759 research outputs found
Unified models of the cosmological dark sector
We model the cosmological substratum by a viscous fluid that is supposed to
provide a unified description of the dark sector and pressureless baryonic
matter. In the homogeneous and isotropic background the \textit{total} energy
density of this mixture behaves as a generalized Chaplygin gas. The
perturbations of this energy density are intrinsically non-adiabatic and source
relative entropy perturbations. The resulting baryonic matter power spectrum is
shown to be compatible with the 2dFGRS and SDSS (DR7) data. A joint statistical
analysis, using also Hubble-function and supernovae Ia data, shows that,
different from other studies, there exists a maximum in the probability
distribution for a negative present value of the
deceleration parameter. Moreover, different from other approaches, the unified
model presented here favors a matter content that is of the order of the
baryonic matter abundance suggested by big-bang nucleosynthesis.Comment: 4 pages, 1 figure, submitted to the Proceedings of the Spanish
Relativity Meeting - ERE 2010, Granada, September 6 - September 10, 201
Dissipation of dark matter
Fluids often display dissipative properties. We explore dissipation in the
form of bulk viscosity in the cold dark matter fluid. We constrain this model
using current data from supernovae, baryon acoustic oscillations and the cosmic
microwave background. Considering the isotropic and homogeneous background
only, viscous dark matter is allowed to have a bulk viscosity
Pas, also consistent with the expected integrated Sachs-Wolfe effect
(which plagues some models with bulk viscosity). We further investigate the
small-scale formation of viscous dark matter halos, which turns out to place
significantly stronger constraints on the dark matter viscosity. The existence
of dwarf galaxies is guaranteed only for much smaller values of the dark matter
viscosity, Pas.Comment: 10 pages, 3 figures, published in PR
Matter power spectrum for the generalized Chaplygin gas model: The relativistic case
The generalized Chaplygin gas (GCG) model is the prototype of a unified model
of dark energy (DE) and dark matter (DM). It is characterized by
equation-of-state (EoS) parameters and . We use a statistical
analysis of the 2dFGRS data to constrain these parameters. In particular, we
find that very small (close to zero) and very large values () of
the equation-of-state parameter are preferred. To test the validity of
this type of unification of the dark sector we admit the existence of a
separate DM component in addition to the Chaplygin gas and calculate the
probability distribution for the fractional contributions of both components to
the total energy density. This analysis favors a model for which the Universe
is nearly entirely made up of the separate DM component with an almost
negligible Chaplygin gas part. This confirms the results of a previous
Newtonian analysis.Comment: Latex file, 8 pages, 15 figures in eps forma
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