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 ₃₀ = 0. 74 0. 05, in good agreement with the recent Busca et al. (2012) determination of z ₃₀ = 0. 82 0. 08 based on 11 H (z) measurements between redshifts 0. 2 < z < 2. 3, almost entirely from BAO-like data.