HUBBLE PARAMETER MEASUREMENT CONSTRAINTS ON THE COSMOLOGICAL DECELERATION-ACCELERATION TRANSITION REDSHIFT

We compile a list of 28 independent measurements of the Hubble parameter between redshifts 0.07 < z < 2.3 and use this to place constraints on model parameters of constant and time-evolving dark energy cosmologies. These H(z) measurements by themselves require a currently accelerating cosmological expansion at about, or better than, 3-sigma confidence. The mean and standard deviation of the 6 best-fit model deceleration-acceleration transition redshifts (for the 3 cosmological models and 2 Hubble constant priors we consider) is $z_{\rm da} = 0.74 \pm 0.05$, in good agreement with the recent Busca et al. (2012) determination of $z_{\rm da} = 0.82 \pm 0.08$ based on 11 H(z) measurements between redshifts 0.2 < z < 2.3, almost entirely from BAO-like data.