Isotropic viscous cosmologies compatible with the standard second law of thermodynamics

Abstract

A model proposed by M. Giovannini (Phys. Rev. D 61, 108302 (2000)) is considered here. The author has shown that there exists a class of viscous cosmological models which violate the dominant energy condition for a limited amount of time after which they are smoothly connected to the ordinary radiation era (which preserves the dominant energy conditions). It was shown that this violation of the dominant energy condition at an early cosmological epoch may influence the slopes of energy spectra of relic gravitons. However, the bulk viscosity coefficient of these cosmologies became negative during the ordinary radiation era, and then the entropy of the sources driving the geometry decreases with time. We show that for viscous sources with a linear barotropic equation of state we get viscous cosmological models with positive bulk viscous stress during all their evolution, and hence the matter entropy increases with the expansion time.