On phantom thermodynamics with negative temperature

We discuss the thermodynamic properties of the Friedmann-Robertson-Walker universe with dark energy fluids labelled by omega = p/rho < -1/3. Using the integrability condition, we show that the phantom phase of omega < -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 rho. (C) 2008 Elsevier B.V. All rights reserved.