From cosmic deceleration to acceleration: new constraints from SN Ia and BAO/CMB

We use type Ia supernovae (SN Ia) data in combination with recent baryonic acoustic oscillations (BAO) and cosmic microwave background (CMB) observations to constrain a kink-like parametrization of the deceleration parameter (q). This q-parametrization can be written in terms of the initial (q sub(i)) and present (q sub(0)) values of the deceleration parameter, the redshift of the cosmic transition from deceleration to acceleration (z sub(t)) and the redshift width of such transition ([tau]). By assuming a flat space geometry, q sub(i) = 1/2 and adopting a likelihood approach to deal with the SN Ia data we obtain, at the 68% confidence level (C.L.), that: z sub(t) = 0.56 super(+0.13) sub(-0.10), [tau] = 0.47 super(+0.16) sub(-0.20) and q sub(0) = -0.31 super(+0.11) sub(-0.11) when we combine BAO/CMB observations with SN Ia data processed with the MLCS2k2 light-curve fitter. When in this combination we use the SALT2 fitter we get instead, at the same C.L.: z sub(t) = 0.64 super(+0.13) sub(-0.07), [tau] = 0.36 super(+0.11) sub(-0.17) and q sub(0) = -0.53 super(+0.17) sub(-0.13). Our results indicate, with a quite general and model independent approach, that MLCS2k2 favors Dvali-Gabadadze-Porrati-like cosmological models, while SALT2 favors [Lambda]CDM-like ones. Progress in determining the transition redshift and/or the present value of the deceleration parameter depends crucially on solving the issue of the difference obtained when using these two light-curve fitters.