We discuss the thermodynamic properties of the Friedmann-Robertson-Walker universe with dark energy fluids labelled by ω = p/ρ < −1/3. Using the integrability condition, we show that the phantom phase of w < −1 can still be thermodynamically allowed even when the temperature takes on negative values because in that case, there exists at least a condition of keeping physical values for p and ρ.

We investigate, in a unified and general way, the thermodynamic properties of dark energy with an arbitrary, varying equation-of-state parameter w(a). We find that all quantities are well defined and regular for every w(a), including at the −1-crossing, with the temperature being negative in the phantom regime (w(a) < −1) and positive in the quintessence one (w(a) > −1). The density and entropy are always positive while the chemical potential can be arbitrary. At the −1-crossing, both temperature and chemical potential are zero. The temperature negativity can only be interpreted in the quantum framework. The regular behavior of all quantities at the −1-crossing, leads to the conclusion that such a crossing does not correspond to a phase transition, but rather to a smooth cross-over. PACS numbers: 95.36.+x,

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