Non-adiabatic dark fluid cosmology

Details Non-adiabatic dark fluid cosmology Non-adiabatic dark fluid cosmology

2009-02-26

arxiv.org/abs/0902.4710v2

JCAP 0906:016,2009

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

25 pages, 9 figures, comments and references added, accepted by JCAP

Scientific paper

10.1088/1475-7516/2009/06/016

We model the dark sector of the cosmic substratum by a viscous fluid with an equation of state $p=-\zeta \Theta$, where $\Theta$ is the fluid-expansion scalar and $\zeta$ is the coefficient of bulk viscosity for which we assume a dependence $\zeta \propto \rho^{\nu}$ on the energy density $\rho$. The homogeneous and isotropic background dynamics coincides with that of a generalized Chaplygin gas with equation of state $p = - A/\rho^{\alpha}$. The perturbation dynamics of the viscous model, however, is intrinsically non-adiabatic and qualitatively different from the Chaplygin-gas case. In particular, it avoids short-scale instabilities and/or oscillations which apparently have ruled out unified models of the Chaplygin-gas type. We calculate the matter power spectrum and demonstrate that the non-adiabatic model is compatible with the data from the 2dFGRS and the SDSS surveys. A $\chi^{2}$-analysis shows, that for certain parameter combinations the viscous-dark-fluid (VDF) model is well competitive with the $\Lambda$CDM model. These results indicate that \textit{non-adiabatic} unified models can be seen as potential contenders for a General-Relativity-based description of the cosmic substratum.

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