arXiv subjects

Bogeun Gwak

Publications and source records attributed to Bogeun Gwak.

At least 19 recordsLinked to original sources

arXiv2026

Phase Transitions with Lyapunov Exponents under Einstein and String Frames in Dilatonic Reissner--Nordström--AdS Black Holes

We investigate Lyapunov exponents as dynamical probes of black hole phase transitions in dilatonic Reissner--Nordström--AdS black holes within Einstein--Maxwell--dilaton theory. The thermodynamic quantities and the Lyapunov exponent of charged probe particles were analyzed in both the Einstein and string frames, thus providing a direct comparison between the thermodynamic phase structure of the black hole and that captured by the Lyapunov exponent. Thermodynamic quantities, including the Hawking temperature and Wald entropy, remained constant under conformal frame transformations, yielding identical phase structures in the two frames. In contrast, the Lyapunov exponent exhibited non-trivial frame dependence for massive probe particles due to dilaton coupling, while no frame dependence was found in the massless limit. Numerical analysis revealed that the phase structure features captured by the Lyapunov exponent, including characteristic cusp behavior and transition points, were independent of the choice of frame, despite the Lyapunov exponent itself being frame-dependent. Therefore, the Lyapunov exponent exhibited frame-dependent values, while the critical structure it captures remained constant across conformal frames.

gr-qc↗

arXiv2026

Correspondence between quasinormal modes and grey-body factors of Schwarzschild--Tangherlini black holes

We investigate the correspondence between the quasinormal modes and grey-body factors of Schwarzschild--Tangherlini black holes. The gravitational perturbations in higher-dimensional black holes can be classified into scalar, vector, and tensor types. Considering the dimension-dependent forms of their effective potentials, the correspondence was examined for each dimension and perturbation mode. The accurate quasinormal modes were computed by suitably adopting the continued fraction and integration-through-midpoints methods, depending on the structure of the singularity. The grey-body factor can be obtained through its correspondence with the quasinormal mode, and its accuracy was analyzed by calculating its difference from the numerically computed grey-body factor. The correspondence failed for $l=2$ scalar gravitational perturbations in $D\ge7$ because the form of the potential is markedly different from that in four dimensions. The vector and tensor perturbation types exhibited good correspondence accuracies in all cases. The breakdown of the correspondence was rigorously demonstrated to stem from multiple potential barriers, and its applicability to each mode in higher dimensions was assessed.

gr-qc↗

arXiv2025

Unbounded Radius of Innermost Stable Circular Orbit in Higher-Dimensional Black Holes

The innermost stable circular orbit (ISCO) offers a fundamental test of spacetime structure. However, its behavior in higher-dimensional black holes influenced by anisotropic energy-momentum tensors remains insufficiently explored. In this work, we investigate the upper bound of the ISCO in higher-dimensional, static, spherically symmetric, and asymptotically flat black hole spacetimes in the presence of an anisotropic energy-momentum tensor. The energy-momentum tensor is assumed to satisfy the weak energy condition, possess a non-positive trace, and obey constraints on radial and tangential pressures, collectively equivalent to the dominant energy condition with additional constraints. By analyzing the effective potential for timelike geodesics and imposing ISCO conditions, we demonstrate the general absence of an upper bound on the ISCO radius in higher-dimensional spacetimes. For dimensions greater than or equal to eight, an ISCO may not exist, depending on the radial and tangential components of the energy-momentum tensor. If an ISCO exists, its radius remains unbounded. These findings advance our understanding of orbital stability in higher-dimensional gravitational systems and highlight fundamental differences from four-dimensional black hole dynamics.

gr-qc↗

arXiv2025

Frame Dependence of Bound on Lyapunov Exponent in Dilatonic Reissner-Nordström-AdS and Kerr-Sen-AdS Black Holes

We investigate the frame dependence of the Lyapunov exponent bound for charged particles in dilatonic Reissner-Nordström-AdS and Kerr-Sen-AdS black hole backgrounds, derived from Einstein-Maxwell-dilaton theory and the low-energy effective action of heterotic string theory, respectively. The analysis is performed in both the Einstein and string (Jordan) frames to examine the influence of conformal transformations on chaotic behavior. For massless particles, the Lyapunov exponent remains invariant under frame transformations, whereas for massive particles, it exhibits frame dependence owing to coupling to the dilaton field. Our results indicate sensitivity of the bound on chaos to the choice of frame. Depending on various parameters, the bound can be satisfied in the Einstein frame and violated in the string frame, while the opposite situation may occur for different parameter values. Numerical computations corroborate the findings of our analysis and demonstrate modifications in the chaotic behavior of string-inspired black holes induced by the dilaton field and the choice of frame.

gr-qc↗

arXiv2025

Correspondence between quasinormal modes and grey-body factors in five-dimensional black holes

We investigate the correspondence between quasinormal modes and grey-body factors for the five-dimensional Schwarzschild--Tangherlini black hole. The quasinormal modes of gravitational perturbations are computed numerically using the continued fraction method. Particularly, we analyze the scalar, vector, and tensor types of perturbations, noting that the tensor type only exists in spacetimes with more than four dimensions. The grey-body factors are then calculated analytically via the correspondence, using data from the fundamental quasinormal mode and the first overtone. We then compute the grey-body factors independently using a numerical method and find that the results from the two methods coincide. This demonstrates that the correspondence holds with high accuracy for all three types of gravitational perturbations. Furthermore, our results extend the validity of the correspondence to the tensor type, which can only be tested in spacetimes with more than four dimensions.

gr-qc↗

arXiv2025

Bound on Lyapunov exponent for a charged particle in Kerr-Sen-AdS Black Hole

We investigate the upper bound of the Lyapunov exponent for a charged particle in the Gibbons--Maeda--Garfinkle--Horowitz--Strominger (GMGHS)--AdS and Kerr--Sen--AdS black hole backgrounds, which originate from the low-energy effective actions of heterotic string theory and gauged supergravity. We analyze the Lyapunov exponent near the unstable orbit to examine possible violations of the bound. Our results indicate that the bound is sensitive to the signs and magnitudes of the charges, the angular momentum of the particle, the black hole spin, and the negative cosmological constant. The violations are pronounced in the extremal or near-extremal regime. Numerical analysis supports the analytical predictions and highlights the interplay between the string-inspired black hole and the charged particle.

gr-qc↗

arXiv2024

Quasinormal modes of Plebański-Demiański black hole in the near-Nariai regime

We investigates the massless scalar perturbations of the Plebański-Demiański black hole considering the general case that admits all nonzero parameters. This case is the most generic black hole spacetime in general relativity, characterized by mass, spin, acceleration, electric and magnetic charges, NUT parameter, and cosmological constant. Employing conformal transformations, we can separate the massless scalar field equation and reduce the effective potential in the radial perturbation equation into the Pöschl--Teller potential in the near-Nariai limit where the event and cosmo-acceleration horizons are close. This allows us to obtain an exact analytical solution of the quasinormal frequency, implying that the decay rate of the field is quantized depending only on the surface gravity of the black hole.

gr-qc↗

arXiv2024

Thermodynamic relation on higher-dimensional black hole with arbitrary cosmological constant

We investigated the Goon--Penco (GP) relation on higher-dimensional Reissner-Nordström black holes with an arbitrary cosmological constant. It was found that the GP relation retained its form in four- and higher-dimensional spacetimes. Thus, the reactions in the black holes are universal with respect to the dimensionality. Furthermore, the GP relation was found to be universal on any state of the black hole including near-extremal and near-Nariai cases. Thus, this study showed that the GP relation was prevalent for higher-dimensional Reissner-Nordström black holes with an arbitrary cosmological constant regardless of the initial state.

gr-qc↗

arXiv2024

Mass fluctuations in non-rotating BTZ black holes

We investigate the impact of oscillations of a black-hole mass around its average value on the three-dimensional black hole geometry. Drawing on a classical framework that conceptualizes fluctuations near an event horizon as mass variations, we introduce a model where the metric of a black hole, formed from the collapse of a massive null shell, exhibits oscillatory behavior in spherical modes. This dynamic is encapsulated by a non-rotating BTZ-Vaidya solution, characterized by the black hole mass fluctuating at a resonant frequency $ω$ and a small amplitude parameter $μ_0$. Using a perturbative approach, solutions to the null geodesic equation are determined up to the second order in $μ_0$. The temporal fluctuations of the event horizon's location induce alterations in the thermodynamic variables' values. Upon calculating the time-averaged values, it is observed that the mean Hawking temperature experiences a slight decrease due to these fluctuations, while the mean entropy exhibits an increase, deviating from trends observed in four- and higher-dimensional spacetimes. Further, the study delves into the influence of these fluctuations on the trajectories of null rays near the horizon, ultimately reaching the anti-de Sitter boundary at late times. The analytical computation of the general solution for the perturbed rays up to the second order underscores the novel approach of this study in examining the effects of mass oscillations on black hole thermodynamics and geometry, contributing a unique perspective to the field.

gr-qc↗

arXiv2024

Universality on thermodynamic relation with corrections in de Sitter black holes

We herein investigate the universal relation proposed by Goon and Penco in de Sitter black holes with electric charge or angular momentum. Our analysis focuses on the cosmological horizon, which only exists in de Sitter and Nariai spacetimes. Because the relation is given in a general case, the overall relationship may be valid. However, we elucidate the details of the relation, highlighting distinctions from those of (anti-)de Sitter black holes while affirming the validity of the relation. Furthermore, based on our analysis of Schwarzschild--de Sitter, Reissner--Nordström--de Sitter, and Kerr--de Sitter black holes, we demonstrate the universality of the thermodynamic relation in de Sitter black holes.

gr-qc↗

arXiv2024

Scalar quasi-normal modes of accelerating Kerr-Newman-AdS black holes

We study linear scalar perturbations of slowly accelerating Kerr-Newman-anti-de Sitter black holes using the method of isomonodromic deformations. The conformally coupled Klein-Gordon equation separates into two second-order ordinary differential equations with five singularities. Nevertheless, the angular equation can be transformed into a Heun equation, for which we provide an asymptotic expansion for the angular eigenvalues in the small acceleration and rotation limit. In the radial case, we recast the boundary value problem in terms of a set of initial conditions for the isomonodromic tau function of Fuchsian systems with five regular singular points. For the sake of illustration, we compute the quasi-normal modes frequencies.

gr-qc↗

arXiv2023

Effects of fluctuations in higher-dimensional AdS black holes

We explored the impact of mass fluctuations on anti-de Sitter black holes in higher dimensions, particularly focusing on their effects on thermodynamic properties and null trajectories of the black holes. Our findings indicate that mass oscillations lead to perturbations in thermodynamic variables and geodesics. These result in the second-order fluctuations for the location of the horizon, thereby altering the Hawking temperature and Bekenstein--Hawking entropy. Furthermore, we derived equations for perturbed null rays near the horizon with arbitrary dimensions and for complete null rays in the large $D$ limit.

gr-qc↗

arXiv2023

Bound on Lyapunov exponent in Kerr-Newman-de Sitter black holes by a charged particle

We investigate the bound on the Lyapunov exponents by a charged particle in Kerr-Newman-de Sitter black holes using analytic and numerical methods. We determine whether the Lyapunov exponent can exceed the bound by an electrically charged particle with an angular momentum. Our tests are applied to the de Sitter spacetime by the positive cosmological constant such as Reissner-Nordström-de Sitter, Kerr-de Sitter, and Kerr-Newman-de Sitter black holes. In particular, we consider Nariai and ultracold limits on these black holes for our tests. From our analysis results, there remain violations on the bound under the positive cosmological constant, and electric charge and angular momentum of the particle significantly impact the Lyapunov exponent.

gr-qc↗

arXiv2023

Absorption cross section in gravity's rainbow from confluent Heun equation

We investigate the scattering of a massless scalar field by a charged non-rotating black hole in the presence of gravity's rainbow. Using the connection coefficients of the confluent Heun equation expressed in terms of the semi-classical confluent conformal blocks and the instanton part of the Nekrasov-Shatashvili (NS) free energy, we obtain an asymptotic expansion for the low-energy absorption cross section.

gr-qc↗

arXiv2023

Metric fluctuations in higher-dimensional black holes

We investigated the impact of metric fluctuations on the higher-dimensional black hole geometry. We generalized the four-dimensional model to higher dimensions to treat quantum vacuum fluctuations by the classical approach. A fluctuating black hole is portrayed by a higher-dimensional Vaidya metric with a spherically oscillating mass. Assuming a small fluctuation amplitude, we employed a perturbation method to obtain a radially outgoing null geodesic equation up to the second order in the fluctuation. Furthermore, the fluctuation of the event horizon up to the second order depends on the number of spacetime dimensions. Therefore, the time-averaged values of the thermodynamic variables defined at the horizon also feature dimension-dependent correction terms. A general solution was obtained for rays propagating near the horizon within a fluctuating geometry. Upon examining this in a large $D$ limit, we found that a complete solution can be obtained in a compact form.

gr-qc↗

arXiv2022

Quasinormal Modes in Near-Extremal Spinning C-Metric

We investigate the quasinormal modes of the spinning C-metric with a massless scalar field conformally coupled with gravity. Conformal transformation is employed to separate and subsequently solve the massless scalar field equations. As the outer and acceleration horizons approach each other, the potential term of the field equation is reduced to the Pöshl-Teller potential because the acceleration horizon is similar to the cosmological horizon of de Sitter spacetime. Finally, we obtain the analytical quasinormal frequency at which the decay rate is quantized.

gr-qc↗

arXiv2022

Weak Cosmic Censorship Conjecture in Myers-Perry Black Hole with Separability

We investigate the weak cosmic censorship conjecture in Myers-Perry black holes with arbitrary rotations in general dimensions based on the scattering of a massless scalar field. From the fluxes of the scalar field flowing into the black hole, the changes in mass and angular momenta of the black hole are obtained. However, the extremal and near-extremal black holes with the aforementioned changes are still black holes in the final state. Hence, the conjecture is valid for our investigation. Furthermore, we analyze the changes in the black hole from a thermodynamic perspective to highlight that the laws of thermodynamics support the conjecture.

gr-qc↗

arXiv2022

Violation of bound on chaos for charged probe in Kerr-Newman-AdS black hole

We investigate the conjectured bound on the Lyapunov exponent for a charged particle with angular motion in the Kerr-Newman-AdS black hole. The Lyapunov exponent is calculated based on the effective Lagrangian. We show that the negative cosmological constant reduces the chaotic behavior of the particle, namely, it decreases the Lyapunov exponent. Hence, the bound is more effective in the AdS spacetime than in the flat spacetime. Nevertheless, we find that the bound can be violated when the angular momenta of the black hole are turned on. Moreover, we show that in an extremal black hole, the bound is more easily violated compared to the nonextremal black hole.

gr-qc↗