Viscous cosmologies and the second law of thermodynamics

Abstract

In the framework of viscous cosmologies, it is possible to smoothly connect a phase where the dominant energy condition (DEC) is violated to a phase where the DEC is preserved. It is well known that, depending upon the sign of the bulk viscosity coefficient, the entropy of the sources driving the geometry can very well decrease. It was recently argued that such behavior is not acceptable. We show that such an assessment might be not justified since it only takes into account the entropy of the sources but not the entropy of the geometry itself. The entropic properties of bulk viscous models violating the DEC are examined in light of previous results on the subject. We discuss the physical ambiguities of the generalized second law of thermodynamics with particular attention to the case of backgrounds with shrinking event horizons.