599 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
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
Newtonian View of General Relativistic Stars
Although general relativistic cosmological solutions, even in the presence of
pressure, can be mimicked by using neo-Newtonian hydrodynamics, it is not clear
whether there exists the same Newtonian correspondence for spherical static
configurations. General relativity solutions for stars are known as the
Tolman-Oppenheimer-Volkoff (TOV) equations. On the other hand, the Newtonian
description does not take into account the total pressure effects and therefore
can not be used in strong field regimes. We discuss how to incorporate pressure
in the stellar equilibrium equations within the neo-Newtonian framework. We
compare the Newtonian, neo-Newtonian and the full relativistic theory by
solving the equilibrium equations for both three approaches and calculating the
mass-radius diagrams for some simple neutron stars equation of state.Comment: 6 pages, 3 figures. v2 matches accepted version (EPJC
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