334 citations · 4k across the 75 of their papers we have counts for
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Quasinormal modes of Kerr-Newman-de Sitter black holes from Heun connection formulae
Hideo Furugori, Naotaka Kubo, Hayato Motohashi +1
We study quasinormal modes governed by the Teukolsky equation in Kerr-Newman-de Sitter spacetime, formally treating the spin weight and field charge as arbitrary parameters. We use…
Pole Skipping, Avoided Crossing, and Resonant Excitation in Kerr Quasinormal Modes near Algebraically Special Frequencies
Kei-ichiro Kubota, Hayato Motohashi
Kerr quasinormal modes near algebraically special frequencies exhibit anomalous behavior, including apparent bifurcation, disappearance, and a nonsmooth connection to the Schwarzsc…
Pole structure of the Kerr Green's function
Hayato Motohashi, Yuto Suichi
We investigate the pole structure of Kerr black-hole perturbations in the frequency domain, focusing on the building blocks of the Green's function for the radial Teukolsky equatio…
Resonance of black hole quasinormal modes in coupled systems
Takuya Takahashi, Hayato Motohashi, Kazufumi Takahashi
Black hole quasinormal modes (QNMs) can exhibit resonant excitations associated with avoided crossings in their complex frequency spectrum. Such resonance phenomena can serve as no…
Spin wave optics for gravitational waves lensed by a Kerr black hole
Kei-ichiro Kubota, Shun Arai, Hayato Motohashi +1
Gravitational waves exhibit the unique signature of their spin-2 nature in processes of wave scattering, due to the interaction between spin and a background spacetime. Since the s…
Resonant Excitation of Quasinormal Modes of Black Holes
Hayato Motohashi
We elucidate that a distinctive resonant excitation between quasinormal modes (QNMs) of black holes emerges as a universal phenomenon at an avoided crossing near the exceptional po…
Exact solution for rotating black holes in parity-violating gravity
Hiroaki W. H. Tahara, Kazufumi Takahashi, Masato Minamitsuji +1
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 cla…
Iterative extraction of overtones from black hole ringdown
Kazuto Takahashi, Hayato Motohashi
Extraction of multiple quasinormal modes from ringdown gravitational waves emitted from a binary black hole coalescence is a touchstone to test whether a remnant black hole is desc…
Black hole ringdown from physically sensible initial value problem in higher-order scalar-tensor theories
Keisuke Nakashi, Masashi Kimura, Hayato Motohashi +1
We study odd-parity perturbations about static and spherically symmetric black hole solutions with a linearly time-dependent scalar field in higher-order scalar-tensor theories. In…
Effective description of generalized disformal theories
Kazufumi Takahashi, Masato Minamitsuji, Hayato Motohashi
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…
Consistency of matter coupling in modified gravity
Kazufumi Takahashi, Rampei Kimura, Hayato Motohashi
Matter coupling in modified gravity theories is a nontrivial issue when the gravitational Lagrangian possesses a degeneracy structure to avoid the problem of the Ostrogradsky ghost…
Generalized disformal Horndeski theories: cosmological perturbations and consistent matter coupling
Kazufumi Takahashi, Masato Minamitsuji, Hayato Motohashi
Invertible disformal transformations are a useful tool to investigate ghost-free scalar-tensor theories. By performing a higher-derivative generalization of the invertible disforma…
Spectroscopy of Kerr-AdS spacetime with the Heun function: Quasinormal modes, greybody factor, and evaporation
Sousuke Noda, Hayato Motohashi
We investigate quasinormal modes, greybody factor, and Hawking evaporation of a five-dimensional Kerr-anti de Sitter (AdS) black hole by solving a wave equation for a test mass…
Quasinormal modes of Schwarzschild black holes on the real axis
Koutarou Kyutoku, Hayato Motohashi, Takahiro Tanaka
We study the scattering of gravitational waves by a Schwarzschild black hole and its perturbed siblings to investigate influences of proposed spectral instability of quasinormal mo…
Evaporation of Echoing Black Holes
Naritaka Oshita, Hayato Motohashi, Sousuke Noda
We compute the graybody factor and evaporation rate of a rotating black hole in the presence of a hypothetical reflective surface slightly outside the outer horizon radius, assumin…
Black hole perturbations in higher-order scalar-tensor theories: initial value problem and dynamical stability
Keisuke Nakashi, Masashi Kimura, Hayato Motohashi +1
We propose a physically sensible formulation of initial value problem for black hole perturbations in higher-order scalar-tensor theories. As a first application, we study monopole…
Invertible disformal transformations with higher derivatives
Kazufumi Takahashi, Hayato Motohashi, Masato Minamitsuji
We consider a higher-derivative generalization of disformal transformations in -dimensional spacetime and clarify the conditions under which they form a group with respect to th…
Black hole perturbations in DHOST theories: Master variables, gradient instability, and strong coupling
Kazufumi Takahashi, Hayato Motohashi
We study linear perturbations about static and spherically symmetric black hole solutions with stealth scalar hair in degenerate higher-order scalar-tensor (DHOST) theories. We cla…
Exact solution for wave scattering from black holes: Formulation
Hayato Motohashi, Sousuke Noda
We establish an exact formulation for wave scattering of a massless field with spin and charge by a Kerr-Newman-de Sitter black hole. Our formulation is based on the exact solution…
Stealth dark energy in scordatura DHOST theory
Mohammad Ali Gorji, Hayato Motohashi, Shinji Mukohyama
A stealth de Sitter solution in scalar-tensor theories has an exact de Sitter background metric and a nontrivial scalar field profile. Recently, in the context of Degenerate Higher…
On Rotating Black Holes in DHOST Theories
Jibril Ben Achour, Hongguang Liu, Hayato Motohashi +2
Using the disformal solution-generating method, we construct new axisymmetric solutions in Degenerate Higher Order Scalar Tensor (DHOST) theories. The method consists in first cons…
General Relativity solutions with stealth scalar hair in quadratic higher-order scalar-tensor theories
Kazufumi Takahashi, Hayato Motohashi
We explore General Relativity solutions with stealth scalar hair in general quadratic higher-order scalar-tensor theories. Adopting the assumption that the scalar field has a const…
Weakly-coupled stealth solution in scordatura degenerate theory
Hayato Motohashi, Shinji Mukohyama
In scalar-tensor theories we revisit the issue of strong coupling of perturbations around stealth solutions, i.e.\ backgrounds with the same forms of the metric as in General Relat…
Constant roll and primordial black holes
Hayato Motohashi, Shinji Mukohyama, Michele Oliosi
The constant-roll inflation with small positive value of the constant-roll parameter has been known to produce a slightly red-tilted cu…