Is dark energy decaying?

We explore the fate of the universe given the possibility that the density associated with ‘dark energy’ may decay slowly with time. We model decaying dark energy by a homogeneous scalar field which couples minimally to gravity and whose potential has at least one local maximum. Dark energy decays as the scalar field rolls down its potential and this allows for several interesting possibilities including: (i) the current acceleration epoch can be a transient, (ii) a spatially flat universe ultimately collapses under the influence of a growing negative potential. (The second possibility is realized by the cosine potential.) We examine both possibilities by comparing our model universe with observations of high redshift type Ia supernovae. A maximum likelihood analysis reveals that such models are viable if the effective mass squared near the potential maximum is small (m ∼ H0) and if the initial field displacement is not very large φ/MP < ∼ 1. The time left until the universe collapses is always larger than ∼ 22 Gyrs for Ω0m = 0.3 and H0 ≃ 70 km/sec/Mpc (at the 95.4% confidence level).