arXiv subjects

Ramón Bécar

Publications and source records attributed to Ramón Bécar.

13 recordsLinked to original sources

arXiv2026

Anomalous Decay Rate and Greybody Factors for Regular Black Holes with Scalar Hair

We study the propagation of massive scalar fields in the background of asymptotically flat regular black holes supported by a phantom scalar field with a scalar charge $A$. This parameter regularizes the geometry by removing the central singularity. Focusing on wave dynamics, we analyze scalar perturbations, quasinormal modes, and greybody factors, emphasizing the role of the regularization parameter on the effective potential and the decay properties of the modes. Using WKB methods beyond the eikonal limit, we show that the presence of scalar hair modifies both the oscillation frequencies and damping rates of quasinormal modes. In particular, we demonstrate the occurrence of an anomalous decay rate for massive scalar perturbations, and above a critical field mass, the longest-lived modes correspond to lower angular momentum, in contrast with the massless case. We derive analytical expressions for the critical mass and study its dependence on the scalar charge and overtone number. Furthermore, we apply the Horowitz-Hubeny method to compute the quasinormal frequencies and show that the results obtained from the WKB and Horowitz-Hubeny approaches exhibit excellent agreement in the regime where both methods are valid. In addition, we compute reflection and transmission coefficients and analyze the corresponding greybody factors, clarifying how regularity effects imprint themselves on black-hole scattering properties. Our results show that regular black holes with scalar hair exhibit distinctive dynamical signatures that can be probed through quasinormal ringing and wave propagation.

gr-qc↗

arXiv2026

Lyapunov Exponents and Phase Transitions in Four-Dimensional AdS Black Holes with a Nonlinear Electrodynamics Source

We investigate the relationship between dynamical instability and thermodynamic phase transitions in four-dimensional Anti--de Sitter black holes in Einstein gravity coupled to a nonlinear power-law electromagnetic field with exponent $p = 3/4$. In the canonical ensemble, we identify a critical electric charge $Q_c$ separating a regime exhibiting a first-order small/large black-hole (SBH/LBH) phase transition from a regime with a single thermodynamically stable phase. For both massless and massive probes, the thermal profile of the Lyapunov exponent $λ(T)$ becomes multivalued in the SBH/LBH coexistence region and exhibits a finite discontinuity at the transition temperature. This jump vanishes continuously as $Q \to Q_c$, signaling the termination of the first-order transition at a second-order critical point. Near criticality, the Lyapunov discontinuity obeys a universal mean-field scaling law with critical exponent $1/2$. For massless probes, we further analyze the critical impact parameter $b_c$, which displays the same multivalued structure and critical behavior as the Lyapunov exponent. We also demonstrate that the spinodal temperatures, defined by the extrema of the $T(r_h)$ curve where the heat capacity at fixed charge diverges, coincide with singular features in the Lyapunov exponent. Our results identify the Lyapunov exponent as a unified dynamical probe capable of capturing both first-order phase coexistence and second-order critical behavior in black-hole thermodynamics.

gr-qc↗

arXiv2025

Massive scalar field perturbations of 4D de Sitter Einstein-Gauss-Bonnet black holes

We investigate the propagation of massive scalar fields in the background of four-dimensional Einstein-Gauss-Bonnet black holes with de Sitter (dS) asymptotics. Our study focuses on the various branches of quasinormal modes present in this background, employing the pseudospectral Chebyshev method and the third-order Wentzel-Kramers-Brillouin approximation. We identify that the introduction of the Gauss-Bonnet coupling constant $α$ gives rise to three branches of modes: the perturbative (in $α$) Schwarzschild branch, the perturbative (in $α$) dS branch, and a non-perturbative (in $α$) dS branch. Our results show that the propagation of a massive scalar field is stable in this background. Furthermore, the Gauss-Bonnet coupling constant induces significant deviations in the Schwarzschild branch and smaller deviations in the perturbative dS branch compared to the corresponding branches in the Schwarzschild-dS limit. Additionally, the non-perturbative dS branch of modes, absent when $α=0$, emerges as a novel feature of the Einstein-Gauss-Bonnet framework.

gr-qc↗

arXiv2024

Massive Scalar Field Perturbations of Black Holes Immersed in Chaplygin-Like Dark Fluid

We consider massive scalar field perturbations in the background of black holes immersed in Chaplygin-like dark fluid (CDF), and we analyze the photon sphere modes, the de Sitter modes as well as the near extremal modes and discuss their dominance, by using the pseudospectral Chebyshev method and the third order Wentzel-Kramers-Brillouin approximation. We also discuss the impact of the parameter representing the intensity of the CDF on the families of quasinormal modes. Mainly, we find that the propagation of a massive scalar field is stable in this background, and it is characterized by quasinormal frequencies with a smaller oscillation frequency and a longer decay time compared to the propagation of the same massive scalar field within the Schwarzschild-de Sitter background.

gr-qc↗

arXiv2024

Massive Scalar Field Perturbations of Black Holes Surrounded by Dark Matter

We consider scalar field perturbations in the background of black holes immersed in perfect fluid dark matter (PFDM). We find, by using the sixth order Wentzel-Kramers-Brillouin (WKB) approximation, that the longest-lived modes are the ones with higher angular number, for a scalar field mass smaller than a critical value, known as anomalous decay rate of the quasinormal modes, while that beyond this critical value the behaviour is inverted. Moreover, we show that it is possible to recover the real part of the quasinormal frequencies (QNFs), the imaginary part of the QNFs, and the critical scalar field mass, of the Schwarzschild background for different values of the PFDM intensity parameter $k$, respectively. For values of $k$ smaller than these values, the mentioned quantities are greater than the Schwarzschild background. However, beyond of these values of $k$, these quantities are smaller than the Schwarzschild background.

gr-qc↗

arXiv2023

Massive scalar field perturbations in Weyl black holes

In this work we consider the propagation of massive scalar fields in the background of Weyl black holes and we mainly study the effect of the scalar field mass in the spectrum of the quasinormal frequencies (QNFs) via the Wentzel-Kramers-Brillouin (WKB) method, and the pseudospectral Chebyshev method. The spectrum of QNFs is described by two families of modes, one of them is the photon sphere and the other one is the de Sitter. Essentially we show, via the WKB method, that the photon sphere modes exhibit an anomalous behaviour of the decay rate of the QNFs, that is, the longest-lived modes are the ones with higher angular number, as well as, there is a critical value of the scalar field mass, beyond which the anomalous behaviour is inverted. Also, we analyze the effect of the scalar field mass on each family of modes, and on their dominance, and we give an estimate value of the scalar field mass where the interchange in the dominance family occurs.

gr-qc↗

arXiv2022

Charged scalar field perturbations in Ernst black holes

We consider the propagation of a charged massive scalar field in the background of a four-dimensional Ernst black hole and study its stability analyzing the quasinormal modes (QNMs), which are calculated using the semi-analytical Wentzel-Kramers-Brillouin method and numerically using the continued fraction method. Mainly, we find that for a scalar field mass less than a critical mass, the decay rate of the QNMs decreases when the harmonic angular number $\ell$ increases; and for a scalar field mass greater than the critical mass the behaviour is inverted, i.e, the longest-lived modes are always the ones with the lowest angular number recovering the standard behaviour. Also, we find a critical value of the external magnetic field, as well as, a critical value of the scalar field charge that exhibit the same behaviour with respect to the angular harmonic numbers. In addition, we show that the spacetime allows stable quasibound states and we observe a splitting of the spectrum due to the Zeeman effect. Finally, we show that the unstable null geodesic in the equatorial plane is connected with the QNMs when the azimuthal quantum number satisfy $m= \pm \ell$ in the eikonal limit.

gr-qc↗

arXiv2021

Massive Dirac quasinormal modes in Schwarzschild-dS black holes: anomalous decay rate and fine structure

Recently, the anomalous decay rate of quasinormal modes has been studied for some geometries under scalar field perturbations, which occurs when the longest-lived modes are the ones with higher angular number, as well as, the existence of a critical scalar field mass, i.e the value of scalar field mass such that the decay rate does not depend appreciably on the angular number, and beyond which the behaviour of the decay rate is inverted. Here, we consider the propagation of fermionic fields in the background of Schwarzschild de Sitter black holes, and we show that the anomalous decay rate behaviour and the fine structure, related to the coupling between the chirality and the mass of the field, can be observed in the fermionic spectrum.

gr-qc↗

arXiv2020

Perturbative and nonperturbative quasinormal modes of 4D Einstein-Gauss-Bonnet black holes

We study the propagation of probe scalar fields in the background of 4D Einstein-Gauss-Bonnet black holes with anti-de Sitter (AdS) asymptotics and calculate the quasinormal modes. Mainly, we show that the quasinormal spectrum consists of two different branches, a branch perturbative in the Gauss-Bonnet coupling constant $α$ and another branch nonperturbative in $α$. The perturbative branch consists of complex quasinormal frequencies that approximate to the quasinormal frequencies of the Schwarzschild AdS black hole in the limit of a null coupling constant. On the other hand, the nonperturbative branch consists of purely imaginary frequencies and is characterized by the growth of the imaginary part when $α$ decreases, diverging in the limit of null coupling constant, therefore they do not exist for the Schwarzschild AdS black hole. Also, we find that the imaginary part of the quasinormal frequencies is always negative for both branches; therefore, the propagation of scalar fields is stable in this background.

gr-qc↗

arXiv2020

Collision of particles near a three-dimensional rotating Hořava AdS black hole

We consider a three-dimensional rotating Hořava AdS black hole, that corresponds to a Lorentz-violating version of the BTZ black hole and we analyze the effect of the breaking of Lorentz invariance on the possibility that the black hole can acts as a particle accelerator by analyzing the energy in the center of mass (CM) frame of two colliding particles in the vicinity of its horizons. We find that the critical angular momentum of particles increases when the Hořava parameter $ξ$ increases and when the aether parameter $b$ increases. Also, the particles can collide on the inner horizon with arbitrarily high CM energy if one of the particles has a critical angular momentum being possible the BSW process, for the non-extremal rotating Hořava AdS black hole. Mainly, while that for the extremal BTZ black hole the particles with critical angular momentum only can exist on the degenerate horizon, for the Lorentz-violating version of the BTZ black hole the particle with critical angular momentum can exist in a region from the degenerate horizon.

gr-qc↗

arXiv2019

Quasinormal modes of three-dimensional rotating Hořava AdS black hole and the approach to thermal equilibrium

We compute the quasinormal modes (QNMs) of a massive scalar field in the background of a rotating three-dimensional Hořava AdS black hole, and we analyze the effect of the breaking of the Lorentz invariance on the QNMs. Imposing on the horizon that there are only ingoing waves and at infinity the Dirichlet boundary conditions and the Neumann boundary condition, we calculate the oscillatory and the decay modes of the QNMs. We find that the propagation of the scalar field is stable in this background and employing the holographic principle we find the different times of the perturbed system to reach thermal equilibrium for the various branches of solutions.

gr-qc↗

arXiv2017

Particle collisions near a three-dimensional warped AdS black hole

In this paper we consider the warped AdS$_{3}$ black hole solution of topologically massive gravity with a negative cosmological constant, and we investigate the possibility that it acts as a particle accelerator by analyzing the energy in the center of mass (CM) frame of two colliding particles in the vicinity of its horizon, which is known as the Bañados, Silk and West (BSW) process. Mainly, we show that the critical angular momentum $(L_c)$ of the particle decreases when the parameter that controls the stretching deformation ($ν$) increases. Also, we show that despite the particle with $L_c$ can exist for certain values of the conserved energy outside the horizon, it will never reach the event horizon; therefore, the black hole can not act as a particle accelerator with arbitrarily high CM energy on the event horizon. However, such particle could also exist inside the outer horizon being the BSW process possible on the inner horizon. On the other hand, for the extremal warped AdS$_{3}$ black hole, the particle with $L_c$ and energy $E$ could exist outside the event horizon and the CM energy blows up on the event horizon if its conserved energy fulfill the condition $E^{2}>\frac{(ν^{2}+3)l^{2}}{3(ν^{2}-1)}$, being the BSW process possible.

gr-qc↗

arXiv2016

Quasinormal modes of non-Abelian hyperscaling violating Lifshitz black holes

We study the quasinormal modes of scalar field perturbations in the background of non-Abelian hyperscaling violating Lifshitz black holes. We find that the quasinormal frequencies have no real part so there is no oscillatory behavior in the perturbations, only exponential decay, that is, the system is always overdamped, which guarantees the mode stability of non-Abelian hyperscaling violating Lifshitz black holes. We determine analytically the quasinormal modes for massless scalar fields for a dynamical exponent $z=2$ and hyperscaling violating exponent $\tildeθ>-2$. Also, we obtain numerically the quasinormal frequencies for different values of the dynamical exponent and the hyperscaling violating exponent by using the improved asymptotic iteration method.

hep-th↗