Thermodynamics of k-essence

TL;DR

This paper explores the thermodynamic properties of k-essence dark energy, demonstrating that its entropy is non-negative and analyzing thermal fluctuations as a free boson gas with an effective degrees of freedom related to the sound speed.

Contribution

It introduces a formalism for thermodynamics of k-essence dark energy and clarifies misconceptions about entropy in phantom energy scenarios.

Findings

Dark energy entropy is always ≥ 0.

Phantom dark energy does not necessarily have negative entropy.

Thermal fluctuations of k-essence resemble a free boson gas with c_s^{-3} degrees of freedom.

Abstract

We discuss thermodynamic properties of dark energy using the formalism of field theory at finite temperature. In particular, we apply our formalism to a purely kinetic type of k-essence. We show quite generally that the entropy associated with dark-energy is always equal or greater than zero. Hence, contrary to often stated claims, a violation of the null energy condition (phantom dark energy) does not necessarily yield a negative entropy. In addition, we find that the thermal fluctuations of a k-essence field may be represented by a free boson gas with an effective number of degrees of freedom equal to c_s^{-3}.