Non-adiabatic dark fluid cosmology

We model the dark sector of the cosmic substratum by a viscous fluid with an equation of state p = - Delta *z Delta *C, where Delta *C is the fluid-expansion scalar and Delta *z is the coefficient of bulk viscosity for which we assume a dependence Delta *z Delta *r Delta *n on the energy density Delta *r. The homogeneous and isotropic background dynamics coincides with that of a generalized Chaplygin gas with equation of state p = -A/ Delta *r Delta *a. 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 Delta *y2-analysis shows, that for certain parameter combinations the viscous-dark-fluid (VDF) model is well competitive with the Delta *LCDM model. These results indicate that non-adiabatic unified models can be seen as potential contenders for a General-Relativity-based description of the cosmic substratum.