On phantom thermodynamics

The thermodynamic properties of dark energy fluids described by an equation of state parameter ω=p/ρ are rediscussed in the context of FRW type geometries. Contrarily to previous claims, it is argued here that the phantom regime ω<−1 is not physically possible since that both the temperature and the entropy of every physical fluids must be always positive definite. This means that one cannot appeal to negative temperature in order to save the phantom dark energy hypothesis as has been recently done in the literature. Such a result remains true as long as the chemical potential is zero. However, if the phantom fluid is endowed with a non-null chemical potential, the phantom field hypothesis becomes thermodynamically consistent, that is, there are macroscopic equilibrium states with T>0 and S>0 in the course of the Universe expansion.