Reconstructing the cosmic expansion with a generalized q(z) parameterization: A decelerating Universe from late-time constraints

Abstract

We present a generalized phenomenological parameterization of the deceleration parameter $q(z)$ that incorporates an effective radiative component (ERC) in addition to a localized late-time contribution. The proposed framework extends previous two-parameter $q(z)$ reconstructions by explicitly regulating the high-redshift behavior while preserving the late-time transition dynamics. We constrain the free parameters $(h,q_0,z_c,z_e)$ using late-time observational data from cosmic chronometers (CC), Pantheon+ Type Ia supernovae (SNIa), H\,\textsc{ii} galaxies (HIIG), and intermediate-luminosity quasars (QSO). For the full data combination (CC+SNIa+HIIG+QSO), we obtain $q_0=-0.25_{-0.04}^{+0.04}$ and a transition redshift $z_T\simeq 0.80$, indicating a currently accelerating Universe with a transition occurring earlier than in the $\hat{I}›$CDM model. Within the redshift range probed by the data, the reconstructed $q(z)$ deviates from the $\hat{I}›$CDM trend, suggesting a possible reduction of the late-time acceleration. Furthermore, the reconstruction favors a relatively high value of the Hubble parameter, $h=0.729\pm 0.006$. The ERC remains weakly constrained by late-time data but ensures a smooth and monotonic evolution of $q(z)$, $j(z)$, and $w_{\mathrm{eff}}(z)$ across a wide redshift range. Within the observed interval, the model effectively reproduces the late-time behavior of the previous parametrization, while providing a controlled extension toward early epochs. Our results show that current low- and intermediate-redshift data are compatible with a reduced late-time acceleration.

Accepted for publication in The European Physical Journal C