Cosmological consequences of a time-dependent Lambda term.

Abstract

The phenomenological approach to investigate the decay of the effective cosmological constant, as recently proposed by Chen and Wu, is generalized to include a term proportional to ${H}^{2}$ on the time dependence of $\ensuremath{\Lambda}$, where $H$ is the Hubble parameter. This new term can modify some features of the standard Friedmann-Robertson-Walker model and its free parameter may be adjusted in accordance with nucleosynthesis constraints. The model also allows a deceleration parameter ${q}_{0}$ assuming negative values so that the density parameter ${\ensuremath{\Omega}}_{0}$ is smaller than ⅔ and the age of the Universe is always bigger than ${H}_{0}^{\ensuremath{-}1}$. In these cases, the usual matter creation rate appearing in models with a decaying vacuum energy is smaller than the one present in the steady-state model.