Transition Redshift: New constraints from parametric and nonparametric methods

TL;DR

This study employs parametric and nonparametric methods to constrain the transition redshift where the universe shifted from decelerating to accelerating expansion, finding it to be less than one.

Contribution

It introduces a model-independent cosmokinematics approach using multiple parametrizations and nonparametric methods to accurately estimate the transition redshift.

Findings

Transition redshift $z_t<1$ from parametric analysis

Consistent $z_t<1$ result from nonparametric approach

Supports accelerated expansion onset at redshift below one

Abstract

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, $q_\I(z)=q_{\textnormal{\tiny\textsc{1}}}+q_{\textnormal{\tiny\textsc{2}}}z$, $q_\II (z) = q_\3 + q_\4 \ln (1 + z)$ and $q_\III(z)=\frac{1}{2}+\frac{q_{\textnormal{\tiny\textsc{5}}}}{(1+z)^2}$. 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.