arXiv subjects

Gilberto Aguilar-Pérez

Publications and source records attributed to Gilberto Aguilar-Pérez.

10 recordsLinked to original sources

arXiv2026

Cosmological Stealth fields and Non-Equilibrium thermodynamics

We investigate the connection between cosmological stealth scalar fields and non-equilibrium thermodynamics in a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) background. We consider a non-minimally coupled scalar field whose energy-momentum tensor vanishes identically, allowing the field to evolve on a dissipative cosmological background without producing gravitational backreaction. We show that the stealth condition leads to a generalized Riccati equation for the scalar-field kinematics, where the dissipative pressure acts as a thermodynamic driving term. In terms of the variable $y=\dotϕ/(Hϕ)$, the system admits two thermodynamic branches: a stable attractor selected by entropy production and an unstable repeller. We also construct the corresponding phase-space structure in the $(y,ω_{\rm eff})$ plane and identify a near-critical regime associated with $ζ=1/4$. Finally, we reconstruct the stealth potential and present a bulk viscous realization in which irreversible entropy production drives the universe toward an asymptotic de Sitter state while the stealth field tracks the dissipative background. We further develop the linear scalar perturbations of the model and show that, although the stealth field is gravitationally invisible at the background level, its perturbation couples directly to the dissipative-pressure perturbation. This coupling provides a mechanism for distinguishing dissipative and non-dissipative sources that share the same background expansion history. Our results suggest that stealth fields can be interpreted as dynamically non-trivial thermodynamic trackers of non-equilibrium cosmological evolution.

gr-qc↗

arXiv2026

Interacting Generalized Chaplygin-Jacobi gas: Thermodynamics approach

This work investigates a cosmological model featuring an interaction between dark energy and dark matter, where the dark energy component is described by the Generalized Chaplygin-Jacobi gas (GCJG). In this study, we establish a system in which the GCJG and a pressureless dark matter fluid exchange energy via a linear interaction term, $Q \propto ρ_x$, being $ρ_{x}$ the dark energy density. By solving the conservation equations, we derive analytical expressions for the evolution of the dark energy and dark matter densities. The thermodynamic properties of this interacting system are then thoroughly analyzed. The thermodynamic analysis reveals that both dark components maintain positive temperatures, ensuring stability. Notably, the dark energy component transitions to a phantom regime in the past, a feature of interest for recent cosmological observations, without violating thermodynamic principles. The total entropy production is shown to be in agreement with the second law of thermodynamics. Furthermore, an analysis of the specific heats suggests that while the dark matter sector remains thermodynamically stable, the dark energy sector undergoes a late-time phase transition, consistent with its entering into the phantom domain at effective level.

gr-qc↗

arXiv2026

Spatial curvature in Unimodular Gravity

We investigate the cosmological implications of unimodular gravity (UG) featuring energy diffusion and spatial curvature. While standard diffusion models often suffer from thermodynamic inconsistencies, we propose a phenomenologically viable power-law Ansatz for the diffusion function, $Q(z) = Q_0(1+z)^β$, which strictly satisfies the second law of thermodynamics by demanding positive entropy production ($βQ_0 > 0$). Using a joint statistical analysis with the Pantheon+ Type Ia Supernova compilation and Baryon Acoustic Oscillation (BAO) measurements, we tightly constrain the parameter space. We find a diffusion exponent of $β= 0.503_{-0.126}^{+0.118}$ and a slight preference for a closed spatial geometry with $Ω_{k0} = -0.109_{-0.071}^{+0.076}$ at present time. Remarkably, the consideration of spatial curvature and diffusion naturally alleviates the Hubble tension, yielding $H_0 = 73.350_{-0.226}^{+0.221}$ km/s/Mpc while maintaining a consistent cosmic age of $t_0 \simeq 13.61$ Gyr. Furthermore, the constrained diffusion scales as a stable, quintessence-like effective dark energy ($ω_{\text{eff}} \simeq -0.832$). Thus, unimodular diffusion provides a thermodynamically consistent phenomenological alternative that can alleviate the Hubble tension while preserving both the cosmic age and the sound-horizon scale, with a preference for a closed spatial geometry.

gr-qc↗

arXiv2026

Correspondence between a decaying dark matter sector scenario and scalar field model

We explore the theoretical viability of modeling a decaying dark matter sector through a unified scalar field approach. Using exact analytical solutions of the Friedmann constraints, we map the fluid phenomenology onto a scalar field potential. Our analysis reveals that physical viability, specifically the existence of a well-defined potential minimum; inevitably forces the dark energy equation of state into the phantom domain. To resolve the kinetic pathologies at late times, we propose reinterpreting the framework within a complex scenario, mapping the imaginary transition to the angular dynamics of a $U(1)$ phase. This mapping naturally yields an ultra-light mass scale of $m_ϕ\sim 10^{-33} \ \text{eV}$, classifying the model as a unified dark fluid. Finally, we employ a dynamical approach to study the effects of non-minimal coupling, proving that the phantom-dominated epoch acts as a stable, late-time cosmic attractor in this kind of cosmological scenario.

astro-ph.CO↗

arXiv2026

Thermodynamics of Geodetic Brane Gravity

In this work, we explore the effect at cosmological level of the extra contribution arising from the Geodetic Brane Gravity model within a thermodynamical perspective. As already known, the universe seen as an extended object embedded within a higher dimensional space time, modifies the dynamical background equations, which in turn results in correction contributions to the entropy and temperature of the apparent horizon. Additionally, we investigate the possibility that the apparent horizon and the bulk remain in thermal equilibrium across various matter contents, demonstrating that such properties are highly sensitive to the equation-of-state parameter.

hep-th↗

arXiv2025

Cosmological evolution driven by polytropic fluids in an inhomogeneous spacetime

Addressing the late-time accelerated expansion of the universe, known as the "dark energy problem", remains a central challenge in cosmology. While the cosmological constant is the standard explanation, alternative models such as quintessence, phantom fluids, and Chaplygin gas have been proposed. This work investigates the generalized Chaplygin gas (GCG) model, which is characterized by a polytropic equation of state. We explore this model within the framework of an anisotropic fluid, by means of a metric that reduces to the standard form of the Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime at cosmological scales. To assess the model's viability, we derive analytical expressions for the scale factor, the Hubble parameter, and the deceleration parameter. Finally, the model is tested against observational data to constrain its parameters and evaluate its consistency.

gr-qc↗

arXiv2025

Exploring the cosmological degeneracy between decaying dark matter model and viscous $Λ$CDM

In the context of a homogeneous and isotropic universe, we consider the degeneracy condition at the background level between two scenarios in which processes out of equilibrium are possible; this consideration allows us to deviate from the perfect fluid description, and in this case bulk viscosity represents a viable candidate to describe entirely such effects. The cosmological model describing an unstable dark matter sector is mapped into a slight modification of the $Λ$CDM model characterized by a viscous dark matter sector; under this consideration our description does not depend on a specific formulation of viscous effects and these can be fully reconstructed and characterized by the parameter that determines the decay ratio of dark matter. However, in this new scenario the cosmic expansion is influenced by the viscous pressure and the dark energy sector given by the cosmological constant is translated into a dynamical one. As a consequence of our formulation, the test against observations of the model indicates consistency with quintessence dark energy, but the crossing of the phantom divide can be accessible.

gr-qc↗

arXiv2024

Black hole in a generalized Chaplygin-Jacobi dark fluid: shadow and light deflection angle

We investigate a generalized Chaplygin-like gas with an anisotropic equation of state, characterizing a dark fluid within which a static spherically symmetric black hole is assumed. By solving the Einstein equations for this black hole spacetime, we explicitly derive the metric function. The spacetime is parametrized by two critical parameters, $\mathcal{B}$ and $α$, which measure the deviation from the Schwarzschild black hole and the extent of the dark fluid's anisotropy, respectively. We explore the behavior of light rays in the vicinity of the black hole by calculating its shadow and comparing our results with the Event Horizon Telescope observations. This comparison constrains the parameters to $0 \leq \mathcal{B} < 0.03$ and $0 < α< 0.1$. Additionally, we calculate the deflection angles to determine the extent to which light is bent by the black hole. These calculations are further utilized to formulate possible Einstein rings, estimating the angular radius of the rings to be approximately $37.6\,\mathrm{μas}$. Throughout this work, we present analytical solutions wherever feasible, and employ reliable approximations where necessary to provide comprehensive insights into the spacetime characteristics and their observable effects.

gr-qc↗

arXiv2022

Qualitative analysis for viscous cosmologies in a non linear regime of the Israel-Stewart formalism

We explore the dynamical properties of a cosmological model that includes viscous effects in the dark matter sector of the fluid equations in a flat Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime. The bulk viscous effects are described by a non linear extension of the full Israel-Stewart model. We allow the interchange of energy in the dark sector by means of the interaction term, namely $Q$. We establish the dynamical system corresponding to Friedmann and fluid set of equations associated to the model and study the linear stability of its critical points. From the dynamical system, we show the appearance of a critical point characterizing a de Sitter universe within the non interacting and interacting dark sector. We focus our study to analyse the stability of this fixed point in a large region of parameter space and derive linearized solutions around it. These approximate and analytical solutions are potentially able to describe the expansion of the universe since they are close to a de Sitter stationary solution. Within this regime with $Q \neq 0$, we realize the existence of regions in the space of parameters where this critical point is stable and describes the behavior of dark energy as quintessence, cosmological constant and phantom like fluids. We perform a comparison between numerical and linearized solutions nearby the critical points within the full non linear regimes and also contrast them against $Λ$CDM model. We find that fully non linear regime is favored by observations and closer to the concordance model due to the non-zero value of the parameter $j$, which controls the non linear effects. In fact, at low redshift values, the expansion rate associated to the full non linear regime is practically indistinguishable from $Λ$CDM. The deceleration parameter obtained in this regime exhibits a transition from decelerated to accelerated cosmic expansion.

gr-qc↗

arXiv2019

Hairy black hole stability under odd parity perturbations in the Einstein-Gauss-Bonnet model

Supported by the use of a regular scalar field we find a black hole solution in the Einstein-Gauss-Bonnet model. From the obtained solution we can recover the Schwarzschild black hole as in other works. Later, by implementing the odd parity perturbations method we study the stability of the linearized equations of motion of the model, we find the explicit form of the Regge-Wheeler potential and we explore the condition of vanishing perturbations at the horizon of the black hole. We test the stability of the obtained solution by checking the positivity condition of the Regge-Wheeler potential. Finally, we show that the stability of model depends on the value of a parameter introduced in the profile for the scalar field.

gr-qc↗