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 i ) and present ( q 0 ) values of the deceleration parameter, the redshift of the cosmic transition from deceleration to acceleration ( z t ) and the redshift width of such transition (τ). By assuming a flat space geometry, q 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 t = 0.56 +0.13 −0.10 , τ = 0.47 +0.16 −0.20 and q 0 = −0.31 +0.11 −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 t = 0.64 +0.13 −0.07 , τ = 0.36 +0.11 −0.17 and q 0 = −0.53 +0.17 −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 ΛCDM-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.