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 $$(h, q_0, z_c, z_e)$$ ( 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 $$q_0 = -0.25^{+0.04}_{-0.04}$$ q 0 = - 0 . 25 - 0.04 + 0.04 and a transition redshift $$z_T \simeq 0.80$$ z T ≃ 0.80 , indicating a currently accelerating Universe with a transition occurring earlier than in the $$\Lambda $$ Λ CDM model. Within the redshift range probed by the data, the reconstructed q ( z ) deviates from the $$\Lambda $$ Λ CDM trend, suggesting a possible reduction of the late-time acceleration. Furthermore, the reconstruction favors a relatively high value of the Hubble parameter, $$h = 0.729 \pm 0.006$$ 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 $$w_\textrm{eff}(z)$$ 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.