On the 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 non-trivial 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 after 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 the Generalized Second Law of Bekenstein and Hawking. (author)