The ringdown part of gravitational waves in the final stage of merger of
compact objects tells us the nature of strong gravity which can be used for
testing the theories of gravity. The ringdown waveform, however, fades out in a
very short time wi...

The "flux-balance formulae" that determine the averaged evolution of energy,
azimuthal angular momentum and Carter constant in terms of the averaged
asymptotic gravitational-wave fluxes for inspirals of small bodies into Kerr
black holes were firs...

Class.Quant.Grav. 36(5) 055005 Mar 2019 [Refereed]

We propose a simple method to prove the linear mode stability of a black hole
when the perturbed field equations take the form of a system of coupled
Schrödinger equations. The linear mode stability of the spacetime is
guaranteed by the existenc...

Parity violation in the gravitational interaction has an important impact on
fundamental observables and the evolution of the universe. We here investigate
for the first time our ability to probe the parity violating nature of the
gravitational in...

Phys. Rev. D 98, 064020 (2018) Jun 2018 [Refereed]

Gravitational waves from spin-precessing binaries exhibit amplitude
oscillations that provide an invaluable method to extract the spins of the
inspiraling compact objects. The spin-spin and spin-orbit interactions that
cause this effect are sensit...

It is known that in the theory of light scalar fields during inflation,
correlation functions suffer from infrared (IR) divergences or large IR loop
corrections, leading to the breakdown of perturbation theory. In order to
understand the physical ...

The -deformation method is a useful way to show the linear mode stability
of a black hole when the perturbed field equation takes the form of the
Schrödinger equation. While previous works where many explicit examples are
studied suggest that...

We study the advantage of combining measurements from future ground and space
based gravitational wave detectors in estimating the parameters of a black-hole
binary coalescence. This is an extension of our previous work (PTEP 053E01
(2016)) where ...

This is a review article on the primordial black holes (PBHs), with
particular focus on the massive ones () which have not
evaporated by the present epoch by the Hawking radiation. By the detections of
gravitational waves b...

We consider the inflationary universe with a spectator scalar field coupled
to a gauge field and calculate curvature perturbation and gravitational
waves (GWs). We find that the sourced GWs can be larger than the one from
vacuum fluctuation...

Astrophys.J. 854 (2018) no.1, 41 854(1) 41 Jan 2018 [Refereed]

It has been proposed that primordial black holes (PBHs) form binaries in the
radiation dominated era. Once formed, some fraction of them may merge within
the age of the Universe by gravitational radiation reaction. We investigate the
merger rate o...

In this study, we formulate a systematic way of deriving an effective
equation of motion(EoM) for long wavelength modes of a massless scalar field
with a general potential on de Sitter background, and investigate
whether or not the effec...

It is widely known that the primordial curvature perturbation has
several universal properties in the infrared (IR) such as the soft theorem,
which is also known as the consistency relation, and the conservation in time.
They are valid in ...

Recently the possibility of detecting echoes of ringdown gravitational waves
from binary black hole mergers was shown. The presence of echoes is expected if
the black hole is surrounded by a mirror that reflects gravitational waves near
the horizo...

We compute the entanglement entropy of a free massive Dirac field between two
causally disconnected open charts in de Sitter space. We first derive the
Bunch-Davies vacuum mode functions of the Dirac field. We find there exists no
supercurvature m...

Class. Quantum Grav. 34 134001 (2017) Dec 2016 [Refereed]

We formulate a Hamiltonian description of the orbital motion of a point
particle in Kerr spacetime for generic (eccentric, inclined) orbits, which
accounts for the effects of the conservative part of the gravitational
self-force. This formulation ...

Class.Quant.Grav. 34(13) 134001 Jun 2017 [Refereed]

We formulate a Hamiltonian description of the orbital motion of a point
particle in Kerr spacetime for generic (eccentric, inclined) orbits, which
accounts for the effects of the conservative part of the gravitational
self-force. This formulation ...

Progress of Theoretical and Experimental Physics 2016(9) Jul 2016 [Refereed]

Pre-DECIGO consists of three spacecraft arranged in an equilateral triangle
with 100km arm lengths orbiting 2000km above the surface of the earth. It is
hoped that the launch date will be in the late 2020s.
Pre-DECIGO has one clear target: binar...

Phys. Rev. D 94, 064059 (2016) May 2016 [Refereed]

We investigate gravitational Cherenkov radiation in a healthy branch of
background solutions in the ghost-free bigravity model. In this model, because
of the modification of dispersion relations, each polarization mode can possess
subluminal phase...

We investigate primordial gravitational waves and curvature perturbations in
de Rham-Gabadadze-Tolley (dRGT) bimetric gravity. We evaluate the power-spectra
in the leading order in slow roll. Taking into account the decay of massive
graviton, we f...

The Sasaki-Nakamura transformation gives a short-ranged potential and a
convergent source term for the master equation of perturbations in the Kerr
space-time. In this paper, we study the asymptotic behavior of the
transformation, and present a ne...

Phys. Rev. Lett. 117, 061101 (2016) Mar 2016 [Refereed]

We point out that the gravitational-wave event GW150914 observed by the LIGO
detectors can be explained by the coalescence of primordial black holes (PBHs).
It is found that the expected PBH merger rate would exceed the rate estimated
by the LIGO ...

Prog. Theor. Exp. Phys. (2016) 031E02 Feb 2016 [Refereed]

Recent numerical relativity simulations of mergers of binary black holes
suggest that the maximum final value of is for the
coalescence of two equal mass black holes with aligned spins of the same
magnitude which is c...

Phys. Rev. D 93, 044048 (2016) Jan 2016 [Refereed]

Recent population synthesis simulations of Pop III stars suggest that the
event rate of coalescence of -- binary black holes
can be high enough for the detection by the second generation gravitational
wave detectors. The...

We discuss how the ghost-free bigravity coupled with a single scalar field
can be derived from a braneworld setup. We consider DGP two-brane model without
radion stabilization. The bulk configuration is solved for given boundary
metrics, and it is...

Phys. Rev. D 92, 064003 (2015) Jun 2015 [Refereed]

There is a forbidden region in the parameter space of quasinormal modes of
black holes in general relativity. Using both inspiral and ringdown phases of
gravitational waves from binary black holes, we propose two methods to test
general relativity...

We study the advantage of the co-existence of future ground and space based
gravitational wave detectors, in estimating the parameters of a binary
coalescence. Using the post-Newtonian waveform for the inspiral of non-spinning
neutron star-black h...

Phys. Rev. D 91, 062007 (2015) Dec 2014 [Refereed]

The gravitational waveforms in the ghost-free bigravity theory exhibit
deviations from those in general relativity. The main difference is caused by
graviton oscillations in the bigravity theory. We investigate the prospects for
the detection of t...

Yasuho Yamashita, Antonio De Felice, Takahiro Tanaka

Int.J.Mod.Phys. D23 (2014) 1443003 Aug 2014

We discuss the ghost freeness in the case when we add matter coupled to two
metrics to the ghost-free bigravity. In this paper we show that the
Boulware-Deser ghost generally revives in the presence of doubly coupled matter
and that ghost freeness...

Soichiro Isoyama, Leor Barack, Sam R. Dolan, Alexandre Le Tiec, Hiroyuki Nakano, Abhay G. Shah, Takahiro Tanaka, Niels Warburton

Phys. Rev. Lett. 113, 161101 (2014) Apr 2014

For a self-gravitating particle of mass \mu in orbit around a Kerr black hole
of mass M >> \mu, we compute the O(\mu/M) shift in the frequency of the
innermost stable circular equatorial orbit (ISCEO) due to the conservative
piece of the gravitati...

Antonio De Felice, A. Emir Gumrukcuoglu, Shinji Mukohyama, Norihiro Tanahashi, Takahiro Tanaka

JCAP 1406 (2014) 037 Apr 2014 [Refereed]

We study cosmological perturbations in bimetric theory with two fluids each
of which is coupled to one of the two metrics. Focusing on a healthy branch of
background solutions, we clarify the stability of the cosmological
perturbations. For this p...

Markus B. Fröb, Jaume Garriga, Sugumi Kanno, Misao Sasaki, Jiro Soda, Takahiro Tanaka, Alexander Vilenkin

Jan 2014 [Refereed]

We consider Schwinger pair production in 1+1 dimensional de Sitter space,
filled with a constant electric field . This can be thought of as a model
for describing false vacuum decay beyond the semiclassical approximation, where
pairs of a quant...

We discuss graviton oscillations based on the ghost free bi-gravity theory.
We point out that this theory possesses a natural cosmological background
solution which is very close to the case of general relativity. Furthermore,
interesting paramete...

Physical Review D - Particles, Fields, Gravitation and Cosmology 87(2) 2013 [Refereed]

A covariant approach to general field space metric in multi-field inflation

J. O. Gong and T. Tanaka

JCAP 1103, 015 Feb 2012 [Refereed]

Modified gravity theory and Gravitational waves

Takahiro Tanaka

Invited, in “PPC2012,” KIAS, Korea, Nov. 2012. 2012 [Refereed]

Evolutionary effects in one-bubble open inflation for string landscape

D. Yamauchi; A. Linde; A. Naruko; M. Sasaki; T. Tanaka

12th Marcel Grossmann Meeting on Recent Dev. in Theoretical and Experimental General Relativity, Astrophysics and Relativistic Field Theories - Proc. of the MG 2009 Meeting on General Relativity 2252-2254 2012 [Refereed]

The gravitational field of a black hole is strongly localized near its
horizon when the number of dimensions D is very large. In this limit, we can
effectively replace the black hole with a surface in a background geometry (eg
Minkowski or Anti-de...

The description of quantum tunnelling in the presence of gravity shows
subtleties in some cases. Here we discuss wormhole production in the context of
the spherically symmetric thin-shell approximation. By presenting a fully
consistent treatment b...

Masashi Kimura, Hideki Ishihara, Ken Matsuno, Takahiro Tanaka

Jul 2014

We propose a simple method to prove non-smoothness of a black hole horizon.
The existence of a extension across the horizon implies that there is no
extension across the horizon if some components of -th covariant
derivative o...

It has been claimed that the super Hubble modes of the graviton generated
during inflation can make loop corrections diverge. Even if we introduce an
infrared (IR) cutoff at a comoving scale as an ad hoc but a practical way for
the regularization,...

We discuss whether or not bigravity theory can be embedded into the
braneworld setup. As a candidate, we consider Dvali-Gabadadze-Porrati two-brane
model with the Goldberger-Wise radion stabilization. We will show that we can
construct a ghost fre...

“An easy way to get the gravitational wave templates for Extreme mass ratio inspiral,”

Takahiro Tanaka

Dept. of Phys., Osaka City Univ., Jan. 2013 2013 [Refereed]

“How to test the Einstein gravity using gravitational waves,”

Takahiro Tanaka

ICRR, Feb 2013 (KAGRA F2F meeting) 2013 [Refereed]

“Adiabatic approximation to the gravitational waveform for extreme mass-ratio inspirals,”

Takahiro Tanaka

INR, Moscow, Russia, March 2013 2013 [Refereed]

Infrared Phenomena of Field Theory in de Sitter space

Takahiro Tanaka

Invited, in “Asia-Pacific School on Gravity and Cosmology,” Jeju, Korea, Feb. 2013. 2013 [Refereed]

Gravitational wave oscillations in bigravity theory