Non-adiabatic dark fluid cosmology

Abstract

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.

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

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