Supernova constraints on decaying vacuum cosmology

Abstract

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 ($\ensuremath{\Lambda}$) seems to be the simplest and the most natural explanation. However, despite its observational successes, such a possibility exacerbates the well known $\ensuremath{\Lambda}$ 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, $\ensuremath{\Lambda}\ensuremath{\propto}H$. We show that this $\ensuremath{\Lambda}(t)\mathrm{CDM}$ model is indistinguishable from the standard one ($\ensuremath{\Lambda}\mathrm{CDM}$) in that the early radiation phase is followed by a long dust-dominated era, and only recently the varying $\ensuremath{\Lambda}$ 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 $0.27\ensuremath{\le}{\ensuremath{\Omega}}_{\mathrm{m}}\ensuremath{\le}0.37$ and $0.68\ensuremath{\le}{H}_{0}\ensuremath{\le}0.72$ (at $2\ensuremath{\sigma}$), which is in good agreement with the currently accepted estimates for these parameters.