in the power spectrum but not negligible in the bispectrum.

arXiv: 21 July 2011

Peculiar motions, accelerated expansion and the cosmological axis

Christos G. Tsagas

Peculiar velocities change the expansion rate of any observer moving relative to the smooth Hubble flow. As a result, observers in a galaxy like our Milky Way can experience accelerated expansion within a globally decelerating universe, even when the drift velocities are small. The effect is local, but the affected scales can be large enough to give the false impression that the whole cosmos has recently entered an accelerating phase. Generally, peculiar velocities are also associated with dipole-like anisotropies, triggered by the fact that they introduce a preferred spatial direction. This implies that observers experiencing locally accelerated expansion, as a result of their own drift motion, may also find that the acceleration is maximised in one direction and minimised in the opposite. We argue that, typically, such a dipole anisotropy should be relatively small and the axis should probably lie fairly close to the one seen in the spectrum of the Cosmic Microwave Background.

arXiv: 20 July 2011

The WiggleZ Dark Energy Survey: constraining the evolution of Newton's constant using the growth rate of structure

Savvas Nesseris, Chris Blake, Tamara Davis, David Parkinson

arXiv:1107.3659v1

We constrain the evolution of Newton's constant using the growth rate of large-scale structure measured by the WiggleZ Dark Energy Survey in the redshift range $0.1 < z < 0.9$. We use this data in two ways. Firstly we constrain the matter density of the Universe, $\Omega_m$ (assuming General Relativity), and use this to construct a diagnostic to detect the presence of an evolving Newton's constant. Secondly we directly measure the evolution of Newton's constant, $G_{eff}$, that appears in Modified Gravity theories, without assuming General Relativity to be true. The novelty of these approaches are that, contrary to other methods, they do not require knowledge of the expansion history of the Universe, $H(z)$, making them model independent tests. Our constraints for the second derivative of Newton's constant at the present day, assuming it is slowly evolving as suggested by Big Bang Nucleosynthesis constraints, using the WiggleZ data is $\ddotGeff(t_0)=-1.19\pm 0.95\cdot 10^{-20}h^2 yr^{-2}$, where $h$ is defined via $H_0=100 h km s^{-1}Mpc^{-1}$, while using both the WiggleZ and the Sloan Digital Sky Survey Luminous Red Galaxy (SDSS LRG) data is $\ddotGeff(t_0)=-3.6\pm 6.8\cdot 10^{-21}h^2 yr^{-2}$, both being consistent with General Relativity. Finally, our constraint for the rms mass fluctuation $\sigma_8$ using the WiggleZ data is $\sigma_8=0.75 \pm 0.08$, while using both the WiggleZ and the SDSS LRG data $\sigma_8=0.77 \pm 0.07$, both in good agreement with the latest measurements from the Cosmic Microwave Background radiation.

arXiv: 19 July 2011

An MCMC approach to extracting the global 21-cm signal during the cosmic dawn from sky-averaged radio observations

Geraint J. A. Harker (1 and 3), Jonathan R. Pritchard (2), Jack O. Burns (1 and 3), Judd D. Bowman (4) ((1) Center for Astrophysics and Space Astronomy, University of Colorado Boulder, USA, (2) Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA, (3) NASA Lunar Science Institute, (4) Arizona State University, School of Earth and Space Exploration, Tempe, AZ, USA)

arXiv:1107.3154v1

Efforts are being made to observe the 21-cm signal from the 'cosmic dawn' using sky-averaged observations with individual radio dipoles. In this paper, we develop a model of the observations accounting for the 21-cm signal, foregrounds, and several major instrumental effects. Given this model, we apply Markov Chain Monte Carlo techniques to demonstrate the ability of these instruments to separate the 21-cm signal from foregrounds and quantify their ability to constrain properties of the first galaxies. For concreteness, we investigate observations between 40 and 120 MHz with the proposed DARE mission in lunar orbit, showing its potential for science return.

arXiv: 18 July 2011

The Baryonic Tully-Fisher Relation of Gas Rich Galaxies as a Test of LCDM and MOND

Stacy McGaugh

arXiv:1107.2934v1

The Baryonic Tully-Fisher Relation (BTFR) is an empirical relation between baryonic mass and rotation velocity in disk galaxies. It provides tests of galaxy formation models in LCDM and of alternative theories like MOND. Observations of gas rich galaxies provide a measure of the slope and normalization of the BTFR that is more accurate (if less precise) than that provided by star dominated spirals, as their masses are insensitive to the details of stellar population modeling. Recent independent data for such galaxies are consistent with Mb = AVf^4 with A = 47+/-6 Msun (km/s)^-4. This is equivalent to MOND with $a_0 = 1.3 +/- 0.3 A/s/s. The scatter in the data is consistent with being due entirely to observational uncertainties. It is unclear why the physics of galaxy formation in LCDM happens to pick out the relation predicted by MOND. We introduce a feedback efficacy parameter E to relate halo properties to those of the galaxies they host. E correlates with star formation rate and gas fraction in the sense that galaxies that have experienced the least star formation have been most impacted by feedback.

arXiv: 15 July 2011

On the prior dependence of constraints on the tensor-to-scalar ratio

Marina Cortês, Andrew R. Liddle, David Parkinson

arXiv:1107.2673v1

We investigate the prior dependence of constraints on cosmic tensor perturbations. Commonly imposed is the strong prior of the single-field inflationary consistency equation, relating the tensor spectral index nT to the tensor-to-scalar ratio r. Dropping it leads to significantly different constraints on nT, with both positive and negative values allowed with comparable likelihood, and substantially increases the upper limit on r on scales k = 0.01 Mpc^-1 to 0.05 Mpc^-1, by a factor of ten or more. Even if the consistency equation is adopted, a uniform prior on r on one scale does not correspond to a uniform one on another; constraints therefore depend on the pivot scale chosen. We assess the size of this effect and determine the optimal scale for constraining the tensor amplitude, both with and without the consistency relation.

arXiv: 16 July 2011

An effective theory of accelerated expansion

Raul Jimenez, P. Talavera, Licia Verde

arXiv:1107.2542v1

We work out an effective theory of accelerated expansion to describe general phenomena of inflation and acceleration (dark energy) in the Universe. Our aim is to determine from theoretical grounds, in a physically-motivated and model independent way, which and how many (free) parameters are needed to broadly capture the physics of a theory describing cosmic acceleration. Our goal is to make as much as possible transparent the physical interpretation of the parameters describing the expansion. We show that, at leading order, there are five independent parameters, of which one can be constrained via general relativity tests. The other four parameters need to be determined by observing and measuring the cosmic expansion rate only, H(z). Therefore we suggest that future cosmology surveys focus on obtaining an accurate as possible measurement of $H(z)$ to constrain the nature of accelerated expansion (dark energy and/or inflation).

arXiv: 13 July 2011

Beyond Einstein: Cosmological Tests of Model Independent Modified Gravity

D.B. Thomas

arXiv:1107.2258v1

Model-independent parametrisations of modified gravity have attracted a lot of attention over the past few years; numerous combinations of experiments and observables have been suggested to constrain these parameterisations, and future surveys look very promising. Galaxy Clusters have been mentioned, but not looked at as extensively in the literature as some other probes. Here we look at adding Galaxy Clusters into the mix of observables and examine whether they could improve the constraints on the modified gravity parameters. In particular, we forecast the constraints from combining the Planck CMB spectrum and SZ cluster catalogue and a DES-like Weak Lensing survey. We've found that adding cluster counts improves the constraints obtained from combining CMB and WL data.

Symmetron Cosmology

Kurt Hinterbichler, Justin Khoury, Aaron Levy, Andrew Matas

arXiv:1107.2112v1

The symmetron is a scalar field associated with the dark sector whose coupling to matter depends on the ambient matter density. The symmetron is decoupled and screened in regions of high density, thereby satisfying local constraints from tests of gravity, but couples with gravitational strength in regions of low density, such as the cosmos. In this paper we derive the cosmological expansion history in the presence of a symmetron field, tracking the evolution through the inflationary, radiation- and matter-dominated epochs, using a combination of analytical approximations and numerical integration. For a broad range of initial conditions at the onset of inflation, the scalar field reaches its symmetry-breaking vacuum by the present epoch, as assumed in the local analysis of spherically-symmetric solutions and tests of gravity. For the simplest form of the potential, the energy scale is too small for the symmetron to act as dark energy, hence we must add a cosmological constant to drive late-time cosmic acceleration. We briefly discuss a class of generalized, non-renormalizable potentials that can have a greater impact on the late-time cosmology, though cosmic acceleration requires a delicate tuning of parameters in this case.

The XENON100 Dark Matter Experiment

XENON100 Collaboration: E. Aprile, K. Arisaka, F. Arneodo, A. Askin, L. Baudis, A. Behrens, E. Brown, J.M.R. Cardoso, B. Choi, D. Cline, S. Fattori, A.D. Ferella, K.L. Giboni, A. Kish, C.W. Lam, R.F. Lang, K.E. Lim, J.A.M. Lopes, T. Marrodan Undagoitia, Y. Mei, A.J. Melgarejo Fernandez, K. Ni, U. Oberlack,S.E.A. Orrigo, E. Pantic, G. Plante, A.C.C Ribeiro, R. Santorelli, J.M.F. dos Santos, M. Schumann, P. Shagin, A. Teymourian, E. Tziaferi, H. Wang, M. Yamashita

arXiv:1107.2155v1

The XENON100 dark matter experiment uses liquid xenon (LXe) in a time projection chamber (TPC) to measure Xe nuclear recoils resulting from the scattering of dark matter Weakly Interacting Massive Particles (WIMPs). In this paper we present a detailed description of the detector design and present performance results, as established during the commissioning phase and during the first science runs. 
The active target of XENON100 contains 62 kg of LXe, surrounded by an LXe veto of 99 kg, both instrumented with photomultiplier tubes (PMTs) operating inside the liquid or in Xe gas. The LXe target and veto are contained in a low-radioactivity stainless steel vessel, embedded in a passive radiation shield. The experiment is installed underground at the Laboratori Nazionali del Gran Sasso (LNGS), Italy and has recently published results from a 100 live-days dark matter search. The ultimate design goal of XENON100 is to achieve a spin-independent WIMP-nucleon scattering cross section sensitivity of \sigma = 2x10^-45 cm^2 for a 100 GeV/c^2 WIMP.

arXiv: 12 July 2011

Cosmology today-A brief review

Jorge L. Cervantes-Cota, George Smoot

arXiv:1107.1789v1

This is a brief review of the standard model of cosmology. We first introduce the FRW models and their flat solutions for energy fluids playing an important role in the dynamics at different epochs. We then introduce different cosmological lengths and some of their applications. The later part is dedicated to the physical processes and concepts necessary to understand the early and very early Universe and observations of it.

Do baryons trace dark matter in the early universe?

Daniel Grin, Olivier Doré, Marc Kamionkowski

arXiv:1107.1716v1

Baryon-density perturbations of large amplitude may exist if they are compensated by dark-matter perturbations so that the total density remains unchanged. Big-bang nucleosynthesis and galaxy clusters allow the amplitudes of these compensated isocurvature perturbations (CIPs) to be as large as $\sim10%$. CIPs will modulate the power spectrum of cosmic microwave background (CMB) fluctuations---those due to the usual adiabatic perturbations---as a function of position on the sky. This leads to correlations between different spherical-harmonic coefficients of the temperature/polarization map, and it induces B modes in the CMB polarization. Here, the magnitude of these effects is calculated and techniques to measure them are introduced. While a CIP of this amplitude can be probed on the largest scales with WMAP, forthcoming CMB experiments should improve the sensitivity to CIPs by at least an order of magnitude.

arXiv: 11 July 2011

How accurate is our knowledge of galaxy bias?

Surhud More

arXiv:1107.1498v1

Observations of the clustering of galaxies can provide useful information about the distribution of dark matter in the Universe. In order to extract accurate cosmological parameters from galaxy surveys, it is important to understand how the distribution of galaxies is biased with respect to the matter distribution. The large-scale bias of galaxies can be quantified either by directly measuring the large-scale ({\lambda} >~ 60 hinv Mpc) power spectrum of galaxies or by modeling the halo occupation distribution of galaxies using their clustering on small scales ({\lambda} <~ 30 hinv Mpc). We compare the luminosity dependence of the galaxy bias (both the shape and the normalization) obtained by these methods and check for consistency. Our comparison reveals that the bias of galaxies obtained by the small scale clustering measurements is systematically larger than that obtained from the large scale power spectrum methods. We also find systematic discrepancies in the shape of the galaxy bias-luminosity relation. We comment on the origin and possible consequences of these discrepancies which had remained unnoticed thus far.

Time-varying G in shift-symmetric scalar-tensor theories with a Vainshtein mechanism

Eugeny Babichev, Cedric Deffayet, Gilles Esposito-Farese

arXiv:1107.1569v1

We show that the current bounds on the time variation of the Newton constant G can put severe constraints on many interesting scalar-tensor theories which possess a shift symmetry and a nonminimal matter-scalar coupling. This includes in particular Galileon-like models with a Vainshtein screening mechanism. We underline that this mechanism, if efficient to hide the effects of the scalar field at short distance and in the static approximation, can in general not alter the cosmological time evolution of the scalar field. This results in a locally measured time variation of G which is too large when the matter-scalar coupling is of order one.

Dark goo: Bulk viscosity as an alternative to dark energy

Jean-Sebastien Gagnon, Julien Lesgourgues

arXiv:1107.1503v1

We present a simple (microscopic) model in which bulk viscosity plays a role in explaining the present acceleration of the universe. The effect of bulk viscosity on the Friedmann equations is to turn the pressure into an "effective" pressure containing the bulk viscosity. For a sufficiently large bulk viscosity, the effective pressure becomes negative and could mimic a dark energy equation of state. Our microscopic model includes self-interacting spin-zero particles (for which the bulk viscosity is known) that are added to the usual energy content of the universe. We study both background equations and linear perturbations in this model. We show that a dark energy behavior is obtained for reasonable values of the two parameters of the model (i.e. the mass and coupling of the spin-zero particles) and that linear perturbations are well-behaved. There is no apparent fine tuning involved. We also discuss the conditions under which hydrodynamics holds, in particular that the spin-zero particles must be in local equilibrium today for viscous effects to be important

arXiv: 8 July 2011

Manifestly gauge invariant theory of the nonlinear cosmological perturbations in the leading order of the gradient expansion

Takashi Hamazaki

arXiv:1107.1281v1

In the full nonlinear cosmological perturbation theory in the leading order of the gradient expansion, all the types of the gauge invariant perturbation variables are defined. The metric junction conditions across the spacelike transition hypersurface are formulated in a manifestly gauge invariant manner. It is manifestly shown that all the physical laws such as the evolution equations, the constraint equations, and the junction conditions can be written using the gauge invariant variables which we defined only. Based on the existence of the universal adiabatic growing mode in the nonlinear perturbation theory and the $\rho$ philosophy where the physical evolution are described using the energy density $\rho$ as the evolution parameter, we give the definitions of the adiabatic perturbation variable and the entropic perturbation variables in the full nonlinear perturbation theory. In order to give the analytic order estimate of the nonlinear parameter $f_{NL}$, we present the exponent evaluation method. As the models where $f_{NL}$ changes continuously and becomes large, using the $\rho$ philosophy, we investigate the non-Gaussianity induced by the entropic perturbation of the component which does not govern the cosmic energy density, and we show that in order to obtain the significant non-Gaussianity it is necessary that the scalar field which supports the entropic perturbation is extremely small compared with the scalar field which supports the adiabatic perturbation.

General conditions for scale-invariant perturbations in an expanding universe

Ghazal Geshnizjani, William H. Kinney, Azadeh Moradinezhad Dizgah (Univ. at Buffalo, SUNY)

arXiv:1107.1241v1

We investigate the general properties of expanding cosmological models which generate scale-invariant curvature perturbations in the presence of a variable speed of sound. We show that in an expanding universe, generation of a super-Hubble, nearly scale-invariant spectrum of perturbations over a range of wavelengths consistent with observation requires at least one of three conditions: (1) accelerating expansion, (2) a speed of sound faster than the speed of light, or (3) super-Planckian energy density.

arXiv: 7 July 2011

Isocurvature modes and Baryon Acoustic Oscillations II: gains from combining CMB and Large Scale Structure

Carmelita Carbone, Anna Mangilli, Licia Verde

arXiv:1107.1211v1

We consider cosmological parameters estimation in the presence of a non-zero isocurvature contribution in the primordial perturbations. A previous analysis showed that even a tiny amount of isocurvature perturbation, if not accounted for, could affect standard rulers calibration from Cosmic Microwave Background observations such as those provided by the Planck mission, affect Baryon Acoustic Oscillations interpretation and introduce biases in the recovered dark energy properties that are larger than forecasted statistical errors from future surveys. Extending on this work, here we adopt a general fiducial cosmology which includes a varying dark energy equation of state parameter and curvature. Beside Baryon Acoustic Oscillations measurements, we include the information from the shape of the galaxy power spectrum and consider a joint analysis of a Planck-like Cosmic Microwave Background probe and a future, space-based, Large Scale Structure probe not too dissimilar from recently proposed surveys. We find that this allows one to break the degeneracies that affect the Cosmic Microwave Background and Baryon Acoustic Oscillations combination. As a result, most of the cosmological parameter systematic biases arising from an incorrect assumption on the isocurvature fraction parameter $f_{iso}$, become negligible with respect to the statistical errors. We find that the Cosmic Microwave Background and Large Scale Structure combination gives a statistical error $\sigma(f_{iso}) \sim 0.008$, even when curvature and a varying dark energy equation of state are included, which is smaller that the error obtained from Cosmic Microwave Background alone when flatness and cosmological constant are assumed. These results confirm the synergy and complementarity between Cosmic Microwave Background and Large Scale Structure, and the great potential of future and planned galaxy surveys.

Light propagation and the average expansion rate in near-FRW universes

Syksy Rasanen

arXiv:1107.1176v1

We consider universes which are close to Friedmann-Robertson-Walker in the sense that metric perturbations, their time derivatives and first spatial derivatives are small, but second spatial derivatives are not constrained. We show that if we in addition assume that the observer four-velocity is close to its background value and close to the four-velocity which defines the hypersurface of averaging, the redshift and the average expansion rate remain close to the FRW case, but this is not true for the angular diameter distance. The four-velocity assumption implies certain conditions on second derivatives of the metric and/or the matter content.

Cosmographic Degeneracy

Arman Shafieloo, Eric V. Linder

arXiv:1107.1033v1

We examine the dark energy and matter densities allowed by precision measurements of distances out to various redshifts, in the presence of spatial curvature and (near) arbitrary behavior of the dark energy equation of state. Degeneracies among the parameters permit a remarkably large variation in their values when using only distance measurements of the late time universe and making no assumptions about the dark energy or curvature. Going beyond distance measurements to a lower limit on the growth of structure bounds the allowed region significantly but still leaves considerable freedom to match a flat LCDM model with dark energy very different from a cosmological constant. The combination of distances with Hubble parameter, gravitational lensing or other large scale structure data is essential to determining robustly the cosmological model.

arXiv: 6 July 2011

Testing model independent modified gravity with future large scale surveys

Daniel B. Thomas, Carlo R. Contaldi

arXiv:1107.0727v1

Model-independent parametrisations of modified gravity have attracted a lot of attention over the past few years and numerous combinations of experiments and observables have been suggested to constrain the parameters used in these models. Galaxy clusters have been mentioned, but not looked at as extensively in the literature as some other probes. Here we look at adding galaxy clusters into the mix of observables and examine how they could improve the constraints on the modified gravity parameters. In particular, we forecast the constraints from combining Planck satellite Cosmic Microwave Background (CMB) measurements and Sunyaev-Zeldovich (SZ) cluster catalogue with a DES-like weak lensing survey. We find that cluster counts significantly improve the constraints over those derived using CMB and WL. We then look at surveys further into the future, to see how much better it may be feasible to make the constraints.

arXiv: 5 July 2011

Towards a fully consistent parameterization of modified gravity

Tessa Baker, Pedro G. Ferreira, Constantinos Skordis, Joe Zuntz

arXiv:1107.0491v1

There is a distinct possibility that current and future cosmological data can be used to constrain Einstein's theory of gravity on the very largest scales. To be able to do this in a model-independent way, it makes sense to work with a general parameterization of modified gravity. Such an approach would be analogous to the Parameterized Post-Newtonian (PPN) approach which is used on the scale of the Solar System. A few such parameterizations have been proposed and preliminary constraints have been obtained. We show that the majority of such parameterizations are only exactly applicable in the quasistatic regime. On larger scales they fail to encapsulate the full behaviour of typical models currently under consideration. We suggest that it may be possible to capture the additions to the `Parameterized Post-Friedmann' (PPF) formalism by treating them akin to fluid perturbations.

Chern-Simons Inflation and Baryogenesis

Stephon Alexander, Antonino Marciano, David Spergel

arXiv:1107.0318v1

We propose a model of inflation where the Chern-Simons interaction and vector fields play a central role in generating an inflationary epoch. As a result, in accord with the APS mechanism, the Sakharov conditions for baryogenesis are self-consistently satisfied, and we calculate the net baryon asymmetry index in terms of the gauge configuration necessary for inflation, based on the chiral anomaly. Inflation begins with a large plasma density of interacting gauge fields and fermions, which interact through gravity and the Chern Simons term. The Chern-Simons term drives power from an initial white-noise spectrum of gauge fields into a narrow-band of superhorizon wave vectors. At the same time, the fermionic current and metric coupling amplifies the gauge field on superhorizon scales. This phase-correlation and amplification of the gauge field produces the correct conditions to maintain more than 60 e-folds of inflation. Eventually the gauge field dissipates by producing the observed baryon asymmetry $\frac{n_{b}}{s} \sim 10^{-10}$, through the chiral anomaly and inflation ends.

arXiv: 4 July 2011

The dark matter assembly of the Local Group in constrained cosmological simulations of a LambdaCDM universe

J. E. Forero-Romero, Y. Hoffman, G. Yepes, S. Gottoeber, R. Piontek, A. Klypin, M. Steinmetz

arXiv:1107.0017v1

We make detailed theoretical predictions for the assembly properties of the Local Group (LG) in the standard LambdaCDM cosmological model. We use three cosmological N-body dark matter simulations from the CLUES project, which are designed to reproduce the main dynamical features of the matter distribution down to the scale of a few Mpc around the LG. Additionally, we use the results of an unconstrained simulation with a sixty times larger volume to calibrate the influence of cosmic variance. We characterize the Mass Aggregation History (MAH) for each halo by three characteristic times, the formation, assembly and last major merger times. A major merger is defined by a minimal mass ratio of 10:1. We find that the three LGs share a similar MAH with formation and last major merger epochs placed on average \approx 10 - 12 Gyr ago. Between 12% and 17% of the halos in the mass range 5 x 10^11 Msol/h < M_h < 5 x 10^12 Msol/h have a similar MAH. In a set of pairs of halos within the same mass range, a fraction of 1% to 3% share similar formation properties as both halos in the simulated LG. An unsolved question posed by our results is the dynamical origin of the MAH of the LGs. The isolation criteria commonly used to define LG-like halos in unconstrained simulations do not narrow down the halo population into a set with quiet MAHs, nor does a further constraint to reside in a low density environment. The quiet MAH of the LGs provides a favorable environment for the formation of disk galaxies like the Milky Way and M31. The timing for the beginning of the last major merger in the Milky Way dark matter halo matches with the gas rich merger origin for the thick component in the galactic disk. Our results support the view that the specific large and mid scale environment around the Local Group play a critical role in shaping its MAH and hence its baryonic structure at present.