VISCOUS-FLUID COSMOLOGY

In this paper we shall examine the effects of both bulk and shear visc osities upon a variety of cosmological models. We assume that the visc ous terms can be modelled by dimensionless equations of stage, and thi s allows us to write the Einstein equations as a system of autonomous differential equations. After briefly discussing the Friedmann-Roberts on-Walker and Bianchi I models, we discuss in some detail the Bianchi V and Kantowski-Sachs models. In all cases we find the critical points of the flow and elucidate their nature, making use of the energy cond itions. Because of our choice of dimensionless variables, (almost) all critial points represent self-similar solutions to the field equation s. We also study the case of a Bianchi V perfect fluid model with a no n-linear equation of state. We find that the models are structurally s table under the addition of shear viscosity, whereas they are structur ally unstable under the introduction of bulk viscosity. Almost all of the Bianchi V models examined have initial singularities where the mat ter is dynamically unimportant; the Kantowski-Sachs models have final singularities where the matter is dynamically important.