arXiv subjects

Da-Shin Lee

Publications and source records attributed to Da-Shin Lee.

At least 19 recordsLinked to original sources

arXiv2026

Dynamical Love numbers of analogue rotating black and white holes

We calculate the dynamical tidal response coefficients (TRCs) of 2+1D analogue black and white holes generated by draining and fountaining bathtub flows, respectively. The parameter space is characterized by the frequency and azimuthal number of the Fourier modes and the rotation of the analogue black or white hole. In general, the TRC is a complex-valued function of these parameters. Its real and imaginary parts are defined as the tidal Love number (TLN) and the tidal dissipation coefficient (TDC), respectively. The TRC of the analogue black hole (ABH) exhibits several interesting features. At certain points in the parameter space, including the static case of vanishing perturbation frequency, the TLN exhibits logarithmic running, while the TDC vanishes. Unlike in Einstein gravity, fluid dynamics allows for the physical existence of an analogue white hole (AWH) event horizon. Time-independent, torque-free, barotropic, inviscid, axisymmetric fluid dynamical equations can yield a pair of transonic background flow solutions. The solutions in the pair, corresponding to an ABH-AWH pair, share the same angular momentum and Bernoulli constant but have opposite mass flow rates, all of which are conserved quantities. For such an ABH-AWH pair, the TRC of the AWH is the complex conjugate of the TRC of the ABH.

gr-qc↗

arXiv2026

Entanglement and firewalls in quantum circuit model of black hole evaporation

We reexamine the quantum circuit model of black hole evaporation proposed in a previous work (Class. Quantum Grav. 35, 235013, 2018) [1]. This tripartite model incorporates the following systems: black hole ($\mathbf{BH}$), just radiation ($\mathbf{JR}$), and early radiation ($\mathbf{ER}$). We apply a scrambling unitary matrix with a single parameter $\theta$ to the ground state of the qubits in infalling matter toward a black hole in order to generate initial qubit states of the black hole that are more general than those in [1]. Specifically, the scrambling unitary matrix reduces to no scrambling and maximum scrambling when $\theta=0$ and $\theta=\pi/2$, respectively. Our aim is to explore the role of quantum monogamy in the firewall formation between the black hole and radiation. In this model, entanglement and firewall formation depend on the black hole mass $M$ and the frequency of Hawking radiation $\omega$. For the initial state with $\theta=\pi/2$, a firewall emerges at an earlier stage of the evolution than with $\theta=0$. We also find that a firewall structure emerges between $\mathbf{BH}$ and $\mathbf{JR}$, and that the information is carried away by radiation for all values of $M\omega$, provided that $\theta$ lies within a certain analytically determined range. Following unitary gate dynamics, the initial black hole qubit state can be retrieved from its imprint on the final radiation state, which was originally hidden behind the black hole's horizon. These results may provide insight into the properties of multipartite entanglement due to the different initial states in the evolution of a quantum circuit model for black hole evaporation.

gr-qc↗

arXiv2026

Analytical solutions for timelike orbits around Damour-Solodukhin wormholes

We investigate timelike geodesics around Damour-Solodukhin wormholes, which are Schwarzschild-like geometries characterized by a deformation parameter $\lambda$ that determines the radius of the throat, $r_{\rm th}$. The radial potential admits four roots, including the throat radius itself, allowing the throat to merge with other roots and form double, triple, and quartic degeneracies. In particular, triple-root configurations associated with the throat determine the innermost stable circular orbit (ISCO), providing a potential observational distinction from Schwarzschild black holes. Using the Mino-time parametrization, we derive particle trajectories with closed-form analytical solutions in terms of incomplete elliptic integrals for both bound and unbound motion. In particular, we focus on double or triple roots are located at the throat, the azimuthal angle and coordinate time exhibit logarithmic or power-law divergences as the particle approaches the throat. By contrast, trajectories remain regular when the throat corresponds to a simple root, allowing particles to traverse smoothly between the two asymptotically flat regions. We also derive exact homoclinic solutions associated with the throat and compute the corresponding Lyapunov exponent. In addition, inspiral and plunge trajectories through the throat are analyzed. These results provide analytic insights into particle dynamics and possible observational signatures of the wormholes.

gr-qc↗

arXiv2025

Motion of spinning particles in the Kerr-Newman black hole exterior

The motion of a spinning particle in the exterior of a Kerr-Newman black hole is studied. The dynamics is governed by the Mathisson-Papapetrou equations in the pole-dipole approximation, which includes spin-curvature coupling to the first order of spin. In terms of conserved quantities, the dynamical equations in Mino time can be transformed into the integral form for both aligned and misaligned spins with respect to the orbital motion. These non-geodesic equations can be solved analytically, and the solutions involve Jacobi elliptic functions. We derive the radial potential to study the parameter space of the particle for various types of orbits based on its roots, corrected by the particle's spin. In the misaligned case, we consider equatorial motion of a particle oscillating between two turning points, which are the two outermost roots of the radial potential. This results in an induced oscillatory motion out of the equatorial plane. In particular, the periods of the motion are obtained explicitly. To validate our analytical solutions further, we compare them with the results of exact numerical integration, demonstrating good agreement. When the orbits become a source of gravitational-wave emission, these periods of motion will provide essential input in determining gravitational-wave signals in the frequency domain. The implications for gravitational-wave emission due to extreme mass-ratio inspirals (EMRIs) are discussed.

gr-qc↗

arXiv2025

Regge poles of analogous rotating black holes in binary Bose-Einstein condensates: The gapped excitations

In this paper, we study the spectrum of the Regge poles (RPs), which are the counterparts of quasinormal modes, in a draining bathtub vortex within a two-component Bose-Einstein condensate (BEC) system. We study the gapped excitations of the condensate with the spatially dependent energy gap term using a spatially tunable Rabi coupling, which will be treated as a perturbation. This model serves as an analogue of a rotating black hole surrounded by an environmental mass shell. We first compute the semiclassical scattering amplitude with the spatially independent mass effect due to the orbital interference. In the case of the mass-shell, bifurcation of the spectrum is observed, resulting in the destabilization of the RPs. We also study the migration of RPs by shifting the bump position. Our results show that the RPs of the co-rotating modes exhibit greater stability than those of the counter-rotating modes. Large migration and overtaking jumps of the overtone (fundamental RP) leave an imprint on the scattering amplitude at small (large) scattering angles. This can be observed in the scattering interference pattern in experiments.

gr-qc↗

arXiv2025

Decoherence by black holes via holography

In this note, we reexamine decoherence effects in quantum field theories with gravity duals. The thought experiment proposed in \cite{DSW_22, DSW_23}, which reveals novel decoherence patterns associated with black holes, also manifests itself from the perspective of the boundary theory. In particular, we consider a moving mirror coupled to quantum critical theories characterized by a dynamical exponent $z$ that are dual to asymptotically Lifshitz geometries. The interference experiment occurs on the boundary, where a superposition of two spatially separated quantum states of a mirror is maintained for a finite time $\tau_0$ before recombination. We find that the interaction with a quantum field at finite temperature, arising from the presence of a Lifshitz black hole, leads to a constant decoherence rate. In contrast, for the zero-temperature case corresponding to pure Lifshitz spacetime, the decoherence rate vanishes in the large-time limit $\tau_0 \to \infty$. Remarkably, in the zero-temperature regime, the decoherence exhibits a power-law decay at large $\tau_0$ as $z \rightarrow \infty$, a behavior reminiscent of the decoherence patterns seen in extremal black hole geometries. In addition, we investigate the decoherence of one particle in an EPR pair constructed holographically. Our results indicate that causality plays a crucial role in determining whether the entanglement leads to the suppression of decoherence in the other particle.

hep-th↗

arXiv2025

Dynamics of spinning particles in Reissner-Nordstr\"om black hole exterior

We study the orbits of a spinning particle in the Reissner-Nordstr\"om black hole exterior through the spin-curvature coupling to leading order in its spin. The dynamics is governed by the Mathisson-Papapetrou equations in the pole-dipole approximation. The equations of motion can be derived and show in particular that in the polar coordinate, the orbits can be restricted to the plane for an aligned spin with the orbital motion, but there is an induced motion out of the plane for a misaligned spin. The radial potential can be defined from the equation of motion along the radial direction, where the roots are studied to construct the parameter space diagram for different types of orbits. We then consider the so-called innermost stable circular orbit (ISCO) due to the triple root to see the effects of the particle spin and the black hole charge. These non-geodesic equations can be solved analytically in terms of the Mino time with the solutions involving Jacobi elliptic functions. One of the bound motions considered is an oscillating orbit between two turning points in the radial direction. The usefulness of the solutions is to obtain the periods of the oscillation along the radial direction as well as the induced motion in the polar coordinate for a misaligned spin in both the Mino time and the coordinate time. Another interesting motions include the inspiral orbit from near ISCO and the homoclinic orbit with the solutions expressed as elementary functions, giving the radial 4-velocity of the inspiral orbit and the Lyapunov exponent associated with the homoclinic orbit. The implications for gravitational wave emission from extreme mass-ratio inspirals (EMRIs) and black hole accretion are discussed.

gr-qc↗

arXiv2025

Out-of-Time-Order-Correlators in Holographic EPR pairs

In this note, we investigate the out-of-time-order correlators (OTOCs) for quantum fields in a holographic framework describing Einstein-Podolsky-Rosen (EPR) pairs. We compute the four-point and six-point OTOCs using the gravity dual, represented by the string worldsheet theory in Anti-de Sitter (AdS) space. These correlators quantify the rate at which information is scrambled, leading to the disentanglement of the EPR pair. We demonstrate consistency between two approaches for calculating OTOCs: the holographic influence functional on worldsheets perturbed by shock waves, and the worldsheet scattering in the eikonal approximation. We show that the OTOCs exhibit an initial phase of exponential growth, with six-point correlators indicating a marginally longer scrambling time compared to four-point correlators.

hep-th↗

arXiv2024

Null geodesics in extremal Kerr-Newman black holes

We study the null geodesics in the extremal Kerr-Newman exterior. We clarify the roots of the radial potential and obtain the parameter space of the azimuthal angular momentum and the Carter constant of the light rays for varieties of the orbits. It is known that one of the unique features of extremal black holes for the null geodesics is the existence of the stable double root at the horizon, giving rise to the stable spherical motion. For the black hole's spin $a<M/2$, the stable double root is isolated from the unstable one. However, for $ a\ge M/2$, the unstable and stable double roots merge at the triple root so that the unstable double root in some parameter region can lie at the horizon, giving a very different shape to the light ring. We then find the analytical expressions of light orbits, which can reach spatial infinity for both nonequatorial and equatorial motions. In particular, for the orbits starting from the near horizon of the extremal Kerr-Newman black holes with the parameters for the unstable double and triple roots, the solutions are remarkably simple in terms of elementary functions. It is also found that the analytical solutions of the equatorial motion can shed light on the deflection of the light by black holes. Varying the azimuthal angular momentum, as either the double or triple root at the horizon is approached from the turning point, the stronger power-law divergence in the deflection angle is found in comparison with the typical logarithmic divergence in nonextremal black holes in the strong deflection limit. This could be another interesting effect of light deflection by extremal black holes.

gr-qc↗

arXiv2024

Throat effects on strong gravitational lensing in Kerr-like wormholes

We study strong gravitational lensing by a specific one-parameter extension of Kerr spacetime, a Kerr-like wormhole, characterized by a single parameter specifying the throat's location. We classify the roots of the radial potential derived from the null geodesic equations. We focus on the conditions required for the throat, together with the other roots, to become either a double root or a triple root, potentially leading to the divergence of the deflection angle of the light rays in the strong deflection limit (SDL). In particular, while a logarithmic divergence of the deflection angle is known to occur as the closest distance $r_0$ of an incident light trajectory around a black hole approaches a double root, a stronger power-law (nonlogarithmic) divergence is found as $r_0$ approaches a triple root especially in a wormhole. In addition, the effective potential in terms of the proper distance from the throat is constructed, with which one can see how the light rays can either travel within a single spacetime, where both the source and the observers are located, or pass from the source through the throat into another spacetime where different observers reside. Observational effects, such as relativistic images resulting from the deflection of light by wormholes, are discussed, and they could serve as a unique feature of wormholes.

gr-qc↗

arXiv2024

Primordial perturbations in Type III hilltop inflation models

We analytically compute the power spectrum of primordial curvature perturbations in Type III hilltop inflation models under the slow-roll approximation. The model parameters are constrained using current Cosmic Microwave Background (CMB) data. The curvature perturbations that exit the horizon at small scales show sufficiently large amplitudes to produce primordial black holes (PBHs). We then consider the quantum one-loop corrections in these models from both the self-interaction of the inflaton and its interaction with the waterfall field. We show the loop corrections in both cases for 60 e-folds of inflation are negligible, ensuring the tree-level results are reliable within the chosen parameter regime.

gr-qc↗

arXiv2024

Acoustic quasibound states and tachyonic instabilities from binary Bose-Einstein condensates

We consider two-component Bose-Einstein condensates (BECs) and introduce the BEC vortex in $1+2$ dimensions. We focus on two types of gapped excitations induced by the modes of two-component BECs with relative phases of $0$ and $\pi$, analogous to the massive scalar field with positive and negative mass squared, respectively. The inclusion of space-dependent Rabi coupling can induce an effective space-dependent mass term. We study superradiant instabilities resulting from the quasibound states corresponding to positive mass squared and the tachyonic instabilities arising from negative mass squared in both the frequency and time domains. These instabilities resemble two possible mechanisms that make Kerr black holes unstable in scalar-tensor gravity with the presence of matter around the black hole. Our proposed phenomena could potentially be implemented in future experiments, drawing from the success of recent analog rotating black hole implementations.

gr-qc↗

arXiv2023

Quantum Loop effects to Primordial perturbations at the end of Type III hilltop inflation models

In this work, we analytically calculate the spectra of primordial perturbations at the end of Type III hilltop inflation models under the slow-roll approximation. We examine the one-loop corrections of the spectra and find that those from the inflaton self-interaction are negligible. On the contrary, the loop effects from the interaction between the inflaton field and the waterfall field can be significant when the vacuum expectation value of the waterfall field is small. The implications are discussed.

astro-ph.CO↗

arXiv2023

Inspiral and Plunging Orbits in Kerr-Newman Spacetimes

We present the analytical solutions for the trajectories of particles that spiral and plunge inward the event horizon along the timelike geodesics following general non-equatorial paths within Kerr-Newman spacetimes. Our studies encompass both bound and unbound motions. The solutions can be written in terms of the elliptical integrals and the Jacobian elliptic functions of manifestly real functions of the Mino time. They can respectively reduce to the Kerr, Reissner-Nordstr$\ddot{o}$m, and Schwarzschild black holes in certain limits of the spin and charge of the black holes, and can be compared with the known ones restricted in equatorial motion. These explicit solutions may have some implications for the gravitational wave emission from extreme mass-ratio inspirals.

gr-qc↗

arXiv2023

Primordial perturbations from ultra-slow-roll single-field inflation with quantum loop effects

It is known that the single-field inflation with a transient ultra-slow-roll phase can produce a large curvature perturbation at small scales for the formation of primordial black holes. In our previous work, we have considered quantum loop corrections to the curvature perturbation and found that the growth of these small-scale modes would affect the curvature perturbation at large scales probed by cosmic microwave background observation. In this work, we will further derive the constraints on the growing modes in the transition between the slow-roll and the ultra-slow-roll phases under the effect of the loop corrections. Our results would help clarify the recent controversy on whether or not the primordial-black-hole formation from the single-field inflation is ruled out at one-loop level.

astro-ph.CO↗

arXiv2023

Homoclinic orbits in Kerr-Newman black holes

We present the exact solutions of the homoclinic orbits for the timelike geodesics of the particle on the general nonequatorial orbits in the Kerr-Newman black holes. The homoclinic orbit is the separatrix between bound and plunging geodesics, a solution that asymptotes to an energetically bound, unstable spherical orbit. The solutions are written in terms of the elliptical integrals and the Jacobi elliptic functions of manifestly real functions of the Mino time where we focus on the effect from the charge of the black hole to the homoclinic orbits. The parameter space of the homoclinic solutions is explored. The nonequatorial homoclinic orbits in Kerr cases can be obtained by setting the charge of the black holes to be zero. The homoclinic orbits and the associated phase portrait as a function of the radial position and its derivation with respect to the Mino time are plotted using the analytical solutions. In particular, the solutions can reduce to the zero azimuthal angular moment homoclinic orbits for understanding the frame dragging effects from the spin as well as the charge of the black hole. The implications of the obtained results to observations are discussed.

gr-qc↗

arXiv2022

Analogous Hawking radiation from gapped excitations in a transonic flow of binary Bose-Einstein condensates

We have studied analytically the approximate solutions to the gapped mode equations in the hydrodynamic regime for a class of binary Bose-Einstein condensate acoustic black holes. The horizon from the transonic flow is formed by manipulating the phonon sound speed and the flow velocity with the experimentally accessible parameters. The asymptotic modes of various scattering processes are constructed from which to obtain scattering coefficients and then to further decompose the field operator in terms of the asymptotic states. Also, the Unruh state is introduced to be the appropriate state for the description of gravitational collapse of the black hole. The particle densities of the outgoing modes are computed. The effective energy gap term in the dispersion relation of the gapped excitations introduces the threshold frequency $\omega_r$ in the subsonic regime, below which the propagating modes do not exist. Thus, the particle spectrum of the analogous Hawking modes in the exterior of the horizon of the subsonic region significantly deviates from that of the gapless cases near the threshold frequency due to the modified graybody factor, which vanishes as the mode frequency is below $\omega_r$. However, in the interior region of the horizon of the supersonic region, the spectrum of the particle production of the Hawking partner has the nonthermal feature. The correlators between the analog Hawking mode and its partner of relevance to the experimental observations are also investigated and show some peaks near the threshold frequency $\omega_r$ resulting from the gap energy term to be seen in future experiments.

gr-qc↗

arXiv2022

Null and time-like geodesics in Kerr-Newman black hole exterior

We study the null and time-like geodesics of the light and the neutral particles respectively in the exterior of Kerr-Newman black holes. The geodesic equations are known to be written as a set of first-order differential equations in Mino time from which the angular and radial potentials can be defined. We classify the roots for both potentials, and mainly focus on those of the radial potential with an emphasis on the effect from the charge of the black holes. We then obtain the solutions of the trajectories in terms of the elliptical integrals and the Jacobian elliptic functions for both null and time-like geodesics, which are manifestly real functions of the Mino time that the initial conditions can be explicitly specified. We also describe the details of how to reduce those solutions into the cases of the spherical orbits. The effect of the black hole's charge decreases the radii of the spherical motion of the light and the particle for both direct and retrograde motions. In particular, we focus on the light/particle boomerang of the spherical orbits due to the frame dragging from the back hole's spin with the effect from the charge of the black hole. To sustain the change of the azimuthal angle of the light rays, say for example $\Delta \phi=\pi$ during the whole trip, the presence of the black hole's charge decreases the radius of the orbit and consequently reduces the needed values of the black hole's spin. As for the particle boomerang, the particle's inertia renders smaller change of the angle $\Delta \phi$ as compared with the light boomerang. Moreover, the black hole's charge also results in the smaller angle change $\Delta \phi$ of the particle than that in the Kerr case. The implications of the obtained results to observations are discussed.

gr-qc↗