A late-time transition in the cosmic dark energy?

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 less than or equal to 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 sigma errors) to all the data are z(t) = 2.0(-0.76)(+2.2), Omega(Q) = 0.73(-0.04)(+0.02) and w(f) = -1+0.2. For z(t) > 5 the likelihood becomes flat, asymptoting to the standard LambdaCDM model.