Viscous dark fluid Universe: a unified model of the dark sector?

TL;DR

This paper proposes a unified dark sector model using a bulk viscous fluid that behaves like a generalized Chaplygin gas, compatible with large-scale structure data and favoring a baryon-like matter content, but faces challenges with CMB spectrum reproduction.

Contribution

It introduces a bulk viscous fluid model for the dark sector that aligns with structure formation data and suggests a baryon-like matter abundance, differing from previous models.

Findings

Compatible with 2dFGRS and SDSS data

Favors a negative present deceleration parameter

Suggests matter content similar to baryonic matter

Abstract

The Universe is modeled as consisting of pressureless baryonic matter and a bulk viscous fluid which is supposed to represent a unified description of the dark sector. 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 nonadiabatic 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, the unified model presented here favors a matter content that is of the order of the baryonic matter abundance suggested by big-bang nucleosynthesis. A problem of simple bulk viscous models, however, is the behavior of the gravitational potential and the reproduction of the CMB power spectrum.