arXiv subjects

A. Hernández-Almada

Publications and source records attributed to A. Hernández-Almada.

At least 19 recordsLinked to original sources

arXiv2026

Constraining a model supported in Moiré gravity with a recent data release

Moiré theory is a revolutionary alternative arising from solid-state physics that can be extended to gravity, forming a framework called Moiré gravity. The idea behind it is that Einsteinian gravity has a validity limit and must be extended to Moiré gravity, in which the Friedmann equation changes and a new scalar field emerges. In this vein, in this paper, we propose an analytic solution for the Moiré scalar field coupled with the modified Friedmann equation, studying the viability and constraining the value for the main parameter of the scalar field and cosmology in general, through the use of datasets that contain cosmic chronometers, type Ia supernovae, intermediate-luminosity quasars, and hydrogen II galaxies. Our results allow for an accelerated phase with two peaks, but also predict an extremely decelerated stage, with $q(z=0)=15.376^{+6.403}_{-4.692}$ when the characteristic parameter of the theory is $\jmath=0.677^{+0.001}_{-0.002}$, in contrast to other models that decelerate at $z=0$.

physics.gen-ph↗

arXiv2026

A comparative study of emergent dark energy models

Late (emergent) dark energy models that involve a hyperbolic tangent in their dynamic equations have achieved success due to their simplicity, the possibility of resolving the Hubble tension, and, in some cases their consistency with the Dark Energy Spectroscopic Instrument (DESI) results. Among the most studied are the Phenomenological and General Emergent Dark Energy Models (PEDE and GEDE, respectively), the variable curvature model, and the graduated dark energy model. Our goal is to include the curvature term in the models and perform a robust comparative study using diverse data samples, in particular, Cosmic Chronometers, Type Ia supernovae, Baryon Acoustic Oscillations from DESI-DR1, and Cosmic Microwave Background Radiation. This study presents the Universe age, the deceleration, the $w_{eff}$ parameter reconstruction, the redshift transition, and the best fit for the free parameters of each model. Our collected evidence from Akaike Information Criterion and Bayesian Information Criterion indicates that the best candidate is the PEDE model. Nevertheless, the variable-curvature model is the only model that exhibits a decelerated phase near $z=0$, consistent with the results from DESI.

astro-ph.CO↗

arXiv2026

Enhancing Gravitational Lens Study with Deep Learning: A Study on Effects of Dropout Regularization

Strong gravitational lensing provides valuable insights into the mass distribution of galaxies and the nature of dark matter. However, its modeling is computationally demanding due to the large volume of strong lensing observations. In this work, we explore the application of Convolutional Neural Networks to infer physical parameters from simulated galaxy-galaxy lens systems, described by the Singular Isothermal Ellipsoid (SIE) profile for the galaxy lens. We construct a dataset of 76,396 synthetic lensing images derived from the China Space Station Telescope catalog and employ it to train a modified CNN model, based on AlexNet architecture, to predict four key SIE parameters, Einstein radius, axis ratio and ellipticity components. We analyze the network performance under three distinct dropout configurations to quantify their influence on generalization and parameter inference accuracy. The results indicate that the incorporation of dropout is critical for enhancing the precision and robustness of the estimated parameters, as demonstrated using a 4-fold cross-validation procedure. When dropout tools are included we obtain yields coefficients of determination up to $R^2 \sim 0.96$ for most SIE parameters and mean Peak Signal-to-Noise Ratios of up to $\sim 37$ dB. Relative to the configuration without dropout, the use of dropout reduces the relative errors in the inferred SIE parameters by approximately $60-76\%$, resulting in errors of at most $\sim 9\%$ at the $90\%$ confidence level for the majority of parameters. These findings highlight the potential of deep learning approaches to enable scalable, computationally efficient, and high-precision modeling of strong gravitational lensing systems.

astro-ph.CO↗

arXiv2025

DESI and fast radio burst used to constrain modified theories of gravity

This paper is devoted to the study of the viability of DESI and the fast radio burst to constrain the free parameters of modified theories of gravity. Thus, we present a model supported in $f(R)$ gravity involving a function of the Ricci scalar named Starobinsky-type with the peculiarity that the non-commutative essence is intrinsic to the coefficients. Additionally, to understand the dynamics within a flat Friedmann-Lemaitre-Robertson-Walker universe, we explore the possibility of deriving a Friedman equation (in measure) that results from an adequate mathematical treatment. As we mentioned previously, to test the outlined model, a Monte Carlo Markov chain analysis is implemented, using cosmic chronometers, type Ia supernovae, Hydrogen II galaxies, Intermediate-luminosity quasars, Baryon Acoustic oscillations and Fast Radio Bursts data, to constraint the free parameters of the model and presenting $H(z)$, $q(z)$ and $ω_{eff}(z)$. The final results are compared with the $Λ$CDM model and a robust discussion is presented about the viability of DESI and fast radio burst to constraint free parameters in specific to a modified theory of gravity.

astro-ph.CO↗

arXiv2025

On a model of variable curvature that mimics the observed Universe acceleration

We present a new model based on General Relativity in where a subtle change of curvature at late times is able to produce the observed Universe acceleration and an oscillating behavior in the effective equation of state. This model aims to test the cosmological principle, by introducing a slight modification in the traditional FLRW metric, through a non-constant curvature parameter. This model is defined by a smooth step-like function with a slight transition between two curvature values, fulfilling the premise that the derivative of this curvature parameter is preserved as approximately zero, $\dotκ\approx0$. To test our model, we implemented a MCMC likelihood analysis using Cosmic Chronometers and Type Ia supernovae data in order to constrain the free parameters of the model and reconstruct $H(z)$, $q(z)$, $w_{eff}(z)$, also comparing the results with the $Λ$CDM model. The main result is that this model provides an alternative to the acceleration of the Universe without the need of a dark energy component. In particular, it gives an equivalent phase transition at $z \sim 0.5$, while obtaining the same fraction of matter density, similar to what is expected for the standard $Λ$CDM model. Remarkably, it also predicts a slight decelerated state at $z=0$ in agreement with diverse Dark Energy parameterizations. We conclude that the behavior of our proposed model points towards a new and intriguing way to investigate slight violations to the cosmological principle, in particular the case of inhomogenities during low phase transitions.

astro-ph.CO↗

arXiv2024

Phenomenological emergent dark energy in the light of DESI Data Release 1

This manuscript revisits the phenomenological emergent dark energy model (PEDE) by confronting it with recent cosmological data from early and late times. In particular we analyze PEDE model by using the baryon acoustic oscillation (BAO) measurements coming from both Dark Energy Spectroscopy Instrument (DESI) data release 1 and Sloan Digital Sky Survey (SDSS). Additionally, the measurements from cosmic chronometers, supernovae type Ia (Pantheon+), quasars, hydrogen II galaxies and cosmic background radiation distance priors are considered. By performing a Bayesian analysis based on Monte Carlo Markov Chain, we find consistent results on the constraints when SDSS and DESI are considered. However, we find higher values on the Hubble constant than Supernova $H_0$ for the Equation of State (SH0ES) does although it is still in agreement, within $1σ$ confidence level, when BAO measurements are added. Furthermore, we estimate the age of the Universe younger $\sim3\%$ than the one predicted by the standard cosmology. Additionally, we report values of $q_0 = -0.771^{+0.007}_{-0.007}$, $z_T = 0.764^{+0.011}_{-0.011}$ for the deceleration parameter today and the deceleration-acceleration transition redshift, respectively. However, PEDE cosmology is disfavoured by the combined samples.

astro-ph.CO↗

arXiv2023

MeV dark energy emission from a de Sitter Universe

The evolution of a de Sitter Universe is the base for both the accelerated Universe and the late stationary Universe. So how do we differentiate between both universes? In this paper, we state that it is not possible to design an experiment using luminous or angular distances to distinguish between both cases because they are the same during the de Sitter phase. However, this equivalence allows us to predict a signal of it a constant dark energy emission with a signal peak around 29.5 MeV, in where according to our astrophysical test of survival probability, the radiation must be non-standard photons. Remarkably, experiments beyond EGRET and COMPTEL could observe an excess of gamma photons in this predicted region, coming from a possible decay process of the dark energy emission, which might constitute the possible smoking gun of a late stationary Universe with the continuous creation of non-standard radiation, an alternative approach to understand the current stages of the Universe evolution.

astro-ph.CO↗

arXiv2023

Constraining uber gravity with recent observations and studying the $H_0$ problem

This paper studies both $Λ$CDM and CDM models under the über gravity theory, named ü$Λ$CDM and üCDM respectively. We report bounds over their parameter phase-space using several cosmological data, in particular, the recent Pantheon+ sample. Based on the joint analysis, the best fit value of the über characteristic parameter is $z_\oplus = 0.046^{+0.047}_{-0.032}$ and $z_\oplus = 1.382^{+0.020}_{-0.021}$ at 68\% confidence level for ü$Λ$CDM and üCDM respectively. Although über gravity can successfully mimics the cosmological constant, we find that the $\mathbb{H}0(z)$ diagnostic suggests the $H_0$ tension is not alleviated. Finally, both models are statistically compared with $Λ$CDM through the Akaike and Bayesian information criteria. Both über gravity models and $Λ$CDM are equally preferred for most of the single samples, in particular, ü$Λ$CDM is not rejected by the CMB data. However, there is strong evidence against them for the joint analysis.

astro-ph.CO↗

arXiv2023

An exponential equation of state of dark energy in the light of 2018 CMB Planck data

The dynamics of the Universe is analyzed using an exponential function for the dark energy equation of state, known as Gong-Zhang parameterization. The phase space of the free parameters presented in the model is constrained using Cosmic Microwave Background radiation, Cosmic Chronometers, modulus distance from Hydrogen II Galaxies, Type Ia Supernovae and measurements from Baryon Acoustic Oscillations, together with a stronger bound from a Joint analysis. The cosmological model is confronted with $Λ$CDM, observing there is a strong evidence for $Λ$CDM in the Joint analysis although the exponential model is preferred when the data are separated. Based on the Joint analysis, a value of $ω_0 = -1.202^{+0.027}_{-0.026}$ is found for the characteristic parameter presented in the equation of state. Additionally, the cosmographic parameters at current times are reported, having $q_0 = -0.789^{+0.034}_{-0.036}$, $j_0=1.779^{+0.130}_{-0.119}$, and a transition deceleration-acceleration redshift $z_T = 0.644^{+0.011}_{-0.012}$. Furthermore, the age of the Universe is estimated as $t_U = 13.788^{+0.019}_{-0.019}$ Gyrs. Finally, we open a discussion if this model could alleviate the $H_0$ and $S_8$ tensions.

astro-ph.CO↗

arXiv2022

Cosmology under the fractional calculus approach

Fractional cosmology modifies the standard derivative to Caputo's fractional derivative of order $μ$, generating changes in General Relativity. Friedmann equations are modified, and the evolution of the species densities depends on $μ$ and the age of the Universe $t_U$. We estimate stringent constraints on $μ$ using cosmic chronometers, Type Ia supernovae, and joint analysis. We obtain $μ=2.839^{+0.117}_{-0.193}$ within the $1σ$ confidence level providing a non-standard cosmic acceleration at late times; consequently, the Universe would be older than the standard estimations. Additionally, we present a stability analysis for different $μ$ values. This analysis identifies a late-time attractor corresponding to a power-law decelerated solution for $μ< 2$. Moreover, a non-relativistic critical point exists for $μ> 1$ and a sink for $μ> 2$. This solution is a decelerated power-law if $1 < μ< 2$ and an accelerated power-law solution if $μ> 2$, consistent with the mean values obtained from the observational analysis. Therefore, for both flat FLRW and Bianchi I metrics, the modified Friedmann equations provide a late cosmic acceleration under this paradigm without introducing a dark energy component. This approach could be a new path to tackling unsolved cosmological problems.

gr-qc↗

arXiv2022

Observational constraints and dynamical analysis of Kaniadakis horizon-entropy cosmology

We study the scenario of Kanadiakis horizon entropy cosmology which arises from the application of the gravity-thermodynamics conjecture using the Kaniadakis modified entropy. The resulting modified Friedmann equations contain extra terms that constitute an effective dark energy sector. We use data from Cosmic chronometers, Supernova Type Ia, HII galaxies, Strong lensing systems, and Baryon acoustic oscillations observations and we apply a Bayesian Markov Chain Monte Carlo analysis to construct the likelihood contours for the model parameters. We find that the Kaniadakis parameter is constrained around 0, namely, around the value where the standard Bekenstein-Hawking is recovered. Concerning the normalized Hubble parameter, we find $h=0.708^{+0.012}_{-0.011}$, a result that is independently verified by applying the $\mathbf{\mathbb{H}}0(z)$ diagnostic and, thus, we conclude that the scenario at hand can alleviate the $H_0$ tension problem. Regarding the transition redshift, the reconstruction of the cosmographic parameters gives $z_T=0.715^{+0.042}_{-0.041}$. Furthermore, we apply the AICc, BIC and DIC information criteria and we find that in most datasets the scenario is statistical equivalent to $Λ$CDM one. Moreover, we examine the Big Bang Nucleosynthesis (BBN) and we show that the scenario satisfies the corresponding requirements. Additionally, we perform a phase-space analysis, and we show that the Universe past attractor is the matter-dominated epoch, while at late times the Universe results in the dark-energy-dominated solution. Finally, we show that Kanadiakis horizon entropy cosmology accepts heteroclinic sequences, but it cannot exhibit bounce and turnaround solutions.

astro-ph.CO↗

arXiv2022

Kaniadakis holographic dark energy: observational constraints and global dynamics

We investigate Kaniadakis-holographic dark energy by confronting it with observations. We perform a Markov Chain Monte Carlo analysis using cosmic chronometers, supernovae type Ia, and Baryon Acoustic Oscillations data. Concerning the Kaniadakis parameter, we find that it is constrained around zero, namely around the value in which Kaniadakis entropy recovers standard Bekenstein-Hawking one. Additionally, for the present matter density parameter $Ω_m^{(0)}$, we obtain a value slightly smaller compared to $Λ$CDM scenario. Furthermore, we reconstruct the evolution of the Hubble, deceleration and jerk parameters extracting the deceleration-acceleration transition redshift as $z_T = 0.86^{+0.21}_{-0.14}$. Finally, performing a detailed local and global dynamical system analysis, we find that the past attractor of the Universe is the matter-dominated solution, while the late-time stable solution is the dark-energy-dominated one.

astro-ph.CO↗

arXiv2021

Barrow Entropy Cosmology: an observational approach with a hint of stability analysis

In this work, we use an observational approach and dynamical system analysis to study the cosmological model recently proposed by Saridakis (2020), which is based on the modification of the entropy-area black hole relation proposed by Barrow (2020). The Friedmann equations governing the dynamics of the Universe under this entropy modification can be calculated through the gravity-thermodynamics conjecture. We investigate two models, one considering only a matter component and the other including matter and radiation, which have new terms compared to the standard model sourcing the late cosmic acceleration. A Bayesian analysis is performed in which we use five cosmological observations (observational Hubble data, type Ia supernovae, HII galaxies, strong lensing systems, and baryon acoustic oscillations) to constrain the free parameters of both models. From a joint analysis, we obtain constraints that are consistent with the standard cosmological paradigm within $2σ$ confidence level. In addition, a complementary dynamical system analysis using local and global variables is developed which allows obtaining a qualitative description of the cosmology. As expected, we found that the dynamical equations have a de Sitter solution at late times.

astro-ph.CO↗

arXiv2021

Linear perturbations spectra for dynamical dark energy inspired by modified gravity

In this paper, we study a particular modified gravity Equation of State, the so-called Jaime-Jaber-Escamilla, that emerges from the first gravity modified action principle and can reproduce three cosmological viable $f(R)$ theories: the Starobinsky, Hu-Sawicki, and Exponential models . This EoS is a suitable candidate to reproduce the dynamical dark energy behaviour already reconstructed by the current data sets. Based on the joint statistical analysis, we found that our results are still in good agreement (within $1σ$) with the $Λ$CDM, while at perturbative level we notice that the matter power spectrum normalisation factor $σ_8$ shows an agreement with SDSS and SNeIa+IRAS at 1-$σ$ for the Starobinsky model and with SDSS-vec for the Hu \& Sawicki and Exponential models. Furthermore, we found that for the $H_0$ values, Starobinsky and Hu \& Sawicki show the least tension in comparison with PL18 TT. All these aspects cannot be observed \textit{directly} from other alternatives theories, were a equation of state is difficult to compute analytically.

gr-qc↗

arXiv2021

The Universe acceleration from the Unimodular gravity view point: Background and linear perturbations

With the goal of studying the cosmological constant (CC) problem, we present an exhaustive analysis of unimodular gravity as a possible candidate to resolve the CC origin and with this, the current Universe acceleration. In this theory, a correction constant (CC-like) in the field equations sources the late cosmic acceleration. This constant is related to a new parameter, $z_{ini}$, which is interpreted as the redshift of CC-like emergence. By comparing with the CC value obtained from Planck and Supernovaes measurements, it is possible to estimate $z_{ini}=11.15^{+0.01}_{-0.02}$ and $z_{ini}=11.43^{+0.03}_{-0.06}$ respectively, which is close to the reionization epoch. Moreover, we use the observational Hubble data (OHD), Type Ia Supernovae (SnIa), Baryon Acoustic Oscillations (BAO) and the Cosmic Microwave Background Radiation (CMB) distance data to constrain the UG cosmological parameters. A Joint analysis (OHD+SnIa+BAO+CMB), results in $z_{ini}=11.47^{+0.074}_{-0.073}$ within $1σ$ confidence level consistent with our estimation from Planck and Supernovae measurements. We also include linear perturbations, starting with scalar and tensor perturbations and complementing with the perturbed Boltzmann equation for photons. We show that the $00$ term in the UG field equations and the Boltzmann equation for photons contains corrections, meanwhile the other equations are similar as those obtained in standard cosmology.

astro-ph.CO↗

arXiv2021

Taxonomy of Dark Energy Models

The accelerated expansion of the Universe is one of the main discoveries of the past decades, indicating the presence of an unknown component: the dark energy. Evidence of its presence is being gathered by a succession of observational experiments with increasing precision in its measurements. However, the most accepted model for explaining the dynamic of our Universe, the so-called Lambda cold dark matter, face several problems related to the nature of such energy component. This has lead to a growing exploration of alternative models attempting to solve those drawbacks. In this review, we briefly summarize the characteristics of a (non-exhaustive) list of dark energy models as well as some of the most used cosmological samples. Next, we discuss how to constrain each model's parameters using observational data. Finally, we summarize the status of dark energy modeling.

astro-ph.CO↗

arXiv2021

A hybrid model of viscous and Chaplygin gas to tackle the Universe acceleration

Motivated by two seminal models proposed to explain the Universe acceleration, this paper is devoted to study a hybrid model which is constructed through a generalized Chaplygin gas with the addition of a bulk viscosity. We call the model a Viscous Generalized Chaplygin Gas (VGCG) and its free parameters are constrained through several cosmological data like the Observational Hubble Parameter, Type Ia Supernovae, Baryon Acoustic Oscillations, Strong Lensing Systems, HII Galaxies and using Joint Bayesian analysis. In addition, we implement a Om-diagnostic to analyze the VGCC dynamics and its difference with the standard cosmological model. The hybrid model shows important differences when compared with the standard cosmological model. Finally, based on our Joint analysis we find that the VGCG could be an interesting candidate to alleviate the well-known Hubble constant tension.

astro-ph.CO↗

arXiv2021

Einstein-Gauss-Bonnet gravity: is it compatible with modern cosmology?

Einstein-Gauss-Bonnet (EGB) model is recently restudied in order to analyze new consequences in gravitation, modifying appropriately the Einstein-Hilbert action. The consequences in EGB cosmology are mainly geometric, with higher order values in the Hubble parameter. In this vein this paper is devoted to contribute with extra evidences about its pros and cons of the model from a cosmological point of view. We start constraining the characteristic parameter, $α$, of the EGB model when a cosmological constant as the catalyst for the acceleration is considered. The constrictions are developed at the background cosmology using Observational Hubble Data, Baryon Acoustic Oscillations, Supernovaes of the Ia type, Strong Lensing Systems and the recent compilation of HII Galaxies. Additionally, we implement a statefinder analysis where we found not only a late acceleration but also an early Universe acceleration which is associated with the parameter $α$. Based on our results of the lensing systems, the Universe evolution never reaches a non accelerated phase, instead it is always presented in an accelerated state, being a possible fault that afflicts the EGB model.

astro-ph.CO↗