A late-time transition in the cosmic dark energy?

Abstract We study constraints from the latest cosmic microwave background (CMB), large-scale structure (2dF, Abell/ACO, PSCz) and SN1a data on dark energy models with a sharp transition in their equation of state, w(z). Such a transition is motivated by models like vacuum metamorphosis where non-perturbative quantum effects are important at late times. We allow the transition to occur at a specific redshift, z t, to a final negative pressure −1 ⩽w f < −1/3. We find that the CMB and supernovae data, in particular, prefer a late-time transition because of the associated delay in cosmic acceleration. The best fits (±1 σ errors) to all the data are z t= 2.0+2.2 −0.76, Ω Q = 0.73+0.02 −0.04 and w f=−1+0.2. For z t > 5 the likelihood becomes flat, asymptoting to the standard ΛCDM model.