Showing papers on "Cosmology published in 2010"

Beyond Einstein Gravity: A Survey of Gravitational Theories for Cosmology and Astrophysics

Abstract: 1. Extended gravity: a primer.- 2. Mathematical Tools.- 3. The landscape beyond Einstein gravity.- 4. Spherical symmetry.- 5. Weak-field limit.- 6. Qualitative analysis and exact solutions in cosmology.- 7. Cosmology.- 8. From the early to the present universe.- A. Physical constants and astrophysical and cosmological parameters.- B. The Noether symmetry approach to f(R) gravity.- C. The weak-field limit of metric f(R) gravity.- References.- Index.

The large-scale bias of dark matter halos: numerical calibration and model tests

Abstract: We measure the clustering of dark matter halos in a large set of collisionless cosmological simulations of the flatCDM cosmology. Halos are identified using the spherical over density algorithm, which finds the mass around isolated peaks in the density field such that the m ean density istimes the background. We calibrate fitting functions for the large scale bias that are adaptable to any value ofwe examine. We find a � 6% scatter about our best fit bias relation. Our fitting functi ons couple to the halo mass functions of Tinker et. al. (2008) such that bias of all dark matter is normalized to unity. We demonstrate that the bias of massive, rare halos is higher than that predicted in the modified ellip soidal collapse model of Sheth, Mo, & Tormen (2001), and approaches the predictions of the spherical collapse model for the rarest halos. Halo bias results based on friends-of-friends halos identified with linking l ength 0.2 are systematically lower than for halos with the canonical � = 200 overdensity by � 10%. In contrast to our previous results on the mass function, we find that the universal bias function evolves very weakly with redshift, if at all. We use our numerical results, both for the mass function and the bias relation, to test the peak- background split model for halo bias. We find that the peak-background split achieves a reasonable agreement with the numerical results, but � 20% residuals remain, both at high and low masses. Subject headings:cosmology:theory — methods:numerical — large scale structure of the universe

f(R) theories

Abstract: Over the past decade, f(R) theories have been extensively studied as one of the simplest modifications to General Relativity. In this article we review various applications of f(R) theories to cosmology and gravity - such as inflation, dark energy, local gravity constraints, cosmological perturbations, and spherically symmetric solutions in weak and strong gravitational backgrounds. We present a number of ways to distinguish those theories from General Relativity observationally and experimentally. We also discuss the extension to other modified gravity theories such as Brans-Dicke theory and Gauss-Bonnet gravity, and address models that can satisfy both cosmological and local gravity constraints.

Reionization and Cosmology with 21-cm Fluctuations

Abstract: Measurement of the spatial distribution of neutral hydrogen via the redshifted 21-cm line promises to revolutionize our knowledge of the epoch of reionization and the first galaxies, and may provide a powerful new tool for observational cosmology from redshifts 1

The Observed Growth of Massive Galaxy Clusters I: Statistical Methods and Cosmological Constraints

Abstract: This is the first of a series of papers in which we derive simultaneous constraints on cosmological parameters and X-ray scaling relations using observations of the growth of massive, X-ray flux-selected galaxy clusters. Our data set consists of 238 cluster detections from the ROSAT All-Sky Survey, and incorporates follow-up observations of 94 of those clusters using the Chandra X-ray Observatory or ROSAT. Here we describe and implement a new statistical framework required to self-consistently produce simultaneous constraints on cosmology and scaling relations from such data, and present results on models of dark energy. In spatially flat models with a constant dark energy equation of state, w, the cluster data yield Ω m = 0.23 ± 0.04, σ 8 = 0.82 ± 0.05 and w = - 1.01 ± 0.20, incorporating standard priors on the Hubble parameter and mean baryon density of the Universe, and marginalizing over conservative allowances for systematic uncertainties. These constraints agree well and are competitive with independent data in the form of cosmic microwave background anisotropies, type Ia supernovae, cluster gas mass fractions, baryon acoustic oscillations, galaxy redshift surveys and cosmic shear. The combination of our data with current microwave background, supernova, gas mass fraction and baryon acoustic oscillation data yields Ω m , = 0.27 ± 0.02, σ 8 = 0.79 ± 0.03 and w = -0.96 ± 0.06 for flat, constant w models. The combined data also allow us to investigate evolving w models. Marginalizing over transition redshifts in the range 0.05-1, we constrain the equation of state at late and early times to be respectively w 0 = -0.88 ± 0.21 and w et = -1.05 +0.20 -0.36 , again including conservative systematic allowances. The combined data provide constraints equivalent to a Dark Energy Task Force figure of merit of 15.5. Our results highlight the power of X-ray studies, which enable the straightforward production of large, complete and pure cluster samples and admit tight scaling relations, to constrain cosmology. However, the new statistical framework we apply to this task is equally applicable to cluster studies at other wavelengths.

Dissecting the Gravitational lens B1608+656. II. Precision Measurements of the Hubble Constant, Spatial Curvature, and the Dark Energy Equation of State

Abstract: Strong gravitational lens systems with measured time delays between the multiple images provide a method for measuring the "time-delay distance" to the lens, and thus the Hubble constant. We present a Bayesian analysis of the strong gravitational lens system B1608+656, incorporating (1) new, deep Hubble Space Telescope (HST) observations, (2) a new velocity-dispersion measurement of 260 ± 15 km s-1 for the primary lens galaxy, and (3) an updated study of the lens' environment. Our analysis of the HST images takes into account the extended source surface brightness, and the dust extinction and optical emission by the interacting lens galaxies. When modeling the stellar dynamics of the primary lens galaxy, the lensing effect, and the environment of the lens, we explicitly include the total mass distribution profile logarithmic slope γ' and the external convergence κext we marginalize over these parameters, assigning well-motivated priors for them, and so turn the major systematic errors into statistical ones. The HST images provide one such prior, constraining the lens mass density profile logarithmic slope to be γ' = 2.08 ± 0.03; a combination of numerical simulations and photometric observations of the B1608+656 field provides an estimate of the prior for κext: 0.10+0.08 -0.05. This latter distribution dominates the final uncertainty on H 0. Fixing the cosmological parameters at Ωm = 0.3, ΩΛ = 0.7, and w = -1 in order to compare with previous work on this system, we find H 0 = 70.6+3.1 -3.1 km s-1 Mpc-1. The new data provide an increase in precision of more than a factor of 2, even including the marginalization over κext. Relaxing the prior probability density function for the cosmological parameters to that derived from the Wilkinson Microwave Anisotropy Probe (WMAP) five-year data set, we find that the B1608+656 data set breaks the degeneracy between Ωm and ΩΛ at w = -1 and constrains the curvature parameter to be -0.031 <Ωk <0.009 (95% CL), a level of precision comparable to that afforded by the current Type Ia SNe sample. Asserting a flat spatial geometry, we find that, in combination with WMAP, H 0 = 69.7+4.9 -5.0 km s-1 Mpc-1 and w = -0.94+0.17 -0.19 (68% CL), suggesting that the observations of B1608+656 constrain w as tightly as the current Baryon Acoustic Oscillation data do. Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program No. GO-10158.

The Hubble Constant

Abstract: Considerable progress has been made in determining the Hubble constant over the past two decades. We discuss the cosmological context and importance of an accurate measurement of the Hubble constant, focusing on six high-precision distance-determination methods: Cepheids, tip of the red giant branch, maser galaxies, surface brightness fluctuations, the Tully-Fisher relation, and Type Ia supernovae. We discuss in detail known systematic errors in the measurement of galaxy distances and how to minimize them. Our best current estimate of the Hubble constant is 73±2 (random) ± 4 (systematic) km s−1 Mpc−1. The importance of improved accuracy in the Hubble constant will increase over the next decade with new missions and experiments designed to increase the precision in other cosmological parameters. We outline the steps that will be required to deliver a value of the Hubble constant to 2% systematic uncertainty and discuss the constraints on other cosmological parameters that will then be possible with such ac...

Galaxy Formation and Evolution

Abstract: The rapidly expanding field of galaxy formation lies at the interface between astronomy, particle physics, and cosmology. Covering diverse topics from these disciplines, all of which are needed to understand how galaxies form and evolve, this book is ideal for researchers entering the field. Individual chapters explore the evolution of the Universe as a whole and its particle and radiation content; linear and nonlinear growth of cosmic structure; processes affecting the gaseous and dark matter components of galaxies and their stellar populations; the formation of spiral and elliptical galaxies; central supermassive black holes and the activity associated with them; galaxy interactions; and the intergalactic medium. Emphasizing both observational and theoretical aspects, this book provides a coherent introduction for astronomers, cosmologists, and astroparticle physicists to the broad range of science underlying the formation and evolution of galaxies.

Confirmation of general relativity on large scales from weak lensing and galaxy velocities

Abstract: Although general relativity underlies modern cosmology, its applicability on cosmological length scales has yet to be stringently tested. Such a test has recently been proposed, using a quantity, EG, that combines measures of large-scale gravitational lensing, galaxy clustering and structure growth rate. The combination is insensitive to `galaxy bias' (the difference between the clustering of visible galaxies and invisible dark matter) and is thus robust to the uncertainty in this parameter. Modified theories of gravity generally predict values of EG different from the general relativistic prediction because, in these theories, the `gravitational slip' (the difference between the two potentials that describe perturbations in the gravitational metric) is non-zero, which leads to changes in the growth of structure and the strength of the gravitational lensing effect. Here we report that EG = 0.39+/-0.06 on length scales of tens of megaparsecs, in agreement with the general relativistic prediction of EG~0.4. The measured value excludes a model within the tensor-vector-scalar gravity theory, which modifies both Newtonian and Einstein gravity. However, the relatively large uncertainty still permits models within f(R) theory, which is an extension of general relativity. A fivefold decrease in uncertainty is needed to rule out these models.

Eddington's theory of gravity and its progeny.

Abstract: We resurrect Eddington's proposal for the gravitational action in the presence of a cosmological constant and extend it to include matter fields. We show that the Newton-Poisson equation is modified in the presence of sources and that charged black holes show great similarities with those arising in Born-Infeld electrodynamics coupled to gravity. When we consider homogeneous and isotropic space-times, we find that there is a minimum length (and maximum density) at early times, clearly pointing to an alternative theory of the big bang. We thus argue that the modern formulation of Eddington's theory, Born-Infeld gravity, presents us with a novel, nonsingular description of the Universe.

Cosmology of a Covariant Galileon Field

Abstract: We study the cosmology of a covariant scalar field respecting a Galilean symmetry in flat space-time. We show the existence of a tracker solution that finally approaches a de Sitter fixed point responsible for cosmic acceleration today. The viable region of model parameters is clarified by deriving conditions under which ghosts and Laplacian instabilities of scalar and tensor perturbations are absent. The field equation of state exhibits a peculiar phantomlike behavior along the tracker, which allows a possibility to observationally distinguish the Galileon gravity from the cold dark matter model with a cosmological constant.

Non-relativistic effective theory of dark matter direct detection

Abstract: Dark matter direct detection searches for signals coming from dark matter scattering against nuclei at a very low recoil energy scale ~ 10 keV In this paper, a simple non-relativistic effective theory is constructed to describe interactions between dark matter and nuclei without referring to any underlying high energy models It contains the minimal set of operators that will be tested by direct detection The effective theory approach highlights the set of distinguishable recoil spectra that could arise from different theoretical models If dark matter is discovered in the near future in direct detection experiments, a measurement of the shape of the recoil spectrum will provide valuable information on the underlying dynamics We bound the coefficients of the operators in our non-relativistic effective theory by the null results of current dark matter direct detection experiments We also discuss the mapping between the non-relativistic effective theory and field theory models or operators, including aspects of the matching of quark and gluon operators to nuclear form factors

Hořava–Lifshitz cosmology: a review

Abstract: Here we review the basic construction and cosmological implications of a power-counting renormalizable theory of gravitation, recently proposed by Hořava. We explain that (i) at low energy this theory does not exactly recover general relativity but instead mimics general relativity plus dark matter; (ii) higher spatial curvature terms allow bouncing and cyclic universes as regular solutions; (iii) the anisotropic scaling with the dynamical critical exponent z = 3 solves the horizon problem and leads to scale-invariant cosmological perturbations even without inflation. We also comment on issues related to an extra scalar degree of freedom called scalar graviton. In particular, for spherically-symmetric, static, vacuum configurations we prove non-perturbative continuity of the λ → 1 + 0 limit, where λ is a parameter in the kinetic action and general relativity has the value λ = 1. We also derive the condition under which linear instability of the scalar graviton does not show up.

General Relativistic Description of the Observed Galaxy Power Spectrum: Do We Understand What We Measure?

Abstract: We extend the general relativistic description of galaxy clustering developed in Yoo, Fitzpatrick, and Zaldarriaga (2009). For the first time we provide a fully general relativistic description of the observed matter power spectrum and the observed galaxy power spectrum with the linear bias ansatz. It is significantly different from the standard Newtonian description on large scales and especially its measurements on large scales can be misinterpreted as the detection of the primordial non-Gaussianity even in the absence thereof. The key difference in the observed galaxy power spectrum arises from the real-space matter fluctuation defined as the matter fluctuation at the hypersurface of the observed redshift. As opposed to the standard description, the shape of the observed galaxy power spectrum evolves in redshift, providing additional cosmological information. While the systematic errors in the standard Newtonian description are negligible in the current galaxy surveys at low redshift, correct general relativistic description is essential for understanding the galaxy power spectrum measurements on large scales in future surveys with redshift depth z ≥ 3. We discuss ways to improve the detection significance in the current galaxy surveys and comment on applications of our general relativistic formalism in future surveys.

Towards a resolution of the cosmological singularity in non-local higher derivative theories of gravity

Abstract: One of the greatest problems of standard cosmology is the Big Bang singularity. Previously it has been shown that non-local ghostfree higher-derivative modifications of Einstein gravity in the ultra-violet regime can admit non-singular bouncing solutions. In this paper we study in more details the dynamical properties of the equations of motion for these theories of gravity in presence of positive and negative cosmological constants and radiation. We find stable inflationary attractor solutions in the presence of a positive cosmological constant which renders inflation geodesically complete, while in the presence of a negative cosmological constant a cyclic universe emerges. We also provide an algorithm for tracking the super-Hubble perturbations during the bounce and show that the bouncing solutions are free from any perturbative instability.

Horava-Lifshitz Cosmology: A Review

Abstract: This article reviews basic construction and cosmological implications of a power-counting renormalizable theory of gravitation recently proposed by Horava. We explain that (i) at low energy this theory does not exactly recover general relativity but instead mimic general relativity plus dark matter; that (ii) higher spatial curvature terms allow bouncing and cyclic universes as regular solutions; and that (iii) the anisotropic scaling with the dynamical critical exponent z=3 solves the horizon problem and leads to scale-invariant cosmological perturbations even without inflation. We also comment on issues related to an extra scalar degree of freedom called scalar graviton. In particular, for spherically-symmetric, static, vacuum configurations we prove non-perturbative continuity of the lambda->1+0 limit, where lambda is a parameter in the kinetic action and general relativity has the value lambda=1. We also derive the condition under which linear instability of the scalar graviton does not show up.

ΛCDM epoch reconstruction from F(R, G) and modified Gauss–Bonnet gravities

Abstract: Dark energy cosmology is considered in a modified Gauss?Bonnet model of gravity with and without a scalar field. It is shown that these generalizations of general relativity endow it with a very rich cosmological structure: it may naturally lead to an effective cosmological constant, quintessence or phantom cosmic acceleration, with the possibility of describing the transition from a decelerating to an accelerating phase explicitly. It is demonstrated here that these modified GB and scalar-GB theories are perfectly viable as cosmological models. They can describe the ?CDM cosmological era without any need for a cosmological constant. The specific properties of these theories of gravity in different particular cases, such as the de Sitter one, are studied.

The Coyote Universe. III. Simulation Suite and Precision Emulator for the Nonlinear Matter Power Spectrum

Abstract: Many of the most exciting questions in astrophysics and cosmology, including the majority of observational probes of dark energy, rely on an understanding of the nonlinear regime of structure formation. In order to fully exploit the information available from this regime and to extract cosmological constraints, accurate theoretical predictions are needed. Currently, such predictions can only be obtained from costly, precision numerical simulations. This paper is the third in a series aimed at constructing an accurate calibration of the nonlinear mass power spectrum on Mpc scales for a wide range of currently viable cosmological models, including dark energy models with w ≠ –1. The first two papers addressed the numerical challenges and the scheme by which an interpolator was built from a carefully chosen set of cosmological models. In this paper, we introduce the "Coyote Universe" simulation suite which comprises nearly 1000 N-body simulations at different force and mass resolutions, spanning 38 w CDM cosmologies. This large simulation suite enables us to construct a prediction scheme, or emulator, for the nonlinear matter power spectrum accurate at the percent level out to k 1 h Mpc–1. We describe the construction of the emulator, explain the tests performed to ensure its accuracy, and discuss how the central ideas may be extended to a wider range of cosmological models and applications. A power spectrum emulator code is released publicly as part of this paper.

LambdaCDM epoch reconstruction from F(R,G) and modified Gauss-Bonnet gravities

Abstract: Dark energy cosmology is considered in a modified Gauss-Bonnet model of gravity with and without a scalar field. It is shown that these generalizations of General Relativity endow it with a very rich cosmological structure: it may naturally lead to an effective cosmological constant, quintessence or phantom cosmic acceleration, with the possibility to describe the transition from a decelerating to an accelerating phase explicitly. It is demonstrated here that these modified GB and scalar-GB theories are perfectly viable as cosmological models. They can describe the LambdaCDM cosmological era without any need for a cosmological constant. Specific properties of these theories of gravity in different particular cases, such as the de Sitter one, are studied.

The Redshift Evolution of LCDM Halo Parameters: Concentration, Spin, and Shape

Abstract: We present a detailed study of the redshift evolution of dark matter halo structural parameters in a LambdaCDM cosmology. We study the mass and redshift dependence of the concentration, shape and spin parameter in Nbody simulations spanning masses from 10^{10} Msun/h to 10^{15} Msun/h and redshifts from 0 to 2. We present a series of fitting formulas that accurately describe the time evolution of the concentration-mass relation since z=2. Using arguments based on the spherical collapse model we study the behaviour of the scale length of the density profile during the assembly history of haloes, obtaining physical insights on the origin of the observed time evolution of the concentration mass relation. We also investigate the evolution with redshift of dark matter halo shape and its dependence on mass. Within the studied redshift range the relation between halo shape and mass can be well fitted by a power law. Finally we show that although for z=0 the spin parameter is practically mass independent, at increasing redshift it shows a increasing correlation with mass.

Evidence for Gamma-ray Halos Around Active Galactic Nuclei and the First Measurement of Intergalactic Magnetic Fields

Abstract: Intergalactic magnetic fields (IGMFs) can cause the appearance of halos around the gamma-ray images of distant objects because an electromagnetic cascade initiated by a high-energy gamma-ray interaction with the photon background is broadened by magnetic deflections. We report evidence of such gamma-ray halos in the stacked images of the 170 brightest active galactic nuclei (AGNs) in the 11 month source catalog of the Fermi Gamma-Ray Space Telescope. Excess over the point-spread function in the surface brightness profile is statistically significant at 3.5σ (99.95% confidence level), for the nearby, hard population of AGNs. The halo size and brightness are consistent with IGMF, B_(IGMF) ≈10^(–15) G. The knowledge of IGMF will facilitate the future gamma-ray and charged-particle astronomy. Furthermore, since IGMFs are likely to originate from the primordial seed fields created shortly after the big bang, this potentially opens a new window on the origin of cosmological magnetic fields, inflation, and the phase transitions in the early universe.

Hořava–Lifshitz dark energy

Abstract: We formulate Hořava–Lifshitz cosmology with an additional scalar field that leads to an effective dark energy sector. We find that, due to the inherited features from the gravitational background, Hořava–Lifshitz dark energy naturally presents very interesting behaviors, possessing a varying equation-of-state parameter, exhibiting phantom behavior and allowing for a realization of the phantom divide crossing. In addition, Hořava–Lifshitz dark energy guarantees for a bounce at small scale factors and it may trigger the turnaround at large scale factors, leading naturally to cyclic cosmology.

Cosmography with the Einstein Telescope

Abstract: The Einstein Telescope, a third-generation gravitational-wave detector under a design study, could detect millions of binary neutron star inspirals each year. A small fraction of these events might be observed as gamma-ray bursts, helping to measure both the luminosity distance DL to and redshift of the source. By fitting these measured values of DL and to a cosmological model, it would be possible to infer the dark energy equation of state to within 1.5% without the need to correct for errors in DL caused by weak lensing. This compares favourably with 0.3–10% accuracy that can be achieved with the Laser Interferometer Space Antenna (where weak lensing will need to be dealt with) as well as with dedicated dark energy missions that have been proposed, where 3.5–11% uncertainty is expected.

ACTPol: a polarization-sensitive receiver for the Atacama Cosmology Telescope.

Abstract: The six-meter Atacama Cosmology Telescope (ACT) in Chile was built to measure the cosmic microwave background (CMB) at arcminute angular scales. We are building a new polarization sensitive receiver for ACT (ACTPol). ACTPol will characterize the gravitational lensing of the CMB and aims to constrain the sum of the neutrino masses with ~ 0.05 eV precision, the running of the spectral index of inflation-induced fluctuations, and the primordial helium abundance to better than 1 %. Our observing fields will overlap with the SDSS BOSS survey at optical wavelengths, enabling a variety of cross-correlation science, including studies of the growth of cosmic structure from Sunyaev-Zel'dovich observations of clusters of galaxies as well as independent constraints on the sum of the neutrino masses. We describe the science objectives and the initial receiver design.

A larger estimate of the entropy of the universe

Abstract: Using recent measurements of the supermassive black hole (SMBH) mass function, we find that SMBHs are the largest contributor to the entropy of the observable universe, contributing at least an order of magnitude more entropy than previously estimated. The total entropy of the observable universe is correspondingly higher, and is S obs = 3.1+3.0 –1.7 × 10104 k. We calculate the entropy of the current cosmic event horizon to be S CEH = 2.6 ± 0.3 × 10122 k, dwarfing the entropy of its interior, S CEH int = 1.2+1.1 –0.7 × 10103 k. We make the first tentative estimate of the entropy of weakly interacting massive particle dark matter within the observable universe, S dm = 1088 ± 1 k. We highlight several caveats pertaining to these estimates and make recommendations for future work.

Simultaneous measurement of cosmology and intrinsic alignments using joint cosmic shear and galaxy number density correlations

Abstract: Aims. Cosmic shear is a powerful method to constrain cosmology, provided that any systematic effects are under control. The intrinsic alignment of galaxies is expected to severely bias parameter estimates if not taken into account. We explore the potential of a joint analysis of tomographic galaxy ellipticity, galaxy number density, and ellipticity-number density cross-correlations to simultaneously constrain cosmology and self-calibrate unknown intrinsic alignment and galaxy bias contributions. Methods. We treat intrinsic alignments and galaxy biasing as free functions of scale and redshift and marginalise over the resulting parameter sets. Constraints on cosmology are calculated by combining the likelihoods from all two-point correlations between galaxy ellipticity and galaxy number density. The information required for these calculations is already available in a standard cosmic shear data set. We include contributions to these functions from cosmic shear, intrinsic alignments, galaxy clustering and magnification effects. Results. In a Fisher matrix analysis we compare our constraints with those from cosmic shear alone in the absence of intrinsic alignments. For a potential future large area survey, such as Euclid, the extra information from the additional correlation functions can make up for the additional free parameters in the intrinsic alignment and galaxy bias terms, depending on the flexibility in the models. For example, the dark energy task force figure of merit is recovered even when more than 100 free parameters are marginalised over. We find that the redshift quality requirements are similar to those calculated in the absence of intrinsic alignments.