Constraining 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), which considers a transition, at $z_t$, from cosmic deceleration to acceleration. In this parametrization the initial, at $z \gg z_t$, value of the q-parameter is $q_i$, its final, z=-1, value is $q_f$ 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, BAO, 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 $H_0$, 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 and Planck. 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, $q_i=1/2$ and expressing the present value of the deceleration parameter $q_0$ as a function of the other three free parameters, we obtain $z_t=0.67^{+0.10}_{-0.08}$, $τ=0.26^{+0.14}_{-0.10}$ and $q_0=-0.48^{+0.11}_{-0.13}$, at 68\% of confidence level, with an uniform prior over $H_0$. If in addition we fix $q_f=-1$, as in flat $Λ$CDM, DGP and Chaplygin quartessence that are special models described by our parametrization, we get $z_t=0.66^{+0.03}_{-0.04}$, $τ=0.33^{+0.04}_{-0.04}$ and $q_0=-0.54^{+0.05}_{-0.07}$, in excellent agreement with flat $Λ$CDM 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.

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