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Esteban González

Publications and source records attributed to Esteban González.

At least 19 recordsLinked to original sources
arXiv2026

Dissipative Unimodular Gravity: Linking Energy Diffusion to Bulk Viscosity as an Alternative to $Λ$CDM under DESI DR2 Data

In this paper, we perform a theoretical and observational study of the presence of viscosity in the Unimodular Gravity formalism, a pioneering approach that, to the best of our knowledge, has not been previously considered within the present context. Specifically, we study a flat FLRW universe at late times, where matter experiences dissipative processes in the form of a bulk viscosity, in the framework of Eckart's theory, which is linked to the energy diffusion function $Q$ through the power law $ξ=ξ_{0}\left|Q\right|^{1/2}$, being $ξ_{0}$ a positive dimensionless parameter. By assuming the Ansatz $Q=νH^{2}$, where $H$ is the Hubble parameter and $ν$ is a dimensionless arbitrary constant, we find analytical solutions for the cosmological evolution. We test these models against the most recent cosmological observations, including type Ia supernovae, baryon acoustic oscillations, cosmic chronometers, gravitational lensing, and black hole shadow data. Our results show that two of the tested models provide a significantly better fit to the data ($χ_{\text{min}}^{2}$) and remain as competitive as the $Λ$CDM model according to the Bayesian Information Criterion. These findings, combined with the inherent ability of Unimodular Gravity to alleviate the cosmological constant problem, position dissipative UG as a robust and compelling alternative to the standard model, potentially suggesting that a very small but nontrivial energy nonconservation is compatible with the late-time observational data.

gr-qc↗
arXiv2026

Varying Gravity from a Modified Fractional Model: Observational Constraints and Slow-Fast Dynamics

We investigate a fractional gravity model in which both the Hubble parameter and the gravitational constant evolve dynamically due to fractional renormalization-group effects. The model incorporates a scalar field coupled to a time-varying $G$, generating nonlocal corrections characteristic of fractional--action cosmology. Analytical and numerical solutions reveal oscillatory regimes, cyclic phases, and rapid variations with implications for BBN and early-universe evolution. A robust numerical framework is developed to integrate the regularized system and compare the resulting $H(z)$ evolution with observational data from the Hubble parameter, baryon acoustic oscillations, type Ia supernovae, gravitational lensing, and black hole shadows, thereby enabling a consistent reconstruction of cosmographic quantities. A Bayesian analysis shows that the Fractional model with $μ=0$ is the only statistically viable variant. The inferred Hubble parameter is stable across models ($h\simeq 0.72$), while the fractional parameters are significantly better constrained in the $μ=0$ case ($α=1.20^{+0.25}_{-0.14}$, $ζ=0.43^{+0.39}_{-0.29}$). The dynamical sector yields $m=30.8^{+28.0}_{-20.9}$ and $Γ=108.3\pm1.1$, leading to a positive discriminant and a well-determined relaxation timescale $τ_{\rm rel}\simeq 9$ Gyr, confirming an overdamped regime. Although the $μ=0$ model attains a slightly lower $χ^2_{\min}$ than $Λ$CDM, the BIC strongly favors $Λ$CDM due to its smaller parameter space. Overall, the model reproduces late-time acceleration and mimics $Λ$CDM while introducing distinctive cosmographic signatures. The dynamical systems analysis clarifies the stability structure and parameter dependence, indicating that fractional nonlocal corrections may offer new pathways toward addressing the $H_0$ and $S_8$ tensions.

physics.gen-ph↗
arXiv2025

A Systematic Literature Review of Machine Learning Techniques for Observational Constraints in Cosmology

This paper presents a systematic literature review focusing on the application of machine learning techniques for deriving observational constraints in cosmology. The goal is to evaluate and synthesize existing research to identify effective methodologies, highlight gaps, and propose future research directions. Our review identifies several key findings: (1) various machine learning techniques, including Bayesian neural networks, Gaussian processes, and deep learning models, have been applied to cosmological data analysis, improving parameter estimation and handling large datasets. However, models achieving significant computational speedups often exhibit worse confidence regions compared to traditional methods, emphasizing the need for future research to enhance both efficiency and measurement precision. (2) Traditional cosmological methods, such as those using Type Ia Supernovae, baryon acoustic oscillations, and cosmic microwave background data, remain fundamental, but most studies focus narrowly on specific datasets. We recommend broader dataset usage to fully validate alternative cosmological models. (3) The reviewed studies mainly address the $H_0$ tension, leaving other cosmological challenges-such as the cosmological constant problem, warm dark matter, phantom dark energy, and others-unexplored. (4) Hybrid methodologies combining machine learning with Markov chain Monte Carlo offer promising results, particularly when machine learning techniques are used to solve differential equations, such as Einstein Boltzmann solvers, as prior to Markov chain Monte Carlo models, accelerating computations while maintaining precision. (5) There is a significant need for standardized evaluation criteria and methodologies, as variability in training processes and experimental setups complicates result comparability and reproducibility (abridged).

astro-ph.CO↗
arXiv2025

FIMP Dark Matter in bulk viscous non-standard cosmologies

In this paper, we revisit the extension of the classical non-standard cosmological model in which dissipative processes are considered through a bulk viscous term in the new field $ϕ$, which interacts with the radiation component during the early universe. Specifically, we consider an interaction term of the form $Γ_ϕ ρ_ϕ$, where $Γ_ϕ$ represents the decay rate of the field and $ρ_ϕ$ denotes its energy density and a bulk viscosity described by $ξ=ξ_{0}ρ_ϕ^{1/2}$, within the framework of Eckart's theory. This extended non-standard cosmology is employed to explore the parameter space for the production of Feebly Interacting Massive Particles (FIMPs) as Dark Matter candidates, assuming a constant thermal averaged Dark Matter production cross-section ($\langleσv\rangle$), as well as a preliminary analysis of the non-constant case. In particular, for certain combinations of the model and Dark Matter parameters, namely ($T_\text{end}$,$κ$) and $(m_χ,\langleσv\rangle)$, where $T_\text{end}$ corresponds to the temperature at which $ϕ$ decays, $κ$ is the ratio between the initial energy density of $ϕ$ and radiation, and $m_χ$ is the Dark Matter mass, we identify extensive new parameter regions where Dark Matter can be successfully established while reproducing the currently observed relic density, in contrast to the predictions of $Λ$CDM and classical non-standard cosmological scenarios.

hep-ph↗
arXiv2024

Fractional Einstein-Gauss-Bonnet scalar field cosmology

Our paper introduces a new theoretical framework called the Fractional Einstein--Gauss--Bonnet scalar field cosmology, which has important physical implications. Using fractional calculus to modify the gravitational action integral, we derived a modified Friedmann equation and a modified Klein--Gordon equation. Our research reveals non-trivial solutions associated with exponential potential, exponential couplings to the Gauss--Bonnet term, and a logarithmic scalar field, which are dependent on two cosmological parameters, $m$ and $α_{0}=t_{0}H_{0}$ and the fractional derivative order $μ$. By employing linear stability theory, we reveal the phase space structure and analyze the dynamic effects of the Gauss--Bonnet couplings. The scaling behavior at some equilibrium points reveals that the geometric corrections in the coupling to the Gauss--Bonnet scalar can mimic the behavior of the dark sector in modified gravity. Using data from cosmic chronometers, type Ia supernovae, supermassive Black Hole Shadows, and strong gravitational lensing, we estimated the values of $m$ and $α_{0}$, indicating that the solution is consistent with an accelerated expansion at late times with the values $α_0=1.38\pm 0.05$, $m=1.44\pm 0.05$, and $μ=1.48 \pm 0.17$ (consistent with $Ω_{m,0}=0.311\pm 0.016$ and $h=0.712\pm 0.007$), resulting in an age of the Universe $t_{0}=19.0\pm 0.7$ [Gyr] at 1$σ$ CL. Ultimately, we obtained late-time accelerating power-law solutions supported by the most recent cosmological data, and we proposed an alternative explanation for the origin of cosmic acceleration other than $Λ$CDM. Our results generalize and significantly improve previous achievements in the literature, highlighting the practical implications of fractional calculus in cosmology.

astro-ph.CO↗
arXiv2024

WIMP dark matter in bulk viscous non-standard cosmologies

In this paper, we explored an extension of the classical non-standard cosmological scenario in which the new field, $ϕ$, which interacts with the radiation component in the early universe, experiences dissipative processes in the form of a bulk viscosity. Assuming an interaction term given by $Γ_ϕρ_ϕ$, where $Γ_ϕ$ accounts for the decay rate of the field and $ρ_ϕ$ corresponds to its energy density, and a bulk viscosity according to the expression $ξ=ξ_{0}ρ_ϕ^{1/2}$ in the framework of Eckart's theory, we apply this novel non-standard cosmology to study the parameters space for WIMPs Dark Matter candidate production. This parameter space shows deviations from the classical non-standard cosmological scenario, obtaining new regions to search for this candidate. In particular, for certain combinations of the free parameters, we found large regions in which the model can establish the DM and reproduce the current observable relic density.

hep-ph↗
arXiv2024

Addressing the Hubble tension in Yukawa cosmology?

In Yukawa cosmology, a recent discovery revealed a relationship between baryonic matter and the dark sector. The relation is described by the parameter α and the long-range interaction parameter λ - an intrinsic property of the graviton. Applying the uncertainty relation to the graviton raises a compelling question: Is there a quantum mechanical limit to the measurement precision of the Hubble constant (H0)? We argue that the uncertainty relation for the graviton wavelength λ can be used to explain a running of H0 with redshift. We show that the uncertainty in time has an inverse correlation with the value of the Hubble constant. That means that the measurement of the Hubble constant is intrinsically linked to length scales (redshift) and is connected to the uncertainty in time. On cosmological scales, we found that the uncertainty in time is related to the look-back time quantity. For measurements with a high redshift value, there is more uncertainty in time, which leads to a smaller value for the Hubble constant. Conversely, there is less uncertainty in time for local measurements with a smaller redshift value, resulting in a higher value for the Hubble constant. Therefore, due to the uncertainty relation, the Hubble tension is believed to arise from fundamental limitations inherent in cosmological measurements. Finally, our findings indicate that the mass of the graviton fluctuates with specific scales, suggesting a possible mass-varying mechanism for the graviton.

astro-ph.CO↗
arXiv2024

Testing a nonlinear solution of the Israel-Stewart theory

In this work, we test the capability of an exact solution found in the framework of a nonlinear extension of the Israel-Stewart theory to fit the supernovae Ia, gravitational lensing, and black hole shadow data. This exact solution is a generalization of one previously found for a dissipative unified dark matter model in the context of the near-equilibrium description of dissipative processes, where we do not have the full regime of the nonlinear picture. This generalized solution is restricted to the case where a positive entropy production is guaranteed and is tested under the condition that ensures its causality, local existence, and uniqueness. From the observational constraints, we found that this generalized solution is a good candidate in the description of the observational late-time data used in this work, with best-fit values $H_{0}=73.2_{-0.9}^{+0.8}\,\frac{km/s}{Mpc}$, $q_{0}=-0.41_{-0.03}^{+0.03}$, $\hatξ_{0}=0.88_{-0.17}^{+0.09}$, $ε=0.34_{-0.04}^{+0.03}$, and $k=0.27_{-0.20}^{+0.37}$. Therefore, we show that the nonlinear regime of the Israel-Stewart theory consistently describes the recent accelerated expansion of the universe without the inclusion of some kind of dark energy component and also provides a more realistic description of the fluids that make up the late Universe.

gr-qc↗
arXiv2023

A non-singular early-time viscous cosmological model

In this paper, we study the thermodynamical and mathematical consistencies for a non-singular early-time viscous cosmological model known as soft-Big Bang, which was previously found in [N. Cruz, E. González, and J. Jovel, Phys. Rev. D \textbf{105}, 024047 (2022)]. This model represents a flat homogeneous and isotropic universe filled with a dissipative radiation fluid and a cosmological constant $Λ$, which is small but not negligible, in the framework of Eckart's theory. In particular, we discuss the capability of the solution in the fulfillment of the three following conditions: (i) the near equilibrium condition, which is assumed in Eckart's theory of non-perfect fluids, (ii) the mathematical stability of the solution under small perturbations, and (iii) the positiveness of the entropy production. We have found that this viscous model can describe the radiation domination era of the $Λ$CDM model and, at the same time, fulfill the three conditions mentioned by the fulfillment of a single constraint on the bulk viscous coefficient $ξ_{0}$, finding also that this non-singular model has a positive energy density in the infinity past which is infinity hotter with a constant entropy.

gr-qc↗
arXiv2023

A new Parametrization for Bulk Viscosity Cosmology as Extension of the $Λ$CDM Model

Bulk viscosity in cold dark matter is an appealing feature that introduces distinctive phenomenological effects in the cosmological setting as compared to the $Λ$CDM model. Under this view, we propose a general parametrization of the bulk viscosity of the form $ξ\sim H^{1-2s} ρ_{m}^{s}$, that covers intriguingly some well-known cases in the Eckart's theory. Some advantages of this novel parametrization are: first, it allows to write the resulting equations of cosmological evolution in the form of an autonomous system for any value of $s$, so a general treatment of the fixed points and stability can be done, and second, the bulk viscosity effect is consistently handled so that it naturally turns off when matter density vanishes. As a main result we find, based on detailed dynamical system analysis, one-parameter family of de-Sitter-like asymptotic solutions with non-vanishing bulk viscosity coefficient during different cosmological periods. Numerical computations are performed jointly along with analytical phase space analysis in order to assess more quantitatively the bulk viscosity effect on the cosmological background evolution. Finally, as a first contact with observation we derive constraints on the free parameters of some bulk viscosity models with specific $s$-exponents from Supernovae Ia and observations of the Hubble parameter, by performing a Bayesian statistical analysis thought the Markov Chain Monte Carlo method.

gr-qc↗
arXiv2023

Observational constraints on Yukawa cosmology and connection with black hole shadows

We confront Yukawa modified cosmology, proposed in arXiv:2304.11492 [Jusufi et al. arXiv:2304.11492], with data from Supernovae Type Ia (SNe Ia) and Hubble parameter (OHD) observations. Yukawa cosmology is obtained from a Yukawa-like gravitational potential, with coupling parameter $α$ and wavelength parameter $λ$, which gives rise to modified Friedmann equations. We show that the agreement with observations is very efficient, and within $1σ$ confidence level we find the best-fit parameters $λ=\left(2693_{-1262}^{+1191}\right)\, \rm Mpc$, $α=0.416_{-0.326}^{+1.137}$, and a graviton mass of $m_{g}=\left(2.374_{-0.728}^{+2.095}\right)\times 10^{-42}\, \text{GeV}$. Additionally, we establish a connection between the effective dark matter and dark energy density parameters and the angular radius of the black hole shadow of the SgrA and M87 black holes in the low-redshift limit, consistent with the Event Horizon Telescope findings.

astro-ph.CO↗
arXiv2023

Scalar field evolution at background and perturbation levels for a broad class of potentials

In this paper, we investigate a non-interacting scalar field cosmology with an arbitrary potential using the $f$-deviser method that relies on the differentiability properties of the potential. Using this alternative mathematical approach, we present a unified dynamical system analysis at a scalar field's background and perturbation levels with arbitrary potentials. For illustration, we consider a monomial and double exponential potential. These two classes of potentials comprise the asymptotic behaviour of several classes of scalar field potentials, and, therefore, they provide the skeleton for the typical behaviour of arbitrary potentials. Moreover, we analyse the linear cosmological perturbations in the matterless case by considering three scalar perturbations: the evolution of the Bardeen potentials, the comoving curvature perturbation, the so-called Sasaki-Mukhanov variable, or the scalar field perturbation in uniform curvature gauge. Finally, an exhaustive dynamical system analysis for each scalar perturbation is presented, including the evolution of Bardeen potentials in the presence of matter.

gr-qc↗
arXiv2023

Exact solutions and cosmological constraints in fractional cosmology

This paper investigates exact solutions of cosmological interest in fractional cosmology. Given $μ$, the order of Caputo's fractional derivative, and $w$, the matter equation of state, we present specific exact power-law solutions. We discuss the exact general solution of the Riccati Equation, where the solution for the scale factor is a combination of power laws. Using cosmological data, we estimate the free parameters. An analysis of type Ia supernovae (SNe Ia) data and the observational Hubble parameter data (OHD), also known as cosmic chronometers, and a joint analysis with data from SNe Ia + OHD leads to best-fit values for the free parameters calculated at $1σ$, $2σ$ and $3σ$ confidence levels (CLs). On the other hand, these best-fit values are used to calculate the age of the Universe, the current deceleration parameter (both at $3σ$ CL) and the current matter density parameter at $1σ$ CL. Finding a Universe roughly twice as old as the one of $Λ$CDM is a distinction of fractional cosmology. Focusing our analysis on these results, we can conclude that the region in which $μ>2$ is not ruled out by observations. This parameter region is relevant because fractional cosmology gives a power-law solution without matter, which is accelerated for $μ>2$. We present a fractional origin model that leads to an accelerated state without appealing to $Λ$ or dark energy.

gr-qc↗
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↗
arXiv2022

Study of a viscous $Λ$WDM model: Near equilibrium condition, entropy production, and cosmological constraints

Extensions to a $Λ$DM model have been explored to face current tensions that occur within its framework, which encompasses broadening the nature of the dark matter (DM) component to include warmness and a non-perfect fluid description. In this paper, we investigated the late-time cosmological evolution of an exact solution recently found in [N. Cruz, E. González, and J. Jovel, Phys. Rev. D \textbf{105}, 024047 (2022)], which describe a viscous warm $Λ$DM model ($Λ$WDM) with a DM component that obeys a polytropic equation of state (EoS), which experience dissipative effects with a bulk viscosity proportional to its energy density, with proportionality constant $ξ_{0}$. This solution has the particularity of having a very similar behavior to the $Λ$CDM model for small values of $ξ_{0}$, evolving also to a de Sitter type expansion in the very far future. We explore firstly the thermodynamic consistences of this solution in the framework of the Eckart's theory of non-perfect fluids, focusing on the fulfillment of the two following conditions: (i) the near equilibrium condition and (ii) the positiveness of the entropy production. We explore the range of parameters of the model that allow to fulfilling these two conditions at the same time, finding that a viscous WDM component is compatible with both ones, being in this sense, a viable model from the thermodynamic point of view. Besides, we constraint the free parameters of the model with the observational data coming form supernovae Ia (SNe Ia) and the observational Hubble parameter data (OHD).(Abridged)

gr-qc↗
arXiv2022

A Perturbative Analysis of Interacting Scalar Field Cosmologies

Scalar field cosmologies with a generalized harmonic potential are investigated in flat and negatively curved Friedmann-Lemaître-Robertson-Walker and Bianchi I metrics. An interaction between the scalar field and matter is considered. Asymptotic methods and averaging theory are used to obtain relevant information about the solution space. In this approach, the Hubble parameter plays the role of a time-dependent perturbation parameter which controls the magnitude of the error between full-system and time-averaged solutions as it decreases. Our approach is used to show that full and time-averaged systems have the same asymptotic behavior. Numerical simulations are presented as evidence of such behavior. Moreover, the asymptotic behavior of the solutions is independent of the coupling function.

gr-qc↗
arXiv2022

Singularities and Soft-Big Bang in a viscous $Λ$CDM model

In this paper we explore the different types of singularities that arise in the $Λ$CDM model when dissipative processes are considered, in the framework of the Eckart's theory. In particular, we study the late-time behavior of $Λ$CDM model with viscous cold dark matter (CDM) and an early-time viscous radiation domination era with cosmological constant (CC). The fluids are described by the barotropic equation of state (EoS) $p=(γ-1)ρ$, where $p$ is the equilibrium pressure of the fluid, $ρ$ their energy density, and $γ$ is the barotropic index. We explore two particular cases for the bulk viscosity $ξ$, a constant bulk viscosity $ξ=ξ_{0}$, and a bulk viscosity proportional to the energy density of the fluid $ξ=ξ_{0}ρ$. Due to some previous investigations that have explored to describe the behavior of the universe with a negative CC, we extend our analysis to this case. We found that future singularities like Big-Rip are allowed but without having a phantom EoS associated to the DE fluid. Big-Crunch singularities also appears when a negative CC is present, but also de Sitter and even Big-Rip types are allowed due to the negative pressure of the viscosity, which opens the possibility of an accelerated expansion in AdS cosmologies. We also discuss a very particular solution without Big Bang singularity that arises in the early-time radiation dominant era of our model known as Soft-Big Bang.

gr-qc↗
arXiv2021

Time-averaging axion-like interacting scalar fields models

In this paper, we study a cosmological model inspired in the axionic matter with two canonical scalar fields $ϕ_1$ and $ϕ_2$ interacting through a term added to its potential. Introducing novel dynamical variables, and a dimensionless time variable, the resulting dynamical system is studied. The main difficulties arising in the standard dynamical systems approach, where expansion normalized dynamical variables are usually adopted, are due to the oscillations entering the nonlinear system through the Klein-Gordon (KG) equations. This motivates the analysis of the oscillations using methods from the theory of averaging nonlinear dynamical systems. We prove that time-dependent systems, and their corresponding time-averaged versions, have the same late-time dynamics. Then, we study the time-averaged system using standard techniques of dynamical systems. We present numerical simulations as evidence of such behavior.

gr-qc↗