When did cosmic acceleration start?

A precise determination, and comparison, of the epoch of the onset of cosmic acceleration, at redshift z_acc, and of dark energy domination, at z_eq, provides an interesting measure with which to parametrize dark energy models. By combining several cosmological data sets, we place constraints on the redshift and age of cosmological acceleration. For a ΛCDM model, we find the constraint z_acc=0.76±0.10 at 95% C.L., occurring 6.7±0.4Gyr ago. Allowing a constant equation of state but different from -1 changes the constraint to z_acc=0.81±0.12 (6.9±0.5Gyr ago), while dynamical models markedly increase the error on the constraint z_acc=0.81±0.30 (6.8±1.4Gyr ago). Unified dark energy models such as silent quartessence yield z_acc=0.8±0.16 (6.8±0.6Gyr ago). Interestingly, we find that the best fit z_acc and z_eq are remarkably insensitive to both the cosmological data sets and theoretical dark energy models considered.