Singularity-free decaying-vacuum cosmologies.

Abstract

Decaying-vacuum singularity-free cosmological models based on the Chen-Wu ansatz of a cosmological term varying as ${R}^{\ensuremath{-}2}$ where $R$ is the scale factor of the Universe are introduced. They describe a closed ever-expanding universe of density parameter $\ensuremath{\Omega}\ensuremath{\ge}1$ and with no entropy, horizon, or monopole problems. They include and extend the critical density cosmology of \"Ozer and Taha. The \"Ozer-Taha period of phase transition during part of which the pressure is negative occurs in these models. In its wake and throughout the radiation-dominated era the Universe is Einstein-de Sitter-like with $\ensuremath{\Omega}$ then and subsequently near unity. Nucleosynthesis proceeds as in the standard model. Consistency with the observed helium abundance and baryon asymmetry allows a maximum vacuum energy close to the radiation energy today. The presence of this vacuum energy could be detrimental to certain theories of galaxy formation. A specific model with initial conditions approaching the hot big bang is studied in detail, particularly as regards the entropy and flatness of the very early Universe.