Unified dark energy thermodynamics: varying w and the -1-crossing
  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
at 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 cross-over.
