Thermodynamics of Decaying Vacuum Cosmologies

The thermodynamic behavior of vacuum decaying cosmologies is investigated within a manifestly covariant formulation. Such a process corresponds to a continuous irreversible energy flow from the vacuum component to the created matter constituents. It is shown that if the specific entropy per particle remains constant during the process, the equilibrium 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 $\rho \sim T^{4}$ and $n \sim T^{3}$. The temperature law is determined and a generalized Planckian type form of the spectrum, which is preserved in the course of the evolution, is also proposed. Some consequences of these results for decaying vacuum FRW type cosmologies as well as for models with ``adiabatic'' photon creation are discussed.Comment: 21 pages, uses LATE

Decaying Vacuum Energy and Deflationary Cosmology in Open and Closed Universes

We consider a nonsingular deflationary cosmological model with decaying vacuum energy density in universes of arbitrary spatial curvature. Irrespective of the value of $k$, the models are characterized by an arbitrary time scale $H_I^{-1}$ which determines the initial temperature of the universe and the largest value of the vacuum energy density, the slow decay of which generates all the presently observed matter-energy of the universe. If $H_I^{-1}$ is of the order of the Planck time, the models begin with the Planck temperature and the present day value of the cosmological constant satisfies $\Lambda_I/\Lambda_0 \simeq 10^{118}$ as theoretically suggested. It is also shown that all models allow a density parameter $\Omega_0 <2/3$ and that the age of the universe is large enough to agree with observations even with the high value of $H_0$ suggested by recent measurements.Comment: 20 pages, 2 figures (available from the authors), uses LaTe