Characteristic properties of two different viscous cosmology models for the future universe

Abstract

We analyze characteristic properties of two different cosmological models: (i) a one-component dark energy model where the bulk viscosity [Formula: see text] is associated with the fluid as a whole, and (ii) a two-component model where [Formula: see text] is associated with a dark matter component [Formula: see text] only, the dark energy component considered inviscid. Shear viscosity is omitted. We assume throughout the simple equation-of-state [Formula: see text] with [Formula: see text] a constant. In the one-component model, we consider two possibilities, either to take [Formula: see text] proportional to the scalar expansion (equivalent to the Hubble parameter), in which case the evolution becomes critically dependent on the value of the small constant [Formula: see text] and the magnitude of [Formula: see text], or we consider the case [Formula: see text], where a de Sitter final stage is reached in the future. In the two-component model, we consider only the case where the dark matter viscosity [Formula: see text] is proportional to the square of [Formula: see text], where again a de Sitter form is found in the future. In this latter case, the formalism is supplemented by a phase space analysis. As a general result of our considerations, we suggest that a value [Formula: see text] for the present viscosity is reasonable, and that the two-component model seems to be favored.