arXiv subjects

Alberto Hernández-Almada

Publications and source records attributed to Alberto Hernández-Almada.

5 recordsLinked to original sources

arXiv2026

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 $(h, q_0, z_c, z_e)$ 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 $Λ$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, $h = 0.729 \pm 0.006$. The ERC remains weakly constrained by late-time data but ensures a smooth and monotonic evolution of $q(z)$, $j(z)$, and $w_{\rm eff}(z)$ 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.

astro-ph.CO↗

arXiv2026

Late-Time Cosmic Acceleration from QCD Confinement Dynamics

We explore a phenomenological extension of the Polyakov-Nambu-Jona-Lasinio (PNJL) model by introducing a curvature-sensitive effective contribution to the Polyakov loop potential, motivated by the hypothesis that the non-perturbative QCD vacuum in the confined phase may retain a residual sensitivity to cosmic expansion. In a spatially flat FLRW background, this modification reduces to a term proportional to $α(H/H_0)^d f(Φ, Φ^*)$, which naturally vanishes in the deconfined regime and behaves as an effective dynamical vacuum component at late times, without invoking a fundamental cosmological constant. The construction provides an effective thermodynamic description of the QCD sector within an adiabatic framework and introduces a minimal phenomenological extension characterized by the exponent $d$ and the amplitude parameter $α$. We analyze the cosmological implications at the background level and confront the model with low-redshift observations, including cosmic chronometers, Type Ia supernovae, HII galaxies, and quasars. Using Bayesian Monte Carlo techniques, we constrain the model parameters and compare its performance with $Λ$CDM. Our results indicate that the modified PNJL cosmology provides a statistically competitive fit to current data while allowing small departures from $Λ$CDM within observational uncertainties. We also investigate the impact of the coupling on the QCD phase diagram and the critical end point. The framework offers a tractable effective approach to connect confinement physics with late-time cosmology and suggests directions for further theoretical development in QCD under curved backgrounds.

physics.gen-ph↗

arXiv2024

ΛCDM-Rastall cosmology revisited: constraints from a recent Quasars datasample

In this paper we study the impact of a recent quasar datasample in the constraint of the free parameters of an extension of general relativity. As a ruler to test, we use Rastall gravity in the context of background cosmology being a simple extension to general relativity. We compare the results from quasars dataset with other known samples such as cosmic chronometers, supernovae of the Ia type, baryon acoustic oscillations, HII galaxies, and also a joint analysis. Results are consistent with the standard cosmological model emphasizing that Rastall gravity is equivalent to General Relativity. According to the constraints provided from the joint sample, the age of the Universe is $τ_U = 12.601^{+0.067}_{-0.066}$ Gyrs and the transition to an accelerated phase occurs at $z_T=0.620\pm0.025$ in the redshift scale, being only the phase transition consistent with the standard paradigm and having a younger Universe. With the quasars sample, the universe age differs with that expected in $Λ$CDM having a result of $τ_U = 11.958^{+0.139}_{-0.109}$ Gyrs with a transition at $z_T=0.652\pm0.032$ this last consistent with standard cosmology. A remarkable result is that quasars constraints has the capability to differentiate among general relativity and Rastall gravity due to the result for the parameter $λ=-2.231^{+0.785}_{-0.546}$. Moreover, the parameter $j$ under quasars constraints suggests that the cause of the late universe's acceleration is a dark energy fluid different from a cosmological constant.

astro-ph.CO↗

arXiv2024

Synchronize your \textit{chrono-brane}: Testing a variable brane tension model with strong gravitational lensing

Brane world models have shown to be promising to understand the late cosmic acceleration, in particular because such acceleration can be naturally derived, mimicking the dark energy behaviour just with a five dimensional geometry. In this paper we present a strong lensing joint analysis using a compilation of early-type galaxies acting as a lenses, united with the power of the well studied strong lensing galaxy cluster Abell\,1689. We use the strong lensing constraints to investigate a brane model with variable brane tension as a function of the redshift. In our joint analysis we found a value $n = 7.8^{+0.9}_{-0.5}$, for the exponent related to the brane tension, showing that $n$ deviates from a Cosmological Constant (CC) scenario (n=6). We obtain a value for the deceleration parameter, $q(z)$ today, $q(0)=-1.2^{+0.6}_{-0.8}$, and a transition redshift, $z_t=0.60\pm0.06$ (when the Universe change from an decelerated phase to an accelerated one). These results are in contrast with previous work that favors CC scenario, nevertheless our lensing analysis is in agreement with a formerly reported conclusion suggesting that the variable brane tension model is able to source a late cosmic acceleration without an extra fluid as in the standard one.

astro-ph.CO↗

arXiv2023

Cosmology under the fractional calculus approach: a possible $H_0$ tension resolution?

Recently, a new field of study called fractional cosmology has emerged. It uses fractional calculus to modify the standard derivative equations and change the Friedmann equations. The evolution of cosmic species densities is also affected by the $μ$ fractional parameter and the age of the Universe $t_0$. This new approach to cosmology modifies the Friedmann equations and allows for a late cosmic acceleration without the need for a dark energy component. This could be a breakthrough in solving longstanding problems in cosmology. By analyzing observational Hubble data and Type Ia supernovae, we have been able to place strict constraints on the fractional and cosmological parameters. Our results suggest that the Universe may be older than previously estimated. We also explore whether fractional cosmology can help resolve the $H_0$ tension.

gr-qc↗