Thermodynamics and dark energy
  A significant observational effort has been directed to unveil the nature of the so-called dark energy. However, given the large number of theoretical possibilities, it is possible that such a task cannot be performed on the basis only of the observational data. In this article we discuss some thermodynamic properties of this energy component by assuming that its constituents are massless quanta with a general time-dependent equation-of-state parameter $\omega(z)=\omega_0 + \omega_a f(z)$, where $\omega_0$ and $\omega_a$ are constants and $f(z)$ may assume different forms. We show that very restrictive bounds can be placed on the $w_0$ - $w_a$ space when current observational data are combined with the thermodynamic constraints derived. 
R. Silva, R. S. Goncalves, J. S. Alcaniz, H. H. B. Silva
astro-ph.CO gr-qc