Thermodynamics of k-essence

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}.

17 pages, minor changes, typos corrected, a comment on causality and speed of sound added, references added

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• Beyond the Cosmological Standard Model
• The Imperfect Fluid behind Kinetic Gravity Braiding
• Thermodynamics of cosmological horizons in $f(T)$ gravity
• Thermodynamics in $F(R)$ gravity with phantom crossing
• Thermodynamics in $f(R)$ gravity in the Palatini formalism
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• Some notes about the density of states for a negative pressure matter
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• Generalized unimodular gravity as a form of k-essence
• CMB power spectrum in the emergent universe with k-essence
• The role of the chemical potential in coupling superfluid dark matter to baryons