On Phantom Thermodynamics

TL;DR

This paper critically examines the thermodynamics of dark energy fluids, arguing that the phantom regime with <-1 is not physically feasible unless the fluid has a non-zero chemical potential, which allows for thermodynamic consistency.

Contribution

It demonstrates that the phantom regime is thermodynamically inconsistent without chemical potential and introduces the possibility of a bosonic nature for phantom particles when chemical potential is considered.

Findings

Phantom regime <-1 is not physically possible with zero chemical potential.

Non-zero chemical potential allows for thermodynamic consistency in phantom fluids.

Phantom particles may be bosonic and massless based on entropy constraints.

Abstract

The thermodynamic properties of dark energy fluids described by an equation of state parameter $\omega=p/\rho$ are rediscussed in the context of FRW type geometries. Contrarily to previous claims, it is argued here that the phantom regime $\omega<-1$ 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. Further, the negative value of the chemical potential resulting from the entropy constraint ($S>0$) suggests a bosonic massless nature to the phantom particles.