Viscous inflationary universe models

The research on viscous cosmological models is reviewed. Inflationary cosmological models of Bianchi type-I with shear, bulk, and nonlinear viscosity are studied. The inflation field energy is represented by a two- component cosmic fluid consisting of a vacuum fluid and a Zel'dovich fluid. It is shown that there exist models in which the viscous Zel'dovich fluid removes the initial singularity of the corresponding viscosity free models. The mean expansion of a Zel'dovich dominated universe is found to be independent of shear viscosity and anisotropy. Bulk viscosity and shear viscosity cause exponential decay of anisotropy, while nonlinear viscosity causes power-law decay of anisotropy. (Author)