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 Lambda CDM model, we find the constraint z(acc)=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 z(acc)=0.81 +/- 0.12 (6.9 +/- 0.5 Gyr ago), while dynamical models markedly increase the error on the constraint z(acc)=0.81 +/- 0.30 (6.8 +/- 1.4 Gyr ago). Unified dark energy models such as silent quartessence yield z(acc)=0.8 +/- 0.16 (6.8 +/- 0.6 Gyr 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.