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.

Cosmological expansion in the Randall-Sundrum brane world scenario

Abstract: The cosmology of the Randall-Sundrum scenario for a positive tension brane in a 5D universe with localized gravity has been studied previously In the radiation-dominated universe, it was suggested that there are two solutions for the cosmic scale factor $a(t):$ the standard solution $a\ensuremath{\sim}{t}^{1/2},$ and a solution $a\ensuremath{\sim}{t}^{1/4},$ which is incompatible with standard big bang nucleosynthesis In this paper, we reconsider expansion of the Universe in this scenario We derive and solve a first order, linear differential equation for ${H}^{2},$ the square of the expansion rate of the Universe, as a function of a The differences between our equation for ${H}^{2}$ and the relationship found in standard cosmology are (i) there is a term proportional to density squared (a fact already known), which is small when the density is small compared to the brane tension, and (ii) there is a contribution which acts like a relativistic fluid We show that this second contribution is due to gravitational degrees of freedom in the bulk Thus, we find that there need not be any conflict between cosmology of the Randall-Sundrum scenario and the standard model of cosmology We discuss how reheating at the end of inflation leads to the correct relationship between matter density and expansion rate, ${H}^{2}\ensuremath{\rightarrow}8\ensuremath{\pi}G{\ensuremath{\rho}}_{m}/3,$ and the conditions that must be met for the expansion rate of the universe to be close to its standard model value around the epoch of cosmological nucleosynthesis

Baryon asymmetry of the universe in the minimal Standard Model

Abstract: We calculate the baryon asymmetry of the Universe which would arise during a first-order electroweak phase transition due to minimal-standard-model processes. It agrees in sign and magnitude with the observed baryonic excess, for reasonable Kobayashi-Maskawa parameters and mt in the expected range, and plausible values of bubble velocity and other high temperature effects. © 1993 The American Physical Society.

The Rh=ct universe

Abstract: The backbone of standard cosmology is the Friedmann-Robertson-Walker solution to Einstein's equations of general relativity (GR). In recent years, observations have largely confirmed many of the properties of this model, which is based on a partitioning of the universe's energy density into three primary constituents: matter, radiation, and a hypothesized dark energy which, in LambdaCDM, is assumed to be a cosmological constant Lambda. Yet with this progress, several unpalatable coincidences (perhaps even inconsistencies) have emerged along with the successful confirmation of expected features. One of these is the observed equality of our gravitational horizon R_h(t_0) with the distance ct_0 light has traveled since the big bang, in terms of the current age t_0 of the universe. This equality is very peculiar because it need not have occurred at all and, if it did, should only have happened once (right now) in the context of LambdaCDM. In this paper, we propose an explanation for why this equality may actually be required by GR, through the application of Birkhoff's theorem and the Weyl postulate, at least in the case of a flat spacetime. If this proposal is correct, R_h(t) should be equal to ct for all cosmic time t, not just its present value t_0. Therefore models such as LambdaCDM would be incomplete because they ascribe the cosmic expansion to variable conditions not consistent with this relativistic constraint. We show that this may be the reason why the observed galaxy correlation function is not consistent with the predictions of the standard model. We suggest that an R_h=ct universe is easily distinguishable from all other models at large redshift (i.e., in the early universe), where the latter all predict a rapid deceleration.

Precision cosmography with stacked voids

Abstract: We present a purely geometrical method for probing the expansion history of the universe from the observation of the shape of stacked voids in spectroscopic redshift surveys. Our method is an Alcock-Paczynski (AP) test based on the average sphericity of voids posited on the local isotropy of the universe. It works by comparing the temporal extent of cosmic voids along the line of sight with their angular, spatial extent. We describe the algorithm that we use to detect and stack voids in redshift shells on the light cone and test it on mock light cones produced from N-body simulations. We establish a robust statistical model for estimating the average stretching of voids in redshift space and quantify the contamination by peculiar velocities. Finally, assuming that the void statistics that we derive from N-body simulations is preserved when considering galaxy surveys, we assess the capability of this approach to constrain dark energy parameters. We report this assessment in terms of the figure of merit (FoM) of the dark energy task force and in particular of the proposed Euclid mission which is particularly suited for this technique since it is a spectroscopic survey. The FoM due to stacked voids from the Euclid wide survey may double that of all other dark energy probes derived from Euclid data alone (combined with Planck priors). In particular, voids seem to outperform baryon acoustic oscillations by an order of magnitude. This result is consistent with simple estimates based on mode counting. The AP test based on stacked voids may be a significant addition to the portfolio of major dark energy probes and its potentialities must be studied in detail.