96 citations · 1.2k across the 37 of their papers we have counts for
31 papers · 1 filter
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…
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…
Invertible disformal transformations with arbitrary higher-order derivatives
Kazufumi Takahashi
Invertible disformal transformations serve as a useful tool to explore ghost-free scalar-tensor theories. In this paper, we construct a generalization of invertible disformal trans…
Quasinormal Modes from EFT of Black Hole Perturbations with Timelike Scalar Profile
Shinji Mukohyama, Kazufumi Takahashi, Keitaro Tomikawa +1
The Effective Field Theory (EFT) of perturbations on an arbitrary background geometry with a timelike scalar profile was recently constructed in the context of scalar-tensor theori…
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 higher derivative couplings to matter fields in scalar-tensor gravity
Tact Ikeda, Kazufumi Takahashi, Tsutomu Kobayashi
Recently, a generalization of invertible disformal transformations containing higher-order derivatives of a scalar field has been proposed in the context of scalar-tensor theories…
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…
Approximately stealth black hole in higher-order scalar-tensor theories
Antonio De Felice, Shinji Mukohyama, Kazufumi Takahashi
We investigate a generic quadratic higher-order scalar-tensor theory with a scordatura term, which is expected to provide a consistent perturbative description of stealth solutions…
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…
Generalized Regge-Wheeler Equation from Effective Field Theory of Black Hole Perturbations with a Timelike Scalar Profile
Shinji Mukohyama, Kazufumi Takahashi, Vicharit Yingcharoenrat
Recently, the Effective Field Theory (EFT) of perturbations on an arbitrary background metric with a timelike scalar profile was formulated in the context of scalar-tensor theories…
Linear stability of black holes with static scalar hair in full Horndeski theories: generic instabilities and surviving models
Masato Minamitsuji, Kazufumi Takahashi, Shinji Tsujikawa
In full Horndeski theories, we show that the static and spherically symmetric black hole (BH) solutions with a static scalar field~ whose kinetic term~ is nonvanishing on the…
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…
Built-in scordatura in U-DHOST
Antonio De Felice, Shinji Mukohyama, Kazufumi Takahashi
Modified gravity theories can accommodate exact solutions, for which the metric has the same form as the one in general relativity, i.e., stealth solutions. One problem with these…
Linear stability of black holes in shift-symmetric Horndeski theories with a time-independent scalar field
Masato Minamitsuji, Kazufumi Takahashi, Shinji Tsujikawa
We study linear perturbations about static and spherically symmetric black holes with a time-independent background scalar field in shift-symmetric Horndeski theories, whose Lagran…
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…
Nonlinear definition of the shadowy mode in higher-order scalar-tensor theories
Antonio De Felice, Shinji Mukohyama, Kazufumi Takahashi
We study U-DHOST theories, i.e., higher-order scalar-tensor theories which are degenerate only in the unitary gauge and yield an apparently unstable extra mode in a generic coordin…
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…
Theories with limited extrinsic curvature and a nonsingular anisotropic universe
Yuki Sakakihara, Daisuke Yoshida, Kazufumi Takahashi +1
We propose a class of theories that can limit scalars constructed from the extrinsic curvature. Applied to cosmology, this framework allows us to control not only the Hubble parame…
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…
Extended Cuscuton as Dark Energy
Aya Iyonaga, Kazufumi Takahashi, Tsutomu Kobayashi
Late-time cosmology in the extended cuscuton theory is studied, in which gravity is modified while one still has no extra dynamical degrees of freedom other than two tensor modes.…
Ghost-free resummation of gravitational interactions of a 2-form gauge field
Kazufumi Takahashi, Daisuke Yoshida
We find that the recently-proposed ghost-free interaction of a 2-form gauge field in four dimensions, which contains derivative couplings in a nonperturbative manner, can be regard…
Linear stability analysis of hairy black holes in quadratic degenerate higher-order scalar-tensor theories: Odd-parity perturbations
Kazufumi Takahashi, Hayato Motohashi, Masato Minamitsuji
We study static spherically symmetric black hole solutions with a linearly time-dependent scalar field and discuss their linear stability in the shift- and reflection-symmetric sub…
Generalized 2D dilaton gravity and KGB
Kazufumi Takahashi, Tsutomu Kobayashi
We show explicitly that the nonminimal coupling between the scalar field and the Ricci scalar in 2D dilaton gravity can be recast in the form of kinetic gravity braiding (KGB). Thi…
Extended Cuscuton: Formulation
Aya Iyonaga, Kazufumi Takahashi, Tsutomu Kobayashi
Among single-field scalar-tensor theories, there is a special class called "cuscuton," which is represented as some limiting case of k-essence in general relativity. This theory ha…