2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/211265We 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}.17 pages, minor changes, typos corrected, a comment on causality and speed of sound added, references addedGeneral Relativity and Quantum CosmologyAstrophysicsHigh Energy Physics - Theorytext