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

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, q₀, zc, zₑ) ( h , q 0 , z c , z e ) using late-time observational data from cosmic chronometers (CC), Pantheon+ Type Ia supernovae (SNIa), H ii galaxies (HIIG), and intermediate-luminosity quasars (QSO). For the full data combination (CC+SNIa+HIIG+QSO), we obtain q₀ = -0.25+0.04-0.04 q 0 = - 0 . 25 - 0.04 + 0.04 and a transition redshift zT 0.80 z T ≃ 0.80 , indicating a currently accelerating Universe with a transition occurring earlier than in the Λ Λ CDM model. Within the redshift range probed by the data, the reconstructed q ( z ) deviates from the Λ Λ 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 ± 0.006 h = 0.729 ± 0.006 . The ERC remains weakly constrained by late-time data but ensures a smooth and monotonic evolution of q ( z ), j ( z ), and wₑff(z) w 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.