arXiv subjects

Kazufumi Takahashi

Publications and source records attributed to Kazufumi Takahashi.

At least 19 recordsLinked to original sources

arXiv2026

Avoided crossings and resonances in black hole ringdown with coupled fields: A case study of the Einstein-Maxwell-axion system

Avoided crossings in black hole quasinormal-mode (QNM) spectra exhibit characteristic resonant properties, including enhanced excitation factors. We extend the study of this phenomenon to perturbation systems with multiple coupled fields and investigate its consequences for ringdown waveforms. As a concrete example, we consider a charged black hole in the Einstein-Maxwell-axion system, whose QNM branches can be classified according to the gravitational, electromagnetic, and axion modes to which they continuously connect in the decoupling limit. We focus on an avoided crossing between the gravity- and electromagnetic-led branches thus defined. We verify that the QNM reconstruction accurately reproduces the ringdown portion of the directly evolved time-domain waveform. Near the avoided crossing, an effectively slower decay emerges for particular initial perturbations, while the resonantly enhanced contributions from the two QNMs largely cancel in the physical waveform. The QNM decomposition further reveals an exchange of the physical characters of the two branches across the resonance. These results characterize the imprint of resonances on black hole ringdown in systems with multiple coupled fields.

gr-qc↗

arXiv2026

Black Hole Quasinormal Mode Resonances and Reconnections in Coupled Systems

Black hole quasinormal modes (QNMs) provide a window into the underlying structure of black holes and the environments in which they arise. QNMs have particularly rich dynamics in coupled systems beyond general relativity, in which they are known to undergo avoided crossings, resonant excitation, and mode reconnections. Recent work has shown that reconnecting QNM trajectories can trace out a distinctive double spiral structure in the complex frequency plane. In this work, we study this phenomenology in a concrete model of perturbed dynamical Chern-Simons (dCS) theory, as well as in a minimal toy model. In the former, we characterize the exceptional points, avoided crossings and reconnections for several values of the perturbation parameters, and find that the limiting frequency of the double spiral endpoints is independent of the perturbation strength. We compute the excitation factors and find that they follow the scaling characteristic of a resonance between the scalar and gravitational modes. To determine whether these phenomena depend on the specific features of dCS, we construct a toy model of coupled degrees of freedom with square potential barriers, and find that we can reproduce the avoided crossings and reconnections, indicating that these phenomena can occur more generally in coupled systems and are highly sensitive to the system parameters.

gr-qc↗

arXiv2026

Effective description of generalized disformal theories

Generalized disformal transformations enable us to construct the generalized disformal Horndeski theories, which form the most general class of ghost-free scalar-tensor theories to this date. We extend the effective field theory (EFT) of cosmological perturbations to incorporate these generalized disformal Horndeski theories. The main difference from the conventional EFT is that our extended EFT involves operators with higher spatial derivatives of the lapse function. Our EFT also accommodates the generalized disformal transformation of U-DHOST theories.

gr-qc↗

arXiv2026

Probing higher curvature gravity via ringdown with overtones

We investigate metric perturbations of a spherically symmetric black hole in higher curvature gravity. We show that higher curvature corrections deform the near-horizon region of the effective potential, and that the deviations of the quasinormal mode (QNM) frequencies from their general relativity (GR) values become more pronounced for overtone modes. We find that, as the order of the higher curvature term increases, the deformations approach the horizon and the deviations of the overtone QNM frequencies grow progressively larger. We also analyze the ringdown waveforms in the higher curvature gravity model. We consider setups in which the deviations from the vacuum-GR QNMs remain mild for the fundamental mode and the first few overtones, and show that these shifted QNMs can be identified in the ringdown signal through waveform fitting.

gr-qc↗

arXiv2026

Inverting no-hair theorems: How requiring General Relativity solutions restricts scalar-tensor theories

Black hole solutions in general scalar-tensor theories are known to permit hair, i.e. non-trivial scalar profiles and/or metric solutions different from the ones of General Relativity (GR). Imposing that some such solutions$\unicode{x2013}$e.g. Schwarzschild or de Sitter solutions motivated in the context of black hole physics or cosmology$\unicode{x2013}$should exist, the space of scalar-tensor theories is strongly restricted. Here we investigate precisely what these restrictions are within general quadratic/cubic higher-order scalar-tensor theories for stealth solutions, whose metric is given by that in GR, supporting time-dependent scalar hair with a constant kinetic term. We derive, in a fully covariant approach, the conditions under which the Euler-Lagrange equations admit all (or a specific set of) exact GR solutions, as the first step toward our understanding of a wider class of theories that admit approximately stealth solutions. Focusing on static and spherically symmetric black hole spacetimes, we study the dynamics of linear odd-parity perturbations and discuss possible deviations from GR. Importantly, we find that requiring the existence of all stealth solutions prevents any deviations from GR in the odd-parity sector. In less restrictive scenarios, in particular for theories only requiring the existence of Schwarzschild(-de Sitter) black holes, we identify allowed deviations from GR, derive the stability conditions for the odd modes, and investigate the generic deviation of a non-trivial speed of gravitational waves. All calculations performed in this paper are reproducible via companion $\texttt {Mathematica}$ notebooks.

gr-qc↗

arXiv2026

EFT of Dark Energy with Cosmic Chronometers: Reconstructing Background EFT Functions

The effective field theory (EFT) of dark energy provides a model-independent framework for studying cosmology within scalar-tensor theories. In this work, we explore how the time evolution of the cosmological background, inferred from cosmic chronometer measurements of the Hubble parameter, can be used to reconstruct the relevant EFT functions. Our approach enables the direct determination of these EFT functions from observational data without assuming any specific cosmological model. This makes it possible to test the background evolution of a wide range of dark energy models, including the $Λ$CDM model. We further demonstrate how the reconstructed EFT functions can be applied to constrain concrete theories, such as the quintessence model.

astro-ph.CO↗

arXiv2026

Dynamical Tidal Response of Non-rotating Black Holes: Connecting the MST Formalism and Worldline EFT

The response of a black hole (BH) to tidal forces encodes key information about the underlying gravitational theory and affects the waveform of gravitational waves emitted during binary inspiral processes. In this paper, we analyze the dynamical tidal response of static and spherically symmetric BHs in a low-frequency regime within general relativity (GR), based on a matching between the Mano-Suzuki-Takasugi (MST) methods for an analytical approach to BH perturbations and the worldline effective field theory (EFT) for an efficient and unified computation of the binary dynamics within the post-Newtonian regime. We show that the renormalized tidal response function is subject to inevitable ambiguities associated with the choice of renormalization scheme and with the initial condition of the renormalization flow equation. Once these ambiguities are fixed, we obtain scheme-dependent dynamical tidal Love numbers. We also discuss possible extensions of our formalism, including generic non-rotating compact objects (e.g., neutron stars) in GR and BHs in theories beyond GR.

gr-qc↗

arXiv2026

Scalar quasinormal modes of rotating black holes in parity-violating gravity

Recently, an exact rotating black hole solution in a parity-violating theory of gravity was obtained via a conformal transformation of the Kerr solution in general relativity, with parity-violating effects encoded in the conformal factor. We study the quasinormal modes (QNMs) of a test scalar field minimally coupled to gravity on this conformal Kerr background, treating the parity-violating effects perturbatively while allowing for arbitrary black hole spin, from the non-rotating case to the near-extremal regime. For low spin, we derive a perturbative formula for the QNM frequencies that includes the leading-order parity-violating correction. For high spin, particularly in the near-extremal regime, we find sizable deviations from the Kerr QNM frequencies. Our results point to a new avenue for probing parity-violating physics in the strong-gravity regime through black hole QNMs.

gr-qc↗

arXiv2025

Quasinormal Modes from EFT of Black Hole Perturbations in Vector-Tensor Gravity

We study the dynamics of odd-parity perturbations on a static and spherically symmetric black hole background with a timelike vector field based on the effective field theory (EFT) approach. We derive the quadratic Lagrangian written in terms of two master variables, corresponding to the tensor and vector gravitons, which are coupled in general, while they can be decoupled on a stealth Schwarzschild(-de Sitter) background. For the stealth Schwarzschild background, we find that the quasinormal mode frequencies for both degrees of freedom are obtained from those in general relativity by simple scaling. Nonetheless, due to the fact that the metric perturbation is a non-trivial linear combination of the two degrees of freedom with different QNM spectra, the ringdown gravitational waves may exhibit characteristic modulation that can in principle be a signature of vector-tensor gravity.

gr-qc↗

arXiv2025

Parametrized Tidal Dissipation Numbers of Non-rotating Black Holes

A set of tidal dissipation numbers (TDNs) quantifies the absorption of the tidal force exerted by a companion during an inspiralling phase of a binary compact object. This tidal dissipation generally affects the gravitational waveform, and measuring the TDNs of a black hole (BH) allows us to test the nature of gravity in the strong-field regime. In this paper, we develop a parametrized formalism for calculating the TDNs of static and spherically symmetric BH backgrounds using the Mano-Suzuki-Takasugi method, which connects the underlying perturbative equations with observable quantities in gravitational-wave observations in a theory-agnostic manner. Our formalism applies to any system where the master equation has the form of the Regge-Wheeler/Zerilli equation with a small correction to the effective potential. As an application of our formalism, we consider three examples: the effective field theory of BH perturbations with timelike scalar profile, the Einstein-Maxwell system, and a higher-curvature extension of general relativity. We also discuss the absence of logarithmic running for the TDNs.

gr-qc↗

arXiv2025

Effective Field Theory of Perturbations on Arbitrary Black Hole Backgrounds with Spacelike Scalar Profile

We develop the effective field theory (EFT) of perturbations in the context of scalar-tensor theories with a spacelike scalar profile on arbitrary black hole backgrounds. Our construction of the EFT is based on the fact that in the unitary gauge, where the scalar field is chosen as one of the spatial coordinates, the background scalar field spontaneously breaks the diffeomorphism invariance along the direction of its gradient. The residual symmetry on a timelike hypersurface of constant scalar field is referred to as the $(2+1)$d diffeomorphism invariance. We then derive a set of consistency relations, imposed on the EFT parameters, by requiring that the EFT action in the unitary gauge be invariant under the $(2+1)$d diffeomorphisms. For concreteness, we apply the EFT to study the background dynamics of a class of non-static and spherically symmetric solutions, focusing in particular on a black hole solution with a time-varying mass. We emphasize that our EFT framework is broadly applicable to any black hole background as long as the scalar field remains spacelike throughout the spacetime region of interest. This formulation provides a model-independent approach for testing scalar-tensor theories as gravity beyond general relativity in the strong-gravity regime.

gr-qc↗

arXiv2025

Spherical black hole perturbations in EFT of scalar-tensor gravity with timelike scalar profile

We study linear even-parity perturbations of static and spherically symmetric black holes with a timelike scalar profile by use of the effective field theory (EFT) approach. For illustrative purposes, we consider a simple subclass of the EFT that accommodates ghost condensate, namely the k-essence model along with the so-called scordatura term, and focus on the spherical (monopole) perturbations about an approximately stealth Schwarzschild solution. The scordatura effect is introduced to avoid the strong coupling problem that typically happens in the scalar sector around stealth solutions with a timelike scalar profile. We argue that the scalar perturbation is decoupled from the metric perturbations at the leading order in the scordatura effect under a particular gauge choice. We stress that this is an important step in understanding the dynamics of even-parity perturbations, paving the way towards deriving a set of master equations -- the generalized Zerilli and the scalar-field equations -- for generic multipoles.

gr-qc↗

arXiv2025

Resonance of black hole quasinormal modes in coupled systems

Black hole quasinormal modes (QNMs) can exhibit resonant excitations associated with avoided crossings in their complex frequency spectrum. Such resonance phenomena can serve as novel signatures for probing new physics, where additional degrees of freedom are commonly introduced. Motivated by this possibility, we investigate QNMs in systems where multiple degrees of freedom are coupled with each other, and introduce a definition of excitation factors suitable for such systems. To demonstrate our formulation, we apply it to a black hole in the Einstein-Maxwell-axion theory, where we find that avoided crossings can appear even between longest-lived modes originating from the fundamental modes of different degrees of freedom, in contrast to the Kerr case in General Relativity. We show that the excitation factors are indeed amplified as a manifestation of resonance at parameter values corresponding to the avoided crossings.

gr-qc↗

arXiv2025

Bridging Dark Energy and Black Holes with EFT: Frame Transformation and Gravitational Wave Speed

Typically, constraints on parameters of the effective field theory (EFT) of dark energy have been obtained in the Jordan frame, where matter fields are minimally coupled to gravity. To connect these constraints with those of the EFT of black hole perturbations with a timelike scalar profile, it is necessary to perform a frame transformation on the EFT in general. In this paper, we study the conformal/disformal transformation of EFT parameters on an arbitrary background. Furthermore, we explore the effect of an EFT operator $M_6(r) \barσ^μ_ν δK^ν_α δK^α_μ$, which is elusive to the LIGO/Virgo bound on gravitational-wave speed, on the dynamics of odd-parity black hole perturbations. Intriguingly, a deviation from luminal propagation shows up only in the vicinity of the black hole, and the speeds of perturbations in the radial and angular directions are different in general due to the traceless part $\barσ^μ_ν$ of the background extrinsic curvature. This study establishes an important link between cosmological constraints and those obtained in the black hole regime.

gr-qc↗

arXiv2024

Tidal Love Numbers from EFT of Black Hole Perturbations with Timelike Scalar Profile

We study static tidal Love numbers (TLNs) of a static and spherically symmetric black hole for odd-parity metric perturbations. We describe black hole perturbations using the effective field theory (EFT), formulated on an arbitrary background with a timelike scalar profile in the context of scalar-tensor theories. In particular, we obtain a static solution for the generalized Regge-Wheeler equation order by order in a modified-gravity parameter and extract the TLNs uniquely by analytic continuation of the multipole index $\ell$ to non-integer values. For a stealth Schwarzschild black hole, the TLNs are vanishing as in the case of Schwarzschild solution in general relativity. We also study the case of Hayward black hole as an example of non-stealth background, where we find that the TLNs are non-zero (or there is a logarithmic running). This result suggests that our EFT allows for non-vanishing TLNs and can in principle leave a detectable imprint on gravitational waves from inspiralling binary systems, which opens a new window for testing gravity in the strong-field regime.

gr-qc↗

arXiv2024

Black holes, multiple propagation speeds, and energy extraction

The Standard Model of particle physics predicts the speed of light to be a universal speed of propagation of massless carriers. However, other possibilities exist -- including Lorentz-violating theories -- where different fundamental fields travel at different speeds. Black holes are interesting probes of such physics, as distinct fields would probe different horizons. Here, we build an exact spacetime for two interacting scalar fields which have different propagation speeds. One of these fields is able to probe the black hole interior of the other, giving rise to energy extraction from the black hole and a characteristic late-time relaxation. Our results provide further stimulus to the search for extra degrees of freedom, black hole instability, and extra ringdown modes in gravitational-wave events.

gr-qc↗

arXiv2024

(In)stability of the black hole greybody factors and ringdowns against a small-bump correction

Recently, it has been proposed that the black hole greybody factors can be important to model ringdown spectral amplitudes. We study the stability of greybody factors against a small-bump correction in the perturbation equation. We find (I) that the greybody factor is stable in the frequency region relevant to ringdown and (II) that it is destabilized at higher frequencies, especially for a sharper bump correction. This behavior is similar to the case of higher overtones, which is also very sensitive to a small correction. We clarify this (in)stability with the WKB analysis. As the greybody factor is stable at the frequency region relevant to the main part of ringdown, we conclude that the greybody factor is suitable to model ringdown amplitude. In order to investigate a bump correction in a self-consistent manner, we consider the small-bump correction that can be realized in the general framework of effective field theory of black hole perturbations.

gr-qc↗

arXiv2024

Exact solution for rotating black holes in parity-violating gravity

It has recently been pointed out that one can construct invertible conformal transformations with a parity-violating conformal factor, which can be employed to generate a novel class of parity-violating ghost-free metric theories from general relativity. We obtain exact solutions for rotating black holes in such theories by performing the conformal transformation on the Kerr solution in general relativity, which we dub conformal Kerr solutions. We explore the geodesic motion of a test particle in the conformal Kerr spacetime. While null geodesics remain the same as those in the Kerr spacetime, timelike geodesics exhibit interesting differences due to an effective external force caused by the parity-violating conformal factor.

gr-qc↗