Constraining the cosmic deceleration - acceleration transition with type Ia supernova , BAO / CMB and H ( z ) data

We revisit the kink-like parametrization of the deceleration parameter q(z) [1], which considers a transition, at redshift zt, from cosmic deceleration to acceleration. In this parametrization the initial, at z zt, value of the q-parameter is qi, its final, z = −1, value is qf and the duration of the transition is parametrized by τ . By assuming a flat space geometry we obtain constraints on the free parameters of the model using recent data from type Ia supernovae (SN Ia), baryon acoustic oscillations (BAO), cosmic microwave background (CMB) and the Hubble parameter H(z). The use of H(z) data introduces an explicit dependence of the combined likelihood on the present value of the Hubble parameter H0, allowing us to explore the influence of different priors when marginalizing over this parameter. We also study the importance of the CMB information in the results by considering data from WMAP7, WMAP9 (Wilkinson Microwave Anisotropy Probe 7 and 9 years) and Planck 2015. We show that the contours and best fit do not depend much on the different CMB data used and that the considered new BAO data is responsible for most of the improvement in the results. Assuming a flat space geometry, qi = 1/2 and expressing the present value of the deceleration parameter q0 as a function of the other three free parameters, we obtain zt = 0.67 +0.10 −0.08, τ = 0.26 −0.10 and q0 = −0.48 +0.11 −0.13, at 68% of confidence level, with an uniform prior over H0. If in addition we fix qf = −1, as in flat ΛCDM, DGP and Chaplygin quartessence that are special models described by our parametrization, we get zt = 0.66 +0.03 −0.04, τ = 0.33 +0.04 −0.04 and q0 = −0.54 −0.07, in excellent agreement with flat ΛCDM for which τ = 1/3. We also obtain for flat wCDM, another dark energy model described by our parametrization, the constraint on the equation of state parameter −1.22 < w < −0.78 at more than 99% confidence level.