arXiv subjects

Ángel Rincón

Publications and source records attributed to Ángel Rincón.

At least 19 recordsLinked to original sources

arXiv2026

Quasinormal modes of a quantum inspired black hole in four dimensions with cosmological constant

We study the scalar and Dirac perturbations of quantum-corrected black holes with cosmological constant. Using two different methods (WKB and double-null characteristic integration) we compute the quasinormal modes (QNMs) of de Sitter and Anti-de Sitter solutions considering linear field perturbations in the background geometry. In the limit of general relativity black holes our methods demonstrate good convergence with the available results in literature. In the presence of an extra quantum parameter we verify that all perturbations are stable evolving in towers of quasinormal oscillations. We scrutinize the spectra of both dS and AdS solutions studying the influence of that extra parameter in the frequencies.

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arXiv2026

Analyzing Quasi-normal modes spectrum in asymptotically de Sitter black bounces and their optical appearance

This work explores asymptotically de Sitter black-bounce spacetimes, obtained as a regular extension of the Schwarzschild-de Sitter geometry via the Simpson-Visser prescription. The resulting family of solutions provides a smooth interpolation between regular black holes, extremal configurations, and traversable wormholes within a single geometrical framework. In particular, we verify the regularity of the spacetime by showing that (some of) curvature invariants, including the Kretschmann and Ricci scalars, remain finite at the origin for any non-vanishing value of the regularization parameter $a$, ensuring geodesic completeness. An analysis of the effective stress-energy tensor reveals a violation of the null energy condition, which can be consistently interpreted in terms of a composite source involving a phantom scalar field coupled to nonlinear electrodynamics. Subsequently, we investigate the observational signatures of these geometries by studying: i) null geodesic propagation and ii) constructing the associated optical appearance. In particular, we determine the shadow radius and characterize the structure of photon rings through numerical ray-tracing procedure. Finally, we compute the spectrum of axial gravitational perturbations using a pseudospectral approach, obtaining the quasinormal mode frequencies and their overtones. Our results show a non-trivial interplay between the cosmological constant and the regularization scale $a$, which imprints itself on both the light-ring structure and the quasinormal spectrum. These features suggest potential observational discriminants that could distinguish these regular spacetimes from their singular counterparts in future gravitational-wave and high-resolution electromagnetic observations.

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arXiv2026

The gravitational wave-black hole imaging correspondence for modified black holes

Black holes (BHs) can be studied via fundamentally different observational channels that probe complementary aspects of their physics. While BH imaging provides access to the quasi-static space-time geometry via the strong bending of light rays, gravitational wave (GW) observations probe the dynamical response of the space-time to time-dependent processes in the inspiral, merger and ringdown phases. Both messengers -- electromagnetic imaging probes and ringdown GW spectroscopy --, provide access to essentially the same region -- the one between the BH event horizon and the photon region --, but they do it via conceptually different methods, encoding different physical information. However, it has been shown in the literature that physical quantities supposedly exclusive of each such messenger are actually tightly related to each another via a correspondence that occurs in the eikonal limit (i.e. large values of the multipole number $\ell$) of the geometric-optics approximation. In this paper we clarify the actual identification of observables within such a correspondence and test its accuracy for modified spherically symmetric BH geometries proposed in the literature. We find that even for low values of $\ell$ the correspondence is surprisingly accurate in relating the real and imaginary parts of quasi-normal modes in the GW ringdown phase with the critical impact parameter and Lyapunov exponent of nearly-bound light trajectories for every such model analyzed. We discuss the applicability of such a result both for each messenger individually, and also for foreseeable tests of BHs combining both messengers.

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arXiv2026

Slowly rotating condensate dark stars beyond the mean-field approximation

We investigate rotational properties and universal relations of slowly rotating Bose-Einstein condensate dark stars in the context of General Relativity, both at the mean-field level and when the leading beyond-mean-field Lee-Huang-Yang correction is retained self-consistently. Adopting the polytropic $n=1$ equation of state appropriate to a dilute, self-interacting Bose gas, parameterised by the boson mass $m$ and the $s$-wave scattering length $a_s$, we integrate the Tolman-Oppenheimer-Volkoff equations together with Hartle's dipole equation for the frame-dragging angular velocity, and we compute the moment of inertia, the gravito-electric tidal Love number and the dimensionless tidal deformability. The resulting equilibrium sequences yield gravitational masses in the $1$--$2\,M_{\odot}$ range with radii of $10$--$20\,\mathrm{km}$, squarely within the window presently probed by NICER and the LIGO-Virgo-KAGRA network. We observe that the LHY term produces a measurable reduction of the dimensionless moment of inertia at fixed compactness, whilst the I-$Λ$ universal relation is preserved to within a few per cent. We supply polynomial fits for the I-$Λ$ and I-$C$ relations, and show that the LHY footprint is large enough to serve as a clean diagnostic of beyond-mean-field quantum physics in a putative dark star population, complementing existing dark matter constraints from pulsar masses and from the equation-of-state interpretation of the unusually light compact remnant HESS~J1731-347.

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arXiv2026

Black hole physics within $f(R)$ gravity: Quasi-normal spectra and greybody factors

We investigate several astrophysical motivated properties of a four-dimensional black hole solution in the framework of $f(R)$ gravity. The model is characterized by a single additional parameter, $α$, which encodes nontrivial deviations from General Relativity. Using this black hole spacetime as the background geometry, we analyze: (i) the quasinormal modes of massless scalar perturbations (employing several complementary methods), and (ii) the greybody factors associated to the propagation of massless test scalar fields. Regarding quasinormal modes, we study the response of black holes to massless scalar perturbations using three independent approaches: the well-established sixth-order WKB semi-analytic method, analytic expressions, and the analytical expression in the eikonal limit. We examine the behavior of both the real and imaginary parts of the quasinormal frequencies as functions of the parameter $α$, the overtone number $n$, and the multipole number $\ell$. In addition to that, we compute the greybody factors within the WKB approximation for various combinations of the relevant parameters. In particular, we focus on the propagation of test massless scalar fields and investigate how the greybody factors depend on the multipole number $\ell$, and the free parameter $α$. The impact of the aforementioned parameters on the absorption cross-section and the reflection and transmission coefficients is studied in detail.

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arXiv2026

Photon spheres in dynamical space-times

The characterization of the photon region -- i.e. the region of space-time filled with unstable bound null geodesics -- is critical to understand the behavior of radiation near a compact-enough object, such as black holes. However, its study has been typically focused on stationary space-times that leave outside interesting theoretical and phenomenological scenarios such as collapsing, accreting, or evaporating black holes, besides time-dependent configurations such as those found in some boson star models. In this work, we present a novel covariant approach to describe radiation in dynamical spherical space-times. This allows a general description of photon surfaces and their dynamics in non-static space-times, recovering well known expressions in the static limit and clarifying their meanings. Furthermore, we illustrate our results via several examples of dynamical scenarios in stellar collapse and accreting/evaporating models, and discuss the open caveats regarding such scenarios.

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arXiv2026

Quasi-normal modes and shadows of scale-dependent regular black holes

In this paper we investigate how a regular scale-dependent black hole, characterized by a single extra parameter $ε$, behaves under perturbations by a test field (quasi-normal modes) and under light imaging (shadows) in a four-dimensional space-time background. On the quasi-normal modes side, we study how it responds to scalar and Dirac perturbations. To do this, we implement the well known WKB semi-analytic method of 6th order for obtaining the quasi-normal frequencies. We discuss the behavior of the real and imaginary parts of the quasi-normal modes for different values of the parameter $ε$ and the overtone $n$ and multipole $\ell$ numbers. On the black hole imaging side, we ray-trace the geometry and illuminate it with a thin-accretion disk. Choosing $ε=1.0$ we compute the size of the central brightness depression and generate full images of the black hole. We discuss the features (i.e. luminosity) of successive photon rings through the Lyapunov exponent of nearly-bound, unstable geodesics. Furthermore we use the correspondence (in the limit $\ell \gg n$) between quasi-normal mode frequencies and unstable bound light orbits to infer the numerical values of the latter using the former and find a remarkable accuracy of the correspondence in providing the right numbers. Our results support the usefulness of this correspondence in order to perform cross-tests of black holes using these two messengers.

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arXiv2025

Quasinormal modes and absorption cross-section of a Bardeen black hole surrounded by perfect fluid dark matter in four dimensions

In this paper we study quasinormal modes and absorption cross sections for the $(1+3)$-dimensional Bardeen black hole surrounded by perfect fluid dark matter. Studies of the massless scalar field is already done in \cite{Sun:2023slzl}. Hence, in this paper we will focus on the massive scalar field perturbations and massless Dirac field perturbations. To compute the quasinormal modes we use the semi-analytical 3rd-order WKB method, which has been shown to be one of the best approaches when the effective potential is adequate and when $n < \ell$ and $n < λ$. We have also utilized the Pöschl-Teller method to compare the valus obtained using the WKB approach. We have computed quasinormal frequencies by varying various parameters of the theory such as the mass of the scalar field $μ$, dark matter parameter $α$ and the magnetic charge $g$. We have summarized our solutions in tables and figures for clarity. As for the absorption cross section, we used third order WKB approach to compute reflection, transmission coefficients and partial absorption cross sections. Graphs are presented to demonstrate the behavior of the above quantities when the dark matter parameter and mass of the massive scalar field are varied.

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arXiv2025

Charged black holes in Kalb-Ramond gravity: Weak Deflection Angle, Shadow cast, Quasinormal Modes and Neutrino annihilation

In this paper, we investigate the phenomenology of electrically charged black holes in a Lorentz-violating gravitational framework mediated by a background Kalb-Ramond (KR) antisymmetric tensor field. Employing the Gauss-Bonnet theorem in a non-asymptotically flat geometry, we derive analytic expressions for the weak deflection angle of light and massive particles, revealing persistent corrections due to the Lorentz-violating parameter $ \ell $. Scalar and Dirac perturbations are also studied using both the WKB approximation and the Poschl-Teller approximation approach to verify the stability of the solution against these types of perturbations. Shadow analysis further uncovers a nontrivial deformation of the photon sphere and critical impact parameter, with KR-induced effects modifying the charge contribution in a manner incompatible with standard Einstein-Maxwell theory. Constraints derived from Event Horizon Telescope data for Sgr A* and M87* validate the model and provide stringent bounds on $ \ell $, establishing the KR framework as an observationally testable extension of General Relativity.

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arXiv2024

Tidal Love numbers of anisotropic stars within the complexity factor formalism

We compute the quadrupolar gravitoelectric tidal Love numbers of spherical configurations made of anisotropic matter. Anisotropies are introduced within the vanishing complexity factor, while interior solutions are obtained adopting the Extended Chaplygin gas equation-of-state. A comparison with a more conventional approach is made as well.

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arXiv2024

An Effective Model for the Quantum Schwarzschild Black Hole: Weak Deflection Angle, Quasinormal Modes and Bounding of Greybody Factor

In this paper, we thoroughly explore two crucial aspects of a quantum Schwarzschild black solution within four-dimensional space-time: i) the weak deflection angle, ii) the rigorous greybody factor and, iii) the Dirac quasinormal modes}. Our investigation involves employing the Gauss-Bonnet theorem to precisely compute the deflection angle and establishing its correlation with the Einstein ring. Additionally, we derive the rigorous bounds for greybody factors through the utilization of general bounds for reflection and transmission coefficients in the context of Schrodinger-like one-dimensional potential scattering. We also compute the corresponding Dirac quasinormal modes using the WKB approximation. We reduce the Dirac equation to a Schrodinger-like differential equation and solve it with appropriate boundary conditions to obtain the quasinormal frequencies. To visually underscore the quantum effect, we present figures that illustrate the impact of varying the parameter $r_0$, or more specifically, in terms of the parameter $α$. This comprehensive examination enhances our understanding of the quantum characteristics inherent in the Schwarzschild black solution, shedding light on both the deflection angle and greybody factors in a four-dimensional space-time framework.

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arXiv2024

Thermodynamics of the quantum Schwarzschild black hole

We discuss some thermodynamic properties as well as the stability of a quantum Schwarzschild black hole, comparing the results with those obtained within a bumblebee gravity model. In particular, the Hawking temperature, $T_H$, the entropy, $S$, the heat capacity, $C$, and the Gibbs free energy, $G$, are computed for both cases. In addition to that, we compute the Brown-York quasilocal energy and compare the solution with the Schwarzschild case. We find that in both cases (quantum Schwarzschild and bumblebee gravity model) the temperature, the entropy, and the heat capacity show the same functional form, under the replacement $λ^2 \rightarrow \ell$ and vice versa. Specifically, the temperature is found to be lower compared to the classical (Schwarzschild) solution, whereas the entropy is computed to be larger. Moreover, the heat capacity becomes more negative. Notably, a distinct contrast emerges in obtaining the Gibbs free energy between these two cases, and this distinction appears to stem from the ADM mass.

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arXiv2024

Shadow and greybody bounding of a regular scale-dependent black hole solution

In this manuscript, we explore the shadow and the greybody bounding characteristics of a regular black hole within 4-dimensional space-time, employing the context of gravity that is scale-dependent. Our focus lies in determining limitations on the parameter denoted as $\tildeε$, which serves as a descriptor for the scale-dependent solution with respect to the classically observed shadow radius $R_\text{sh}$ as indicated in the documented data from the Event Horizon Telescope (EHT). Our result indicates that there is a unique value for $\tildeε$ occurring at the reported mean of $R_\text{sh}$ and the uncertainties could be the result of the fluctuating value of the scale-dependent parameter. We found that $\tildeε > 0$ is positive for Sgr. A*, but $\tildeε<0$ for M87*. Utilizing M87* as a reference model, we scrutinized the shadow radius and weak deflection angle within the specified constraints. Discrepancies were observed not only in the shadow but also in the deflection angle of photons, particularly when the photon's impact parameter closely approached the critical impact parameter.

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arXiv2023

Black Holes with Abelian and Non-Abelian Charges and Their Impact on Matter Accretion Flows

We study the black hole spacetime structure of a model consisting of the standard Maxwell theory and a $p$-power-Yang-Mills term. This non-linear contribution introduces a non-Abelian charge into the global solution, resulting in a modified structure of the standard Reissner-Nordström black hole. Specifically, we focus on the model with $p=1/2$, which gives rise to a new type of modified Reissner-Nordström black hole. For this class of black holes, we compute the event horizon, the innermost stable circular orbit, and the conditions to preserve the weak cosmic censorship conjecture. The latter condition sets a well-established relation between the electric and the Yang-Mills charges. As a first astrophysical implication, the accretion properties of spherical steady flows are investigated in detail. Extensive numerical examples of how the Yang-Mills charge affects the accretion process of an isothermal fluid in comparison to the standard Reissner-Nordström and Schwarzschild black holes are displayed. Finally, analytical solutions in the fully relativistic regime, along with numerical computations, of the mass accretion rate for a polytropic fluid in terms of the electric and Yang-Mills charges are obtained. As a main result, the mass accretion rate efficiency is considerably improved, with respect to the standard Reissner-Nordström and Schwarzschild solutions, for negative values of the Yang-Mills charge.

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arXiv2023

Regular charged black holes, energy conditions and quasinormal modes

We discuss energy conditions and quasinormal modes for scalar perturbations of regular charged black holes within the framework of General Relativity coupled to non-linear electrodynamics. The frequencies are computed numerically adopting the WKB method, while in the eikonal limit an analytic expression for the spectra is obtained. The impact of the electric charge, the angular degree, and the overtone number on the spectra is investigated in detail. We find that all frequencies are characterized by a negative imaginary part, and that each type of energy conditions imply a different quasinormal spectrum.

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arXiv2023

Gravitationally decoupled Non-Schwarzschild black holes and wormhole space-times

In this article, using gravitational decoupling by means of minimal geometric deformation approach, we obtain a new spherically symmetric and static black hole solution. To progress, we close the system by assuming that the average pressure of the $θ$-sector is vanishing. Also, we tackle the problem regarding how, for a given minimally deformed black hole solution, one can connect it to a wormhole space-time.

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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.

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arXiv2023

4D scale-dependent Schwarzschild-AdS/dS black holes: study of shadow and weak deflection angle and greybody bounding

We investigate the shadow, deflection angle, and the greybody bounding of a Schwarzschild-AdS/dS black hole in scale-dependent gravity. We used the EHT data to constrain the parameter $ε$. We have found that within the $1σ$ level of uncertainty, $εM_0$ ranges at $10^{-11} - 10^{-16}$ orders of magnitude for Sgr. A*, and $10^{-11} - 10^{-17}$ for M87*. Using these parameters, we explored how the shadow radius behaves as perceived by a static observer. Using the known parameters for Sgr. A* and M87*, we found different shadow cast behavior near the cosmological horizon even if the same scaling parameters were used. We explored the deflection angle $\hatα$ in the weak field limit for both massive and null particles, in addition to the effect of finite distances using the non-asymptotically flat version of the Gauss-Bonnet theorem. We have found that the $\hatα$ strongly depends on massive particles. Tests on $ε$'s effect using Sgr. A* is only along points of low impact parameters. For M87*, only the scaled BH in AdS type can be tested at high impact parameters at the expense of very low value for $\hatα$. We study the rigorous bound on the greybody factor for scalar field emitted from black holes in the theory and show the bound on the transmission probability. The effects of the scale-dependent gravity on the greybody factors are investigated, and the results indicate that the bound on the greybody factor in this case is less than the bound for the Schwarzschild black hole.

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