EQUATIONS OF STATE AND TRANSPORT-EQUATIONS IN VISCOUS COSMOLOGICAL MODELS

The aim of this paper is to compare the evolution of viscous cosmological models for three different constitutive equations for the viscous pressure sigma, namely: Eckart theory (sigma = -xi del (mu)u(mu)), Maxwell-Cattaneo theory sigma + tausigma = -xi del (mu)u(mu)), and generalized second-order Israel-Stewart theory (sigma + tausigma = -xi del (mu)u(mu)-1/2xisigmaTrhou(mu) del mu(nutau/Txi)). In the last case, the nonlinear terms may be interpreted as a consequence of nonlinear equations of state for absolute temperature and thermodynamic pressure. We show that the nonlinear terms of the latter equation are compatible with a viscosity-driven inflationary expansion.