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

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Title: Reconstructing the cosmic expansion with a generalized q(z) parameterization: a decelerating Universe from late-time constraints.
Authors: Verdugo, Tomás1 (AUTHOR) tomasv@astro.unam.mx, Hernández-Almada, Alberto2 (AUTHOR) ahalmada@uaq.mx, García-Aspeitia, Miguel A.3 (AUTHOR) angel.garcia@ibero.mx, Magaña, Juan4 (AUTHOR) juan.maga@ucentral.cl, Motta, Verónica5 (AUTHOR) veronica.motta@uv.cl
Source: European Physical Journal C -- Particles & Fields. May2026, Vol. 86 Issue 5, p1-13. 13p.
Subjects: Parameterization, Hubble constant, Redshift, Expanding universe, Physical cosmology, Universe, Metaphysical cosmology
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 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 ≃ 0.80 , indicating a currently accelerating Universe with a transition occurring earlier than in the Λ CDM model. Within the redshift range probed by the data, the reconstructed q(z) deviates from the 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 . The ERC remains weakly constrained by late-time data but ensures a smooth and monotonic evolution of q(z), j(z), and 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. [ABSTRACT FROM AUTHOR]
Copyright of European Physical Journal C -- Particles & Fields is the property of Springer Nature and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Reconstructing the cosmic expansion with a generalized q(z) parameterization: a decelerating Universe from late-time constraints.
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  Data: <searchLink fieldCode="AR" term="%22Verdugo%2C+Tomás%22">Verdugo, Tomás</searchLink><relatesTo>1</relatesTo> (AUTHOR)<i> tomasv@astro.unam.mx</i><br /><searchLink fieldCode="AR" term="%22Hernández-Almada%2C+Alberto%22">Hernández-Almada, Alberto</searchLink><relatesTo>2</relatesTo> (AUTHOR)<i> ahalmada@uaq.mx</i><br /><searchLink fieldCode="AR" term="%22García-Aspeitia%2C+Miguel+A%2E%22">García-Aspeitia, Miguel A.</searchLink><relatesTo>3</relatesTo> (AUTHOR)<i> angel.garcia@ibero.mx</i><br /><searchLink fieldCode="AR" term="%22Magaña%2C+Juan%22">Magaña, Juan</searchLink><relatesTo>4</relatesTo> (AUTHOR)<i> juan.maga@ucentral.cl</i><br /><searchLink fieldCode="AR" term="%22Motta%2C+Verónica%22">Motta, Verónica</searchLink><relatesTo>5</relatesTo> (AUTHOR)<i> veronica.motta@uv.cl</i>
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  Data: <searchLink fieldCode="JN" term="%22European+Physical+Journal+C+--+Particles+%26+Fields%22">European Physical Journal C -- Particles & Fields</searchLink>. May2026, Vol. 86 Issue 5, p1-13. 13p.
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  Data: <searchLink fieldCode="DE" term="%22Parameterization%22">Parameterization</searchLink><br /><searchLink fieldCode="DE" term="%22Hubble+constant%22">Hubble constant</searchLink><br /><searchLink fieldCode="DE" term="%22Redshift%22">Redshift</searchLink><br /><searchLink fieldCode="DE" term="%22Expanding+universe%22">Expanding universe</searchLink><br /><searchLink fieldCode="DE" term="%22Physical+cosmology%22">Physical cosmology</searchLink><br /><searchLink fieldCode="DE" term="%22Universe%22">Universe</searchLink><br /><searchLink fieldCode="DE" term="%22Metaphysical+cosmology%22">Metaphysical cosmology</searchLink>
– Name: Abstract
  Label: Abstract
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  Data: 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 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 ≃ 0.80 , indicating a currently accelerating Universe with a transition occurring earlier than in the Λ CDM model. Within the redshift range probed by the data, the reconstructed q(z) deviates from the 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 . The ERC remains weakly constrained by late-time data but ensures a smooth and monotonic evolution of q(z), j(z), and 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. [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of European Physical Journal C -- Particles & Fields is the property of Springer Nature and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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        Value: 10.1140/epjc/s10052-026-15766-1
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      – SubjectFull: Redshift
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      – SubjectFull: Physical cosmology
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      – SubjectFull: Universe
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      – SubjectFull: Metaphysical cosmology
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      – TitleFull: Reconstructing the cosmic expansion with a generalized q(z) parameterization: a decelerating Universe from late-time constraints.
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              Text: May2026
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