THERMODYNAMICS OF DECAYING VACUUM COSMOLOGIES

The thermodynamic behavior of decaying vacuum cosmologies is investiga ted within a manifestly covariant formulation. Such a process correspo nds to a continuous, irreversible energy flow from the vacuum componen t to the created matter constituents. It is shown that if the specific entropy per particle remains constant during the process, the equilib rium relations are preserved. In particular, if the vacuum decays into photons, the energy density rho and average number density of photons n scale with the temperature as rho similar to T-4 and n similar to T -3. The temperature law is determined and a generalized Planckian-type form of the spectrum, which is preserved in the course of the evoluti on, is also proposed. Some consequences of these results for decaying vacuum FRW-type cosmologies as well as for models with ''adiabatic'' p hoton creation are discussed.