Transition redshift: new constraints from parametric and nonparametric methods

In this paper, we use the cosmokinematics approach to study the accelerated expansion of the Universe. This is a model independent approach and depends only on the assumption that the Universe is homogeneous and isotropic and is described by the FRW metric. We parametrize the deceleration parameter, q ( z ), to constrain the transition redshift ( z t ) at which the expansion of the Universe goes from a decelerating to an accelerating phase. We use three different parametrizations of q ( z ) namely, q I ( z )= q 1 + q 2 z , q II ( z ) = q 3 + q 4 ln (1 + z ) and q III ( z )=½+ q 5 /(1+ z ) 2 . A joint analysis of the age of galaxies, strong lensing and supernovae Ia data indicates that the transition redshift is less than unity i.e. z t < 1. We also use a nonparametric approach (LOESS+SIMEX) to constrain z t . This too gives z t < 1 which is consistent with the value obtained by the parametric approach.