When did cosmic acceleration start?

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