Cosmology

About: Cosmology is a research topic. Over the lifetime, 18004 publications have been published within this topic receiving 631028 citations. The topic is also known as: physical cosmology & cosmologies.

Direct determination of the kinematics of the universe and properties of the dark energy as functions of redshift

Abstract: Understanding the nature of dark energy, which appears to drive the expansion of the universe, is one of the central problems of physical cosmology today. In an earlier paper we proposed a novel method to determine the expansion rate E(z) and the deceleration parameter q(z) in a largely model-independent way, directly from the data on coordinate distances y(z). Here we expand this methodology to include measurements of the pressure of dark energy p(z), its normalized energy density fraction f(z), and the equation-of-state parameter w(z). We then apply this methodology to a new, combined data set of distances to supernovae and radio galaxies. In evaluating E(z) and q(z), we make only the assumptions that the FRW metric applies and that the universe is spatially flat (an assumption strongly supported by modern cosmic microwave background radiation measurements). The determinations of E(z) and q(z) are independent of any theory of gravity. For evaluations of p(z), f(z), and w(z), a theory of gravity must be adopted, and general relativity is assumed here. No a priori assumptions regarding the properties or redshift evolution of the dark energy are needed. We obtain trends for y(z) and E(z) that are fully consistent with the standard Friedmann-Lemaitre concordance cosmology with Ω_0 = 0.3 and Λ_0 = 0.7. The measured trend for q(z) deviates systematically from the predictions of this model on a ~1-2 σ level but may be consistent for smaller values of Λ_0. We confirm our previous result that the universe transitions from acceleration to deceleration at a redshift zT ≈ 0.4. The trends for p(z), f(z), and w(z) are consistent with being constant at least out to z ~ 0.3-0.5 and broadly consistent with being constant out to higher redshifts, but with large uncertainties. For the present values of these parameters we obtain E_0 = 0.97 ± 0.03, q_0 = -0.35 ± 0.15, p_0 = -0.6 ± 0.15, f_0 = -0.62 - (Ω_0 - 0.3) ± 0.05, and w_0 = -0.9 - e(Ω_0 - 0.3) ± 0.1, where Ω_0 is the density parameter for nonrelativistic matter and e ≈ 1.5 ± 0.1. We note that in the standard Friedmann-Lemaitre models p_0 = -Λ_0, and thus we can measure the value of the cosmological constant directly and obtain results in agreement with other contemporary results.

The Damping Tail of Cosmic Microwave Background Anisotropies

Abstract: By decomposing the damping tail of cosmic microwave background (CMB) anisotropies into a series of transfer functions representing individual physical effects, we provide ingredients that will aid in the reconstruction of the cosmological model from small-scale CMB anisotropy data. We accurately calibrate the model-independent effects of diffusion and reionization damping, which provide potentially the most robust information on the background cosmology. Removing these effects, we uncover model-dependent processes, such as the acoustic peak modulation and gravitational enhancement, that can help distinguish between alternate models of structure formation and provide windows into the evolution of fluctuations at various stages in their growth.

Cosmological parameters from large scale structure - geometric versus shape information

Abstract: The matter power spectrum as derived from large scale structure (LSS) surveys contains two important and distinct pieces of information: an overall smooth shape and the imprint of baryon acoustic oscillations (BAO). We investigate the separate impact of these two types of information on cosmological parameter estimation for current data, and show that for the simplest cosmological models, the broad-band shape information currently contained in the SDSS DR7 halo power spectrum (HPS) is by far superseded by geometric information derived from the baryonic features. An immediate corollary is that contrary to popular beliefs, the upper limit on the neutrino mass m(nu) presently derived from LSS combined with cosmic microwave background (CMB) data does not in fact arise from the possible small-scale power suppression due to neutrino free-streaming, if we limit the model framework to minimal Lambda CDM+m(nu). However, in more complicated models, such as those extended with extra light degrees of freedom and a dark energy equation of state parameter w differing from -1, shape information becomes crucial for the resolution of parameter degeneracies. This conclusion will remain true even when data from the Planck spacecraft are combined with SDSS DR7 data. In the course of our analysis, we update both the BAO likelihood function by including an exact numerical calculation of the time of decoupling, as well as the HPS likelihood, by introducing a new dewiggling procedure that generalises the previous approach to models with an arbitrary sound horizon at decoupling. These changes allow a consistent application of the BAO and HPS data sets to a much wider class of models, including the ones considered in this work. All the cases considered here are compatible with the conservative 95%-bounds Sigma m(nu) < 1.16 eV, N-eff = 4.8 +/- 2.0.

Average observational quantities in the timescape cosmology

Abstract: We examine the properties of a recently proposed observationally viable alternative to homogeneous cosmology with smooth dark energy, the timescape cosmology. In the timescape model cosmic acceleration is realized as an apparent effect related to the calibration of clocks and rods of observers in bound systems relative to volume-average observers in an inhomogeneous geometry in ordinary general relativity. The model is based on an exact solution to a Buchert average of the Einstein equations with backreaction. The present paper examines a number of observational tests which will enable the timescape model to be distinguished from homogeneous cosmologies with a cosmological constant or other smooth dark energy, in current and future generations of dark energy experiments. Predictions are presented for comoving distance measures; $H(z)$; the equivalent of the dark energy equation of state, $w(z)$; the $Om(z)$ measure of Sahni, Shafieloo, and Starobinsky; the Alcock-Paczy\ifmmode \acute{n}\else \'{n}\fi{}ski test; the baryon acoustic oscillation measure, ${D}_{V}$; the inhomogeneity test of Clarkson, Bassett, and Lu; and the time drift of cosmological redshifts. Where possible, the predictions are compared to recent independent studies of similar measures in homogeneous cosmologies with dark energy. Three separate tests with indications of results in possible tension with the $\ensuremath{\Lambda}\mathrm{CDM}$ model are found to be consistent with the expectations of the timescape cosmology.

Imprint of gravitational waves on the cosmic microwave background

Abstract: Long-wavelength gravitational waves can induce significant temperature anisotropy in the cosmic microwave background. Distinguishing this from anisotropy induced by energy density fluctuations is critical for testing inflationary cosmology and theories of large-scale structure formation. We describe full radiative transport calculations of the two contributions and show that they differ dramatically at angular scales below a few degrees. We show how anisotropy experiments probing large- and small-angular scales can combine to distinguish the imprint due to gravitational waves.