Thermodynamics of the vacuum

The fact that the energy-momentum tensor has nonzero vacuum expectation value in some space-times (the so-called back reaction) indicates a nontrivial thermodynamics of such vacua. A consequent thermodynamic analysis of the problem in Robertson-Walker space-times shows that, in the generic case, the number of the independent extensives is 2, in spite of the fact that the energy density is completely determined by the time-evolution of the geometry since the preparation of the vacuum state. The appearance of a second independent extensive seems to be in direct connection with the extra entropy term in theGeneralized Second Law of Bekenstein and Hawking.