Unified dark energy thermodynamics: varying w and the −1-crossing

Abstract

We investigate, in a unified and general way, the thermodynamic properties of dark energy with an arbitrary, varying equation-of-state parameter w(a). We find that all quantities are well defined and regular for every w(a), including the −1-crossing, with the temperature being negative in the phantom regime (w(a) < −1) and positive in the quintessence one (w(a) > −1). The density and entropy are always positive while the chemical potential can be arbitrary. At the −1-crossing, both temperature and chemical potential are zero. The temperature negativity can only be interpreted in the quantum framework. The regular behavior of all quantities at the −1-crossing leads to the conclusion that such a crossing does not correspond to a phase transition, but rather to a smooth crossover.