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Bum-Hoon Lee

Publications and source records attributed to Bum-Hoon Lee.

At least 19 recordsLinked to original sources
arXiv2026

Phase transition for a black hole with matter fields and the relation with the Lyapunov exponent

We construct static black hole solutions coexisting with anisotropic matter in asymptotically (anti-)de Sitter spacetime and investigate how the matter field modifies their thermodynamic and dynamical properties. The anisotropic matter generates an exponentially decaying charge-like contribution to the metric, causing the geometry to interpolate between the Schwarzschild black hole in anti-de Sitter spacetime and the Reissner-Nordström black hole in anti-de Sitter spacetime. In anti-de Sitter black hole spacetime, we derive the complete thermodynamic description, including the Hawking temperature, heat capacity, Smarr relation, generalized first law, and Helmholtz free energy. The system exhibits a van der Waals-type small/large black hole phase transition with a critical point determined by the matter field parameters. Local thermodynamic stability is characterized by the heat capacity, whereas global stability is determined through the free energy. We further investigate unstable homoclinic orbits by evaluating the Lyapunov exponent associated with null geodesics. Our analysis reveals that different thermodynamic branches possess distinct dynamical instabilities, and that the thermodynamically preferred phase is accompanied by a smaller Lyapunov exponent. These results demonstrate that thermodynamic stability and geodesic instability are correlated because both originate from the same underlying spacetime geometry.

gr-qc↗
arXiv2026

Chaotic behaviors of particles around the black hole with an anisotropic matter immersed in a magnetic field

We present an exact solution to the Einstein-Maxwell equations that describes a static black hole coexisting with anisotropic matter immersed in an external magnetic field, obtained via the Harrison transformation. Our findings reveal that an increase in the anisotropic matter parameter systematically suppresses the local chaotic behavior, as indicated by a reduction in the Lyapunov exponent. Conversely, variations in the external magnetic field lead to qualitative changes in global chaotic behavior. This is analyzed through Poincaré sections, which demonstrate transitions between regular and chaotic trajectories resulting from the nonlinear gravitational-magnetic interactions. These factors play distinct yet complementary roles in shaping chaotic particle dynamics around black holes. This study would offer a new theoretical framework for exploring non-integrable particle motion within magnetized black hole spacetimes and for probing a black hole at the galactic center, where magnetic fields may arise from plasma effects surrounding astrophysical black holes.

gr-qc↗
arXiv2026

Equation of State Parameters for Fluid of Stringy Extended Objects in Cosmology with Cosmological Constant

We construct the strong energy conditions (SECs) for both massive and massless stringy extended objects in the higher dimensional cosmology (HDC) with cosmological constant $Λ$. Exploiting these conditions, we find the equation of state (EoS) parameters \mbox{$w\geq -(D-4)/D$} for both the massive and massless stringy extended objects in $D$ {$(D\geq 5)$} dimensional cosmology. The stringy SECs impose a universal constraint on $w$ that remains valid across both radiation- and matter-dominated eras. We elucidate the relations between the EoS parameter in the HDC with cosmological constant and that of Hawking--Penrose limit for the massive and massless point particles in the four dimensions. We evaluate the EoS parameters in terms of the contributions from the point particle property, cosmological constant, and extended object degrees of freedom, respectively. We also investigate the weak energy condition for the massive and massless stringy extended objects in the HDC, and those for the massive and massless point particles in the four dimensions,~respectively.

gr-qc↗
arXiv2026

Extreme-Mass-Ratio Inspirals Embedded in Dark Matter Halo: Existence of Homoclinic Orbit and Horizon-Induced Chaos

We study the existence of homoclinic orbit and the onset of chaotic motion for a massive particle moving around a Schwarzschild-like black hole embedded in a Dehnen-(1,4,5/2) type dark matter halo, within the extreme-mass-ratio limit q=m/M<<1, where m and M are the masses of the particle and the central black hole, respectively. The presence of the halo modifies the spacetime curvature and consequently deforms the effective potential governing the particle's motion. Using the Hamiltonian formulation, we derive the conditions under which unstable circular orbit and the associated homoclinic trajectory arise, marking the separatrix between bound and plunging motion. By analyzing the effective potential and the corresponding phase-space structure, we identify the transition from regular to chaotic dynamics in the near-horizon region. Numerical analyses through Poincare sections and Lyapunov exponents calculations demonstrate that increasing the halo density, scale radius along with energy amplifies nonlinear effects which leads to chaos eventually. We demonstrate that within a dark matter halo environment, the dynamical stability of particle motion can be significantly altered without violating the universal surface gravity bound on chaos. This work provides a deeper understanding of horizon-induced chaos in astrophysically realistic environments and serves as a theoretical basis for exploring its possible imprints on gravitational wave signals in extreme-mass-ratio inspirals system.

gr-qc↗
arXiv2026

Resonant transmission of scalar waves through rotating traversable wormhole

The viability of traversable wormholes as exotic compact objects requires the identification of signatures that distinguish them from other compact objects. Given recent advances in observing rotating black hole signatures, identifying characteristic imprints that reflect the absence of an event horizon and the presence of a throat structure is of considerable significance. Motivated by this, in the present work, we analyze the propagation of a massless scalar field in a rotating traversable wormhole spacetime described by Teo's class of solutions. We numerically compute the transmission (greybody) factor and the corresponding absorption spectrum across a broad range of frequencies. The spectrum exhibits a series of sharp peaks in the amplitudes, which we identify as Breit-Wigner-type resonances. The emergence of such peaks can be attributed to the scalar modes temporarily trapped within the potential well formed by barriers on either side of the throat. These resonant features, previously identified in static wormhole backgrounds, persist in the rotating case. In particular, for Teo's class of wormholes, we find that rotation enhances the strength of the resonances. Overall, our results demonstrate the role of rotation in shaping the resonance effect and indicate these features as characteristic signatures of wormhole geometries.

gr-qc↗
arXiv2026

Gauss-Bonnet corrected string/black hole transition in large dimensions

We develop a unified analytic treatment of the Horowitz--Polchinski string/black hole correspondence that systematically incorporates higher-derivative corrections to gravity. Working in Euclidean signature -- where the Euclidean black hole and the thermal scalar arise as competing saddles of the same finite-temperature ensemble -- we include the Gauss--Bonnet term. The analysis is rendered tractable in this UV--sensitive regime by the large-\(D\) expansion, which sharply separates the geometry into a universal near-zone and an asymptotic far-zone. In the near-zone, the coupled large-\(D\) equations reduce the thermal-scalar sector to an exactly solvable Schrödinger problem, from which we extract the \(α'\)-corrected decay exponent and the corresponding shift of the Hagedorn temperature. In the far-zone, we construct closed-form Euclidean solutions of Einstein--Gauss--Bonnet theory at leading order in both \(1/D\) and \(α'\). Matching the two regions yields the complete corrected saddle -- fixing its temperature, horizon data, and on--shell action -- and permits a fully analytic comparison of free energies between the thermal-scalar and black hole phases. This provides a controlled derivation of the HP correspondence point with explicit higher-curvature corrections.

hep-th↗
arXiv2026

There and back again -- Closed timelike curves as EFT selection principle

Modified gravity is often approached in the context of effective-field theory (EFT), with the view that the EFT corrections permit a more desirable theory. In this paper, we posit that this should extend to the causal structure of curved spacetime in addition to the standard demands such that of flat spacetime positivity and unitarity. We propose a new guiding principle for modified-gravity theories, namely that closed timelike curves should always be {\it harder} to obtain than in General Relativity. By demanding this, one can place powerful constraints on modified gravity. To elucidate this claim, we investigate modified-gravity EFTs on rotating black-hole backgrounds, focusing on the appearance/disappearance of closed timelike curves, and provide parameter bounds which only partly overlap with other approaches based on time delay. We construct perturbative rotating black-hole solutions in modified-gravity EFTs based on the Horndeski class and provide parameter bounds necessary to preserve causality and stability. Finally, we present a novel probe for the existence of closed timelike curves through quasinormal modes and black-hole echoes. This can be used to diagnose spacetime causality once next-generation gravitational-wave data becomes available.

gr-qc↗
arXiv2026

Charged traversable wormholes: charge without charge

We present and investigate charged wormhole solutions of the Einstein-Maxwell equations supported by anisotropic matter fields, with the purpose of establishing their physical plausibility as traversable wormholes. To this end, we examine the flare-out condition and evaluate tidal forces to confirm their traversability. We also analyze light deflection around these wormholes to provide observational implications. Additionally, we attempt to construct rotating generalizations of the solutions by applying and modifying the Newman-Janis algorithm. Our results suggest that the obtained geometries offer a concrete realization of the concept of ``charge without charge".

gr-qc↗
arXiv2025

Extreme-Mass-Ratio Inspirals Embedded in Dark Matter Halo II: Chaotic Imprints in Gravitational Waves

We investigate the imprints of chaos in gravitational waves from extreme-mass-ratio inspirals configuration, where a stellar massive object, confined in a harmonic potential, orbits a supermassive Schwarzschild-like black hole embedded in a Dehnen-type dark matter halo. In our first paper [1], we demonstrated the system's transition from non-chaotic to chaotic dynamics by analyzing Poincaré sections, orbital evolution, and Lyapunov exponents across different energies and dark matter halo parameters. In this work, we compute the gravitational waveforms of the small celestial object along different chaotic and non-chaotic orbits by implementing the numerical kludge scheme. We further perform a spectral analysis of the gravitational waveforms from such orbits. In particular, we show that when the system is in a chaotic state, the gravitational wave signals are characterized by broader frequency spectra with finite widths, enhanced amplitude and energy emission rate, distinctly differentiating them from the signals generated during the system's non-chaotic state. Through recurrence analysis we also show that the time series of gravitational waveforms strain carry unique information on the motion of chaotic dynamics, which can be used to distinctly differentiate from non-chaotic to chaotic motion of the source. Furthermore, we discuss the potential detectability of these orbits for upcoming observatories like LISA, TianQin, and Taiji, emphasizing the significant potential for detecting chaotic imprints in gravitational waves to substantially enhance our understanding of chaotic dynamics in black hole physics and the dark matter environments of galactic nuclei.

gr-qc↗
arXiv2025

Constraints on Cosmic Birefringence from SPIDER, Planck, and ACT observations

The Early Dark Energy (EDE) model has been proposed as a candidate mechanism to generate cosmic birefringence through a Chern-Simons coupling between a dynamical scalar field and the cosmic microwave background (CMB) photon. Such birefringence induces a nonzero cross-correlation between the CMB $E$- and $B$-modes, providing a direct observational signature of parity violation. Recent measurements of the $EB$ and $TB$ power spectra, however, cannot yet unambiguously separate instrumental miscalibration ($α$) from a true cosmic-rotation angle ($β$). For this reason, we perform a model-independent analysis in terms of the total effective rotation angle $α+β$. We analyze the latest $EB$ and $TB$ measurements from the SPIDER, Planck, and ACT experiments and derive constraints on the Chern-Simons coupling constant $gM_{Pl}$ and on the polarization rotation angle $α+β$. We find that the coupling $gM_{Pl}$ is not compatible with the SPIDER data, while it provides reasonable fits to the Planck and ACT measurements. The fits for $α+β$ prefer a value larger than zero: when combined, Planck+ACT yield a detection significance of approximately 7$σ$. We also find that ACT data alone do not provide sufficiently tight constraints on either $gM_{Pl}$ or $α+β$, whereas the combination Planck+ACT improves the statistical consistency of ACT's high-$\ell$ results and leads to a better PTE for those measurements.

astro-ph.CO↗
arXiv2025

Black holes immersed in modified Chaplygin-like dark fluid and cloud of strings: geodesics, shadows, and images

This study investigates a black hole surrounded by a cloud of strings and a cosmological dark fluid characterized by a modified Chaplygin-like equation of state (MCDF), $p=Aρ-B/ρ^β$. We analyze its geodesic structure, shadow, and optical appearance. Analysis of the effective potential and epicyclic frequencies reveals that the existence of innermost/outermost stable circular orbits (ISCOs/OSCOs) for timelike particles is controlled by the parameters of the MCDF and the cloud of strings. The behavior of orbital conserved quantities and the Keplerian frequency are also examined. By equating the influence of the MCDF on the spacetime metric at spatial infinity with that of a cosmological constant, we constrain the MCDF parameters using the observed shadow radii of Sgr A* and M87*. We investigate the effects of the cloud of strings and MCDF on the black hole's shadows and optical images, assuming various thin disk accretion profiles. Using the method developed by Wald and collaborators, light trajectories are classified by their impact parameters into direct emission, the lensing ring, and the photon ring. The presence of OSCOs can lead to the existence of outer edges in the direct emission and lensing ring images. Observed brightness primarily originates from direct emission, with a minor contribution from the lensing ring, while the photon ring's contribution is negligible due to extreme demagnification. The influence of the cloud of strings and MCDF parameters on all results is analyzed throughout the study.

gr-qc↗
arXiv2025

Stability and topological nature of charged Gauss-Bonnet AdS black holes in five dimensions

We investigate the thermodynamics of Reissner-Nordstrom Gauss-Bonnet (RN-GB) black holes in anti-de Sitter (AdS) space with three horizon geometries (k=+1,0,-1) within the grand canonical ensemble. Using the recently developed topological approach to black hole thermodynamics, inspired by Duan's $ϕ$-mapping theory, we analyze the black holes by treating both critical points in the phase diagram and black hole solutions as defects in the thermodynamic parameter space. Our results show that the Gauss-Bonnet coupling significantly alters the topological classification of RN-GB AdS black holes, distinguishing them from their RN AdS counterparts in the grand canonical ensemble, while aligning with their canonical ensemble counterparts. Complementary analyses of local stability using specific heat validate the implication of topological analysis. Furthermore, an evaluation of global stability via Gibbs free energy provides a comprehensive understanding on the system's phase structure. Notably, for k=+1, topological analysis suggests liquid-gas type phase transitions, whereas global analysis favors Hawking-Page transitions. For k=-1, topology indicates a single stable black hole branch, yet the global analysis reveals the presence of Hawking-Page transition.

hep-th↗
arXiv2024

Observational evidence for Early Dark Energy as a unified explanation for Cosmic Birefringence and the Hubble tension

We test the $n$=3 Ultralight Axion-like model of Early Dark Energy (EDE) with the observationsof the $EB$ mode of the cosmic microwave background (CMB) radiation, and local expansion rate measurements. Our results show that the shape of the CMB $EB$ angular power spectrum is sensitive to the background cosmological parameters. We run Markov chain Monte Carlo (MCMC) simulations to fit the $Λ$CDM + EDE parameters simultaneously, and find that the EDE model with $n$=3 can provide a good fit to the observed CMB $EB$ spectra, consistent with the locally measured value of the Hubble constant. Our result is the first to show that axion-like EDE can provide a unified explanation for the observed cosmic birefringence and the Hubble tension.

astro-ph.CO↗
arXiv2024

Gauss-Bonnet Cosmology: large-temperature behaviour and bounds from Gravitational Waves

We provide a transparent discussion of the high temperature asymptotic behaviour of Cosmology in a dilaton-Einstein-Gauss-Bonnet (dEGB) scenario of modified gravity with vanishing scalar potential. In particular, we show that it has a clear interpretation in terms of only three attractors (stable critical points) of a set of autonomous differential equations: $w=-\frac{1}{3}$, $w=1$ and $1<w<\frac{7}{3}$, where $w\equiv p/ρ$ is the equation of state, defined as the ratio of the total pressure and the total energy density. All the possible different high-temperature evolution histories of the model are exhausted by only eight paths in the flow of the set of the autonomous differential equations. Our discussion clearly explains why five out of them are characterized by a swift transition of the system toward the attractor, while the remaining three show a more convoluted evolution, where the system follows a meta-stable equation of state at intermediate temperatures before eventually jumping to the real attractor at higher temperatures. Compared to standard Cosmology, the regions of the dEGB parameter space with $w=-\frac{1}{3}$ show a strong enhancement of the expected Gravitational Wave stochastic background produced by the primordial plasma of relativistic particles of the Standard Model. This is due to the very peculiar fact that dEGB allows to have an epoch when the energy density $ρ_{\rm rad}$ of the relativistic plasma dominates the energy of the Universe while at the same time the rate of dilution with $T$ of the total energy density is slower than what usually expected during radiation dominance. This allows to use the bound from BBN to put in dEGB a constraint $T_{\rm RH}\lesssim 10^8 - 10^9$ GeV on the reheating temperature of the Universe $T_{\rm RH}$. Such BBN bound is complementary to late-time constraints from compact binary mergers.

hep-ph↗
arXiv2024

Induced Cosmological Anisotropies and CMB Anomalies by a non-Abelian Gauge-Gravity Interaction

We present a non-abelian cousin of the model presented in \cite{Lee:2022rtz} which induces cosmological anisotropies on top of standard FLRW geometry. This is in some sense doing a cosmological mean field approximation, where the mean field cosmological model under consideration would be the standard FLRW, and the induced anisotropies are small perturbative corrections on top of it. Here we mostly focus on the non-abelian $SU(2)$ gauge fields coupled to the gravity to generate the anisotropies, which can be a viable model for the axion-like particle (ALP) dark sector. The induced anisotropies are consequences of the non-trivial back-reaction of the gauge fields on the gravity sector, and by a clever choice of the parametrization, one can generate the Bianchi model we have studied in this note. We also show that the anisotropies influence the Sachs-Wolfe effect and we discuss the implications.

gr-qc↗
arXiv2024

Induced cosmological anisotropy by a gauge-gravity interaction

We present a simple model which generates cosmological anisotropies on top of standard FLRW geometry. This is in some sense reminiscent of the mean field approximation, where the mean field cosmological model under consideration would be the standard FLRW, and the anisotropy is a small perturbative correction on top of it. Using a supergravity-inspired model, we confirm that the stable fixed point of our model corresponds to standard FLRW cosmology. We use a Bianchi VII$_0$-type model supplemented with a scalar and $U(1)$ gauge fields, and we show that the anisotropies of the geometry are generated by the non-trivial interaction between the gravity sector and the $U(1)$ gauge sector. Studying the attractor flow, we show that the anisotropies are present at early times (high redshift) and decay asymptotically to an FLRW attractor fixed point. With such a mechanism, observations of non-isotropy are not contradictory to FLRW geometry or indeed the $Λ$CDM model. Such models could in principle shed some insights on the present cosmological tensions.

gr-qc↗
arXiv2023

Superradiance in the Kerr-Taub-NUT spacetime

Superradiance is the effect of field waves being amplified during reflection from a charged or rotating black hole. In this paper, we study the low-energy dynamics of super-radiant scattering of massive scalar and massless higher spin field perturbations in a generic axisymmetric stationary Kerr-Taub-NUT (Newman-Unti-Tamburino) spacetime, which represents sources with both gravitomagnetic monopole moment (magnetic mass) and gravitomagnetic dipole moment (angular momentum). We obtain a generalized Teukolsky master equation for all spin perturbation fields. The equations are separated into their angular and radial parts. The angular equations lead to spin-weighted spheroidal harmonic functions that generalize those in Kerr spacetime. We identify an effective spin as a coupling between frequency (or energy) and the NUT parameter. The behaviors of the radial wave function near the horizon and at the infinite boundary are studied. We provide analytical expressions for low-energy observables such as emission rates and cross sections of all massless fields with spin, including scalar, neutrino, electromagnetic, Rarita-Schwinger, and gravitational waves.

gr-qc↗