COSMOLOGICAL CONSEQUENCES OF A TIME-DEPENDENT LAMBDA TERM

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 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 q0 assuming negative values so that the density parameter OMEGA-0 is smaller than 2/3 and the age of the Universe is always bigger than H0(-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.