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 $$(h, q_0, z_c, z_e)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>h</mml:mi> <mml:mo>,</mml:mo> <mml:msub> <mml:mi>q</mml:mi> <mml:mn>0</mml:mn> </mml:msub> <mml:mo>,</mml:mo> <mml:msub> <mml:mi>z</mml:mi> <mml:mi>c</mml:mi> </mml:msub> <mml:mo>,</mml:mo> <mml:msub> <mml:mi>z</mml:mi> <mml:mi>e</mml:mi> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> 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 $$q_0 = -0.25^{+0.04}_{-0.04}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>q</mml:mi> <mml:mn>0</mml:mn> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>-</mml:mo> <mml:mn>0</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mn>25</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> and a transition redshift $$z_T \simeq 0.80$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>z</mml:mi> <mml:mi>T</mml:mi> </mml:msub> <mml:mo>≃</mml:mo> <mml:mn>0.80</mml:mn> </mml:mrow> </mml:math> , indicating a currently accelerating Universe with a transition occurring earlier than in the $$\Lambda $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Λ</mml:mi> </mml:math> CDM model. Within the redshift range probed by the data, the reconstructed q ( z ) deviates from the $$\Lambda $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Λ</mml:mi> </mml:math> CDM trend, suggesting a possible reduction of the late-time acceleration. Furthermore, the reconstruction favors a relatively high value of the Hubble parameter, $$h = 0.729 \pm 0.006$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>h</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.729</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.006</mml:mn> </mml:mrow> </mml:math> . The ERC remains weakly constrained by late-time data but ensures a smooth and monotonic evolution of q ( z ), j ( z ), and $$w_\textrm{eff}(z)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>w</mml:mi> <mml:mtext>eff</mml:mtext> </mml:msub> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>z</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> 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.