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.

Cosmographic analysis of the equation of state of the universe through Padé approximations

Abstract: Cosmography is used in cosmological data processing in order to constrain the kinematics of the universe in a model-independent way, providing an objective means to evaluate the agreement of a model with observations. In this paper, we extend the conventional methodology of cosmography employing Taylor expansions of observables by an alternative approach using Pad\'e approximations. Due to the superior convergence properties of Pad\'e expansions, it is possible to improve the fitting analysis to obtain numerical values for the parameters of the cosmographic series. From the results, we can derive the equation of state parameter of the universe and its first derivative and thus acquire information about the thermodynamic state of the universe. We carry out statistical analyses using observations of the distance modulus of type 1a supernovae, provided by the union 2.1 compilation of the supernova cosmology project, employing a Markov chain Monte Carlo approach with an implemented Metropolis algorithm. We compare the results of the original Taylor approach to the newly introduced Pad\'e formalism. The analyses show that experimental data constrain the observable universe well, finding an accelerating universe and a positive jerk parameter. We demonstrate that the Pad\'e convergence radii are greater than standard Taylor convergence radii, and infer a lower limit on the acceleration of the universe solely by requiring the positivity of the Pad\'e expansion. We obtain fairly good agreement with the Planck results, confirming the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model at small redshifts, although we cannot exclude a dark energy density varying in time with negligible speed of sound.

Warm-hot baryons comprise 5-10 per cent of filaments in the cosmic web.

Abstract: Observations of the cosmic microwave background indicate that baryons account for 5 per cent of the Universe's total energy content. In the local Universe, the census of all observed baryons falls short of this estimate by a factor of two. Cosmological simulations indicate that the missing baryons have not condensed into virialized haloes, but reside throughout the filaments of the cosmic web (where matter density is larger than average) as a low-density plasma at temperatures of 10(5)-10(7) kelvin, known as the warm-hot intergalactic medium. There have been previous claims of the detection of warm-hot baryons along the line of sight to distant blazars and of hot gas between interacting clusters. These observations were, however, unable to trace the large-scale filamentary structure, or to estimate the total amount of warm-hot baryons in a representative volume of the Universe. Here we report X-ray observations of filamentary structures of gas at 10(7) kelvin associated with the galaxy cluster Abell 2744. Previous observations of this cluster were unable to resolve and remove coincidental X-ray point sources. After subtracting these, we find hot gas structures that are coherent over scales of 8 megaparsecs. The filaments coincide with over-densities of galaxies and dark matter, with 5-10 per cent of their mass in baryonic gas. This gas has been heated up by the cluster's gravitational pull and is now feeding its core. Our findings strengthen evidence for a picture of the Universe in which a large fraction of the missing baryons reside in the filaments of the cosmic web.

DBI Galileons in the Einstein frame: Local gravity and cosmology

Abstract: It is shown that a disformally coupled theory in which the gravitational sector has the Einstein-Hilbert form is equivalent to a quartic Dirac-Born-Infeld Galileon Lagrangian, possessing nonlinear higher derivative interactions, and hence allowing for the Vainshtein effect. This Einstein frame description considerably simplifies the dynamical equations and highlights the role of the different terms. The study of highly dense, nonrelativistic environments within this description unravels the existence of a disformal screening mechanism, while the study of static vacuum configurations reveals the existence of a Vainshtein radius, at which the asymptotic solution breaks down. Disformal couplings to matter also allow the construction of dark energy models, which behave differently than conformally coupled ones and introduce new effects on the growth of large scale structure over cosmological scales, on which the scalar force is not screened. We consider a simple disformally coupled dark matter model in detail, in which standard model particles follow geodesics of the gravitational metric and only dark matter is affected by the disformal scalar field. This particular model is not compatible with observations in the linearly perturbed regime. Nonetheless, disformally coupled theories offer enough freedom to construct realistic cosmological scenarios, which can be distinguished from the standard model through characteristic signatures.

Can a vector field be responsible for the curvature perturbation in the Universe

Abstract: I investigate the possibility that the observed curvature perturbation is due to a massive vector field. To avoid generating a large scale anisotropy the vector field is not taken to be driving inflation. Instead it is assumed to become important after inflation when it may dominate the Universe and imprint its perturbation spectrum before its decay, as in the curvaton scenario. It is found that, to generate a scale invariant spectrum of perturbations, the mass-squared of the vector field has to be negative and comparable to the Hubble scale during inflation. After inflation the mass-squared must become positive so that the vector field engages into oscillations. It is shown that such an oscillating vector field behaves as pressureless matter and does not lead to large scale anisotropy when it dominates the Universe. The possibility of realizing this scenario in supergravity is also outlined.

Abundance of Primordial Holes Produced by Cosmological First-Order Phase Transition

Abstract: One important prediction of a cosmological model based on grand unified theories is the production of primordial black holes and wormholes through a first· order phase transition. In this paper we estimate the abundance of these holes and their effects on cosmology. We show that they play several important roles in the subsequent evolution of the universe and that the observational information of the present universe, conversely, yields constraints on the phenomenological parameters in grand unified theories. Especially we find that both the thermal and quantum nucleations of vacuum bubbles are the necessary ingredients in construct· ing a cosmological model consistent with the present observations.