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 parameterize dark energy models .
By combining several cosmological datasets we place constraints on the redshift and age of cosmological acceleration .
For a \Lambda CDM model , we find the constraint z _ { acc } = 0.76 \pm 0.10 at 95 \% c.l. , occurring 6.7 \pm 0.4 Gyrs ago .
Allowing a constant equation of state but different from -1 changes the constraint to z _ { acc } = 0.81 \pm 0.12 ( 6.9 \pm 0.5 Gyrs ago ) and z _ { eq } = 0.48 \pm 0.14 ( 4.9 \pm 0.9 Gyrs ago ) , while dynamical models markedly increase the error on the constraint with z _ { acc } = 0.81 \pm 0.30 ( 6.8 \pm 1.4 Gyrs ago ) and z _ { eq } = 0.44 \pm 0.20 ( 4.5 \pm 1.0 Gyrs ago ) .
Unified dark energy models as Silent Quartessence yield : z _ { acc } = 0.80 \pm 0.16 ( 6.8 \pm 0.6 Gyrs ago ) .