PHYSICAL REVIEW D

There is mounting observational evidence that the expansion of our Universe is undergoing a late-time acceleration. Among many proposals to describe this phenomenon, the cosmological constant (Λ) seems to be the simplest and the most natural explanation. However, despite its observational successes, such a possibility exacerbates the well known Λ problem, requiring a natural explanation for its small, but nonzero, value. In this paper we consider a cosmological scenario driven by a varying cosmological term, in which the vacuum energy density decays linearly with the Hubble parameter, Λ∝H. We show that this Λ(t)CDM model is indistinguishable from the standard one (ΛCDM) in that the early radiation phase is followed by a long dust-dominated era, and only recently the varying Λ term becomes dominant, accelerating the cosmic expansion. In order to test the viability of this scenario, we have used the most recent type Ia supernova data, i.e., the High-Z SN Search Team and the Supernova Legacy Survey (SNLS) Collaboration data. In particular, for the SNLS sample we have found 0.27≤Ωm≤0.37 and 0.68≤H0≤0.72 (at 2σ), which is in good agreement with the currently accepted estimates for these parameters