Misao Sasaki

National Taiwan University

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Co-Authors

Takahiro Tanaka

Kyoto University

Hideyuki Tagoshi

The University of Tokyo

Hiroyuki Nakano

Ryukoku University

Guillem Domènech

Leibniz University Hannover

Chul‐Moon Yoo

Nagoya University

Atsushi Naruko

Kyoto University

Masaki Ando

The University of Tokyo

Nobuyuki Kanda

Osaka City University

O. Miyakawa

The University of Tokyo

Seiji Kawamura

Obayashi (Japan)

Cooperative Institutions

The University of Tokyo

530

California Institute of Technology

365

Centre National de la Recherche Scientifique

288

Cardiff University

226

Kyoto University

207

National Astronomical Observatory of Japan

200

LIGO Scientific Collaboration

181

High Energy Accelerator Research Organization

169

Kavli Institute for the Physics and Mathematics of the Universe

157

Australian National University

149

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Gauss-Bonnet dark energy

Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology (2005)

Shin’ichi Nojiri Sergei D. Odintsov Misao Sasaki

We propose the Gauss-Bonnet dark energy model inspired by string/M-theory where standard gravity with scalar contains additional scalar-dependent coupling with a Gauss-Bonnet invariant. It is demonstrated that the effective phantom (or quintessence) phase of the late universe may occur in the presence of such a term when the scalar is phantom or for nonzero potential (for canonical scalar). However, with the increase of the curvature, the Gauss-Bonnet term may become dominant so that the phantom phase is transient and the $w=\ensuremath{-}1$ barrier may be passed. Hence, the current acceleration of the universe may be caused by a mixture of scalar phantom and/or potential or stringy effects. It is remarkable that scalar-Gauss-Bonnet coupling acts against the big rip occurrence also in phantom cosmology.

Gauss–Bonnet theorem

Quintessence

Scalar potential

10.1103/physrevd.71.123509

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Citations (678)

A general proof of the conservation of the curvature perturbation

Journal of Cosmology and Astroparticle Physics (2005)

David H. Lyth Karim A. Malik Misao Sasaki

Without invoking a perturbative expansion, we define the cosmological curvature perturbation, and consider its behaviour assuming that the universe is smooth over a sufficiently large comoving scale. The equations are simple, resembling closely the first-order equations, and they lead to results which generalize those already proven in linear perturbation theory and (in part) in second-order perturbation theory. In particular, the curvature perturbation is conserved provided that the pressure is a unique function of the energy density.

Cosmological perturbation theory

Conservation law

Poincaré–Lindstedt method

10.1088/1475-7516/2005/05/004

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Citations (703)

Probing Primordial Black Hole Scenarios with Terrestrial Gravitational Wave Detectors

arXiv (Cornell University) (2024)

Guillem Domènech Misao Sasaki

It is possible that primordial black holes consitute (or consituted) a significant fraction of the energy budget of our universe. Terrestrial gravitational wave detectors offer the opportunity to test the existence of primordial black holes in two different mass ranges, from $10^2\,{\rm g}-10^{16}\,{\rm g}$ to $10^{-6}\,M_\odot-100 \,M_\odot$. The first mass window is open via induced gravitational waves and the second one by gravitational waves from binary mergers. In this review, we outline and explain the different gravitational wave signatures of primordial black holes that may be probed by terrestrial gravitational wave detectors, such as the current LIGO/Virgo/KAGRA and future ones like Einstein Telescope and Cosmic Explorer. We provide rough estimates for the frequency and amplitude of the associated GW background signals. We also discuss complementary probes for these primordial black hole mass ranges.

Einstein Telescope

Primordial black hole

Black hole (networking)

10.48550/arxiv.2401.07615

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Citations (0)

Cosmological Perturbation Theory

Progress of Theoretical Physics Supplement (1984)

Hideo Kodama Misao Sasaki

The linear perturbation theory of spatially homogeneous and isotropic universes is reviewed and reformulated extensively. In the first half of the article, a gauge-invariant formulation of the theory is carried out with special attention paid to the geometrical meaning of the perturbation. In the second half of the article, the application of the theory to some important cosmological models is discussed.

Cosmological perturbation theory

Poincaré–Lindstedt method

10.1143/ptps.78.1

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Citations (1,530)

Cosmological disformal invariance

Journal of Cosmology and Astroparticle Physics (2015)

Guillem Domènech Atsushi Naruko Misao Sasaki

The invariance of physical observables under disformal transformations is considered. It is known that conformal transformations leave physical observables invariant. However, whether it is true for disformal transformations is still an open question. In this paper, it is shown that a pure disformal transformation without any conformal factor is equivalent to rescaling the time coordinate. Since this rescaling applies equally to all the physical quantities, physics must be invariant under a disformal transformation, that is, neither causal structure, propagation speed nor any other property of the fields are affected by a disformal transformation itself. This fact is presented at the action level for gravitational and matter fields and it is illustrated with some examples of observable quantities. We also find the physical invariance for cosmological perturbations at linear and high orders in perturbation, extending previous studies. Finally, a comparison with Horndeski and beyond Horndeski theories under a disformal transformation is made.

Cosmological perturbation theory

10.1088/1475-7516/2015/10/067

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Citations (56)

Analytic Black Hole Perturbation Approach to Gravitational Radiation

Deleted Journal (2003)

Misao Sasaki Hideyuki Tagoshi

We review the analytic methods used to perform the post-Newtonian expansion of gravitational waves induced by a particle orbiting a massive, compact body, based on black hole perturbation theory. There exist two different methods of performing the post-Newtonian expansion. Both are based on the Teukolsky equation. In one method, the Teukolsky equation is transformed into a Regge-Wheeler type equation that reduces to the standard Klein Gordon equation in the flat-space limit, while in the other method (which was introduced by Mano, Suzuki, and Takasugi relatively recently, the Teukolsky equation is used directly in its original form. The former’s advantage is that it is intuitively easy to understand how various curved space effects come into play. However, it becomes increasingly complicated when one goes to higher and higher post-Newtonian orders. In contrast, the latter’s advantage is that a systematic calculation to higher post-Newtonian orders can be implemented relatively easily, but otherwise, it is so mathematical that it is hard to understand the interplay of higher order terms. In this paper, we review both methods so that their pros and cons may be seen clearly. We also review some results of calculations of gravitational radiation emitted by a particle orbiting a black hole.

Black hole (networking)

Newtonian limit

10.12942/lrr-2003-6

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Citations (195)

Gravitational radiation from a particle in circular orbit around a black hole. V. Black-hole absorption and tail corrections

Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D. Particles and fields (1995)

Eric Poisson Misao Sasaki

A particle of mass \ensuremath{\mu} moves on a circular orbit of a nonrotating black hole of mass M. Under the restrictions \ensuremath{\mu}/M\ensuremath{\ll}1 and v\ensuremath{\ll}1, where v is the orbital veloity (in units in which c=1), we consider the gravitational waves emitted by such a binary system. The framework is that of black-hole perturbation theory. We calculate E\ifmmode \dot{}\else \.{}\fi{}, the rate at which the gravitational waves remove energy from the system. The total energy loss is given by E\ifmmode \dot{}\else \.{}\fi{}=E\ifmmode \dot{}\else \.{}\fi{} $^{\mathrm{\ensuremath{\infty}}}$+E\ifmmode \dot{}\else \.{}\fi{} $^{\mathit{H}}$, where E\ifmmode \dot{}\else \.{}\fi{} $^{\mathrm{\ensuremath{\infty}}}$ denotes that part of the gravitational-wave energy which is carried off to infinity, while E\ifmmode \dot{}\else \.{}\fi{} $^{\mathit{H}}$ denotes the part which is absorbed by the black hole. We show that the black-hole absorption is a small effect: E\ifmmode \dot{}\else \.{}\fi{} $^{\mathit{H}}$/E\ifmmode \dot{}\else \.{}\fi{}\ensuremath{\simeq}${\mathit{v}}^{8}$. This is explained by the presence of a potential barrier in the vicinity of the black hole: Most of the waves propagating initially toward the black hole are reflected off the barrier; the black hole is therefore unable to absorb much. The black-hole absorption, and indeed any other effect resulting from imposing ingoing-wave boundary conditions at the event horizon, are sufficiently small to be irrelevant to the construction of matched filters for gravitational-wave measurements. To derive this result we extend the techniques previously developed by Poisson and Sasaki for integrating the Regge-Wheeler equation. The extension consists of an explicit consideration of the horizon boundary conditions, which were largely ignored in the previous work. Finally, we compare the wave generation formalism which derives from perturbation theory to the post-Newtonian formalism of Blanchet and Damour. Among other things we consider the corrections to the asymptotic gravitational-wave field which are due to wave-propagation (tail) effects. The results obtained using perturbation theory are identical to that of post-Newtonian theory.

Black hole (networking)

10.1103/physrevd.51.5753

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Citations (175)

Chapter 1. Black Hole Perturbation

Progress of Theoretical Physics Supplement (1997)

Yasushi Mino Misao Sasaki Masaru Shibata Hideyuki Tagoshi Takahiro Tanaka

In this chapter, we present analytic calculations of gravitational waves from a particle orbiting a black hole. We first review the Teukolsky formalism for dealing with the gravitational perturbation of a black hole. Then we develop a systematic method to calculate higher order post-Newtonian corrections to the gravitational waves emitted by an orbiting particle. As applications of this method, we consider orbits that are nearly circular, including exactly circular ones, slightly eccentric ones and slightly inclined orbits off the equatorial plane of a Kerr black hole and give the energy flux and angular momentum flux formulas at infinity with higher order post-Newtonian corrections. Using a different method that makes use of an analytic series representation of the solution of the Teukolsky equation, we also give a post-Newtonian expanded formula for the energy flux absorbed by a Kerr black hole for a circular orbit.

Circular orbit

Black hole (networking)

10.1143/ptps.128.1

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Citations (139)

Analytic description of primordial black hole formation from scalar field fragmentation

Journal of Cosmology and Astroparticle Physics (2019)

Eric Cotner Alexander Kusenko Misao Sasaki Volodymyr Takhistov

Primordial black hole (PBH) formation is a more generic phenomenon than was once thought. The dynamics of a scalar field in inflationary universe can produce PBHs under mild assumptions regarding the scalar potential. In the early universe, light scalar fields develop large expectation values during inflation and subsequently relax to the minimum of the effective potential at a later time. During the relaxation process, an initially homogeneous scalar condensate can fragment into lumps via an instability similar to the gravitational (Jeans) instability, where the scalar self-interactions, rather than gravity, play the leading role. The fragmentation of the scalar field into lumps (e.g. Q-balls or oscillons) creates matter composed of relatively few heavy "particles", whose distribution is subject to significant fluctuations unconstrained by comic microwave background (CMB) observations and unrelated to the large-scale structure. If this matter component comes to temporarily dominate the energy density before the scalar lumps decay, PBHs can be efficiently produced during the temporary matter-dominated era. We develop a general analytic framework for description of PBH formation in this class of models. We highlight the differences between the scalar fragmentation scenario and other commonly considered PBH formation models. Given the existence of the Higgs field and the preponderance of scalar fields within supersymmetric and other models of new physics, PBHs constitute an appealing and plausible candidate for dark matter.

Primordial black hole

Structure formation

10.1088/1475-7516/2019/10/077

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Citations (145)

Classical and quantum radiation from a moving charge in an expanding universe

Journal of Cosmology and Astroparticle Physics (2006)

Hidenori Nomura Misao Sasaki Kazuhiro Yamamoto

We investigate photon emission from a moving particle in an expanding universe. This process is analogous to the radiation from an accelerated charge in the classical electromagnetic theory. Using the framework of quantum field theory in curved spacetime, we demonstrate that the Wentzel–Kramers–Brillouin (WKB) approximation leads to the Larmor formula for the rate of the radiation of energy from a moving charge in an expanding universe. Using exactly solvable models in a radiation-dominated universe and in a Milne universe, we examine the validity of the WKB formula. It is shown that the quantum effect suppresses the radiation energy in comparison with the WKB formula.

10.1088/1475-7516/2006/11/013

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Citations (16)