Static spherically symmetric solutions in the IR limit of nonrelativistic quantum gravity (2009)
Harada, Tomohiro, Miyamoto, Umpei, Tsukamoto, Naoki
We investigate static spherically symmetric vacuum solutions in the IR limit of projectable nonrelativistic quantum gravity, including the renormalisable quantum gravity recently proposed by...
Accretion process onto super-spinning objects (2009)
Bambi, Cosimo, Freese, Katherine, Harada, Tomohiro, Takahashi, Rohta, Yoshida, Naoki
The accretion process onto spinning objects in Kerr spacetimes is studied with numerical simulations. Our results show that accretion onto compact objects with Kerr parameter (characterizing the...
Observational constraints of a power spectrum from super-inflation in Loop Quantum Cosmology (2009)
Shimano, Masahiro, Harada, Tomohiro
In loop quantum cosmology there may be a super-inflation phase in the very early universe, in which a single scalar field with a negative power-law potential $V= -M^4(\phi/M)^\beta$ plays important...
Nakao, Ken-ichi, Harada, Tomohiro, Kurita, Yasunari, Morisawa, Yoshiyuki
We numerically study the dynamics of an imploding hollow cylinder composed of dust. Since there is no cylindrical black hole in 4-dimensional spacetime with physically reasonable energy conditions, a...
Ando, Masaki, Kawamura, Seiji, Sato, Shuichi, Nakamura, Takashi, Tsubono, Kimio, Araya, Akito, ...
DECIGO pathfinder (DPF) is a milestone satellite mission for DECIGO (DECi-hertz Interferometer Gravitational wave Observatory), which is a future space gravitational wave antenna. DECIGO is expected...
Singularities and self-similarity in gravitational collapse (2009)
Einstein's field equations in general relativity admit a variety of solutions with spacetime singularities. Numerical relativity has recently revealed the properties of somewhat generic spacetime...
Maeda, Hideki, Harada, Tomohiro, Carr, B. J.
Motivated by the cosmological wormhole solutions obtained from our recent numerical investigations, we provide a definition of a wormhole which applies to dynamical situations. Our numerical...
Einstein-Rosen waves and the self-similarity hypothesis in cylindrical symmetry (2008)
Harada, Tomohiro, Nakao, Ken-ichi, Nolan, Brien C.
The self-similarity hypothesis claims that in classical general relativity spherically symmetric solutions may naturally evolve to a self-similar form in certain circumstances. In this context, the...
High-Speed Collapse of a Hollow Sphere of Type I Matter (2008)
Ahmad, Zahid, Harada, Tomohiro, Nakao, Ken-ichi, Sharif, M.
In this paper, we study the dynamics of a hollow spherical matter collapsing with very large initial velocity. The spacetime is initially very similar to the Vaidya solution, and the deviations from...
Ando, Masaki, Kawamura, Seiji, Nakamura, Takashi, Tsubono, Kimio, Tanaka, Takahiro, Funaki, Ikkoh, ...
DECIGO pathfinder (DPF) is a milestone satellite mission for DECIGO (DECi-hertz Interferometer Gravitational wave Observatory) which is a future space gravitational wave antenna. DECIGO is expected...
The Japanese space gravitational wave antenna; DECIGO (2008)
Kawamura, Seiji, Ando, Masaki, Nakamura, Takashi, Tsubono, Kimio, Tanaka, Takahiro, Funaki, Ikkoh, ...
DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is the future Japanese space gravitational wave antenna. DECIGO is expected to open a new window of observation for gravitational...
Lemaitre-Tolman-Bondi collapse from the perspective of loop quantum gravity (2008)
Bojowald, Martin, Harada, Tomohiro, Tibrewala, Rakesh
Lemaitre-Tolman-Bondi models as specific spherically symmetric solutions of general relativity simplify in their reduced form some of the mathematical ingredients of black hole or cosmological...
Asymptotically Friedmann self-similar scalar field solutions with potential (2008)
Kyo, Masanori, Harada, Tomohiro, Maeda, Hideki
We investigate self-similar solutions which are asymptotic to the Friedmann universe at spatial infinity and contain a scalar field with potential. The potential is required to be exponential by...
Critical Collapse of Einstein Cluster (2007)
Mahajan, Ashutosh, Harada, Tomohiro, Joshi, Pankaj S., Nakao, Ken-ichi
We observe critical phenomena in spherically symmetric gravitational collapse of Einstein Cluster. We show analytically that the collapse evolution ends either in formation of a black hole or in...
Analytical solutions for black-hole critical behaviour (2007)
Harada, Tomohiro, Mahajan, Ashutosh
Dynamical Einstein cluster is a spherical self-gravitating system of counterrotating particles, which may expand, oscillate and collapse. This system exhibits critical behaviour in its collapse at...
Self-similar cosmological solutions with dark energy. I: formulation and asymptotic analysis (2007)
Harada, Tomohiro, Maeda, Hideki, Carr, B. J.
Based on the asymptotic analysis of ordinary differential equations, we classify all spherically symmetric self-similar solutions to the Einstein equations which are asymptotically Friedmann at large...
Maeda, Hideki, Harada, Tomohiro, Carr, B. J.
We use a combination of numerical and analytical methods, exploiting the equations derived in a preceding paper, to classify all spherically symmetric self-similar solutions which are asymptotically...
Black Hole Evaporation in an Expanding Universe (2007)
Saida, Hiromi, Harada, Tomohiro, Maeda, Hideki
We calculate the quantum radiation power of black holes which are asymptotic to the Einstein-de Sitter universe at spatial and null infinities. We consider two limiting mass accretion scenarios, no...
Relativistic Gravitational Collapse of a Cylindrical Shell of Dust (2006)
Nakao, Ken-ichi, Kurita, Yasunari, Morisawa, Yoshiyuki, Harada, Tomohiro
The gravitational collapse of a thick cylindrical shell of dust matter is investigated. It is found that a spacetime singularity forms on the symmetry axis and that it is necessarily naked, i.e.,...
Is there a black hole minimum mass? (2006)
Applying the first and generalised second laws of thermodynamics for a realistic process of near critical black hole formation, we derive an entropy bound, which is identical to Bekenstein's one for...
Harada, Tomohiro, Maeda, Hideki, Carr, B. J.
We consider the possible existence of self-similar solutions containing black holes in a Friedmann background with a stiff fluid or a scalar field. We carefully study the relationship between the...
Super-horizon primordial black holes (2005)
We discuss a new class of solutions to the Einstein equations which describe a primordial black hole (PBH) in a flat Friedmann background. Such solutions arise if a Schwarzschild black hole is...
Novel Cauchy-horizon instability (2005)
Maeda, Hideki, Torii, Takashi, Harada, Tomohiro
The evolution of weak discontinuity is investigated on horizons in the $n$-dimensional static solutions in the Einstein-Maxwell-scalar-$\Lambda$ system, including the Reissner-Nordstr\"om-(anti) de...
Kink instability and stabilization of the Friedmann universe with scalar fields (2004)
Maeda, Hideki, Harada, Tomohiro
The evolution of weak discontinuity is investigated in the flat FRW universe with a single scalar field and with multiple scalar fields. We consider both massless scalar fields and scalar fields with...
Upper limits on the size of a primordial black hole (2004)
We provide precise constraints on the size of any black holes forming in the early Universe for a variety of formation scenarios. In particular, we prove that the size of the apparent horizon of a...
Growth of primordial black holes in a universe containing a massless scalar field (2004)
The evolution of primordial black holes in a flat Friedmann universe with a massless scalar field is investigated in fully general relativistic numerical relativity. A primordial black hole is...
Quantum Effect and Curvature Strength of Naked Singularities (2004)
Miyamoto, Umpei, Maeda, Hideki, Harada, Tomohiro
There are many solutions to the Einstein field equations that demonstrate naked singularity (NS) formation after regular evolution. It is possible, however, that such a quantum effect as particle...
Gravitational collapse and naked singularities (2004)
Gravitational collapse is one of the most striking phenomena in gravitational physics. The cosmic censorship conjecture has provided strong motivation for researches in this field. In the absence of...
Harada, Tomohiro, Nakao, Ken-ichi
It is still uncertain whether the cosmic censorship conjecture is true or not. To get a new insight into this issue, we propose the concept of the border of spacetime as a generalization of the...
Kinematic self-similar solutions in general relativity (2004)
Maeda, Hideki, Harada, Tomohiro
The gravitational interaction is scale-free in both Newtonian gravity and general theory of relativity. The concept of self-similarity arises from this nature. Self-similar solutions reproduce...
Spherically symmetric perfect fluid collapse in area-radial coordinates (2004)
We study the dynamics in the spherical system of a perfect fluid. If we assume the analytical mass function with respect to the comoving radial coordinate and the area radius, we find that the...
Horizon Instability: a Local Analysis (2004)
Maeda, Hideki; Advanced Research Institute For Science And Engineering, Was; Hideki@gravity.phys.waseda.ac.jp, Torii, Takashi; Graduate School Of Science, Waseda University, Tokyo; Torii@gravity.phys.waseda.ac.jp, Harada, Tomohiro; School Of Mathematical Sciences, Queen Mary, University Of London; T.Harada@qmul.ac.uk
The evolution of weak discontinuity is investigated on horizons in the n-dimensional static solutions in the Einstein-Maxwell-scalar-\Lambda system, including the Reissner-Nordstr"om-(anti) de Sitter...
Spherically symmetric perfect fluid in area-radial coordinates (2004)
Iguchi, Hideo, Harada, Tomohiro, Mena, Filipe C
We study the spherically symmetric collapse of a perfect fluid using area-radial coordinates. We show that analytic mass functions describe a static regular centre in these coordinates. In this case,...
Semiclassical Instability of the Cauchy Horizon in Self-Similar Collapse (2003)
Miyamoto, Umpei, Harada, Tomohiro
Generic spherically symmetric self-similar collapse results in strong naked-singularity formation. In this paper we are concerned with particle creation during a naked-singularity formation in...
Harada, Tomohiro, Maeda, Hideki
A stability criterion is derived in general relativity for self-similar solutions with a scalar field and those with a stiff fluid, which is a perfect fluid with the equation of state $P=\rho$. A...
Restricting quark matter models by gravitational wave observation (2003)
Sotani, Hajime, Kohri, Kazunori, Harada, Tomohiro
We consider the possibilities for obtaining information about the equation of state for quark matter by using future direct observational data on gravitational waves. We study the nonradial...
Nonradial oscillations of quark stars (2003)
Sotani, Hajime, Harada, Tomohiro
Recently, it has been reported that a candidate for a quark star may have been observed. In this article, we pay attention to quark stars with radiation radii in the reported range. We calculate...
Harada, Tomohiro, Iguchi, Hideo, Shibata, Masaru
Nonspherical stellar collapse to a black hole is one of the most promising gravitational wave sources for gravitational wave detectors. We numerically study gravitational waves from a slightly...
A Classification of Spherically Symmetric Kinematic Self-Similar Perfect-Fluid Solutions. II (2003)
Maeda, Hideki, Harada, Tomohiro, Iguchi, Hideo, Okuyama, Naoya
We give a classification of spherically symmetric kinematic self-similar solutions. This classification is complementary to that given in a previous work by the present authors [Prog. Theor. Phys....
General relativity as well as Newtonian gravity admits self-similar solutions due to its scale-invariance. This is a review on these self-similar solutions and their relevance to gravitational...
Criticality and convergence in Newtonian collapse (2002)
Harada, Tomohiro, Maeda, Hideki, Semelin, Benoit
We study through numerical simulation the spherical collapse of isothermal gas in Newtonian gravity. We observe a critical behavior which occurs at the threshold of gravitational instability leading...
Particle velocity in noncommutative space-time (2002)
Tamaki, Takashi, Harada, Tomohiro, Miyamoto, Umpei, Torii, Takashi
We investigate a particle velocity in the $\kappa$-Minkowski space-time, which is one of the realization of a noncommutative space-time. We emphasize that arrival time analyses by high-energy...
A Classification of Spherically Symmetric Kinematic Self-Similar Perfect-Fluid Solutions (2002)
Maeda, Hideki, Harada, Tomohiro, Iguchi, Hideo, Okuyama, Naoya
We classify all spherically symmetric spacetimes admitting a kinematic self-similar vector of the second, zeroth or infinite kind. We assume that the perfect fluid obeys either a polytropic equation...
No Go Theorem for Kinematic Self-Similarity with A Polytropic Equation of State (2002)
Maeda, Hideki, Harada, Tomohiro, Iguchi, Hideo, Okuyama, Naoya
We have investigated spherically symmetric spacetimes which contain a perfect fluid obeying the polytropic equation of state and admit a kinematic self-similar vector of the second kind which is...
Physical Processes in Naked Singularity Formation (2002)
Harada, Tomohiro, Iguchi, Hideo, Nakao, Ken-ichi
Gravitational collapse is one of the most fruitful subjects in gravitational physics. It is well known that singularity formation is inevitable in complete gravitational collapse. It was conjectured...
Have we already detected astrophysical symptoms of space-time noncommutativity ? (2001)
Tamaki, Takashi, Harada, Tomohiro, Miyamoto, Umpei, Torii, Takashi
We discuss astrophysical implications of $\kappa$-Minkowski space-time, in which there appears space-time noncommutativity. We first derive a velocity formula for particles based on the motion of a...
Critical phenomena in Newtonian gravity (2001)
Maeda, Hideki, Harada, Tomohiro
We investigate the stability of self-similar solutions for a gravitationally collapsing isothermal sphere in Newtonian gravity by means of a normal mode analysis. It is found that the Hunter series...
Stability criterion for self-similar solutions with perfect fluids in general relativity (2001)
A stability criterion is derived for self-similar solutions with perfect fluids which obey the equation of state $P=k\rho$ in general relativity. A wide class of self-similar solutions turn out to be...
A direct method is developed to reconstruct the equation of state for high-density nuclear matter from the relationship between any two properties of neutron stars, such as masses, radii, moments of...
Physical aspects of naked singularity explosion - How does a naked singularity explode? -- (2001)
Iguchi, Hideo, Harada, Tomohiro
The behaviors of quantum stress tensor for the scalar field on the classical background of spherical dust collapse is studied. In the previous works diverging flux of quantum radiation was predicted....
Convergence to a self-similar solution in general relativistic gravitational collapse (2001)
Harada, Tomohiro, Maeda, Hideki
We study the spherical collapse of a perfect fluid with an equation of state $P=k\rho$ by full general relativistic numerical simulations. For $0
Naked singularities and quantum gravity (2000)
Harada, Tomohiro, Iguchi, Hideo, Nakao, Ken-ichi, Singh, T. P., Tanaka, Takahiro, Vaz, Cenalo
There are known models of spherical gravitational collapse in which the collapse ends in a naked shell-focusing singularity for some initial data. If a massless scalar field is quantized on the...
Response of Interferometric Detectors to Scalar Gravitational Waves (2000)
Nakao, Ken-ichi, Harada, Tomohiro, Shibata, Masaru, Kawamura, Seiji, Nakamura, Takashi
We rigorously analyze the frequency response functions and antenna sensitivity patterns of three types of interferometric detectors to scalar mode of gravitational waves which is predicted to exist...
Power, energy, and spectrum of a naked singularity explosion (2000)
Harada, Tomohiro, Iguchi, Hideo, Nakao, Ken-ichi
Naked singularity occurs in the gravitational collapse of an inhomogeneous dust ball from an initial density profile which is physically reasonable. We show that explosive radiation is emitted during...
Naked Singularity Explosion (2000)
Harada, Tomohiro, Iguchi, Hideo, Nakao, Ken-ichi
It is known that the gravitational collapse of a dust ball results in naked singularity formation from an initial density profile which is physically reasonable. In this paper, we show that explosive...
Sugiura, Norimasa, Nakao, Ken-ichi, Harada, Tomohiro
The relation between the angular diameter distance and redshift in a spherically symmetric dust-shell universe is studied. We have discovered that the relation agrees with that of an appropriate...
Gravitational Radiation from a Naked Singularity. II - Even-Parity Perturbation - (1999)
Iguchi, Hideo, Harada, Tomohiro, Nakao, Ken-ichi
A naked singularity occurs in the generic collapse of an inhomogeneous dust ball. We study the even-parity mode of gravitational waves from a naked singularity of the Lema\^{\i}tre-Tolman-Bondi...
Causal Structure of an Inflating Magnetic Monopole (1999)
Sakai, Nobuyuki, Nakao, Ken-ichi, Harada, Tomohiro
We clarify the causal structure of an inflating magnetic monopole. The spacetime diagram shows explicitly that this model is free from ``graceful exit'' problem, while the monopole itself undergoes...
Harada, Tomohiro, Nakao, Ken-ichi, Iguchi, Hideo
It was recently shown that the metric functions which describe a spherically symmetric space-time with vanishing radial pressure can be explicitly integrated. We investigate the nakedness and...
Gravitational Radiation from a Naked Singularity -- Odd-Parity Perturbation -- (1999)
Iguchi, Hideo, Harada, Tomohiro, Nakao, Ken-ichi
It has been suggested that a naked singularity may be a good candidate for a strong gravitational wave burster. The naked singularity occurs in the generic collapse of an inhomogeneous dust ball. We...
Sugiura, Norimasa, Nakao, Ken-ichi, Harada, Tomohiro
The relation between angular diameter distance and redshift in a spherically symmetric dust-shell universe is studied. This model has large inhomogeneities of matter distribution on small scales. We...
Final fate of the spherically symmetric collapse of a perfect fluid (1998)
The final fate of the spherically symmetric collapse of a perfect fluid which follows the $\gamma$-law equation of state and adiabatic condition is investigated. Full general relativistic...
Naked singularity formation in the collapse of a spherical cloud of counterrotating particles (1998)
Harada, Tomohiro, Iguchi, Hideo, Nakao, Ken-ichi
We investigate collapse of a spherical cloud of counter-rotating particles. An explicit solution is given using an elliptic integral. If the specific angular momentum $L(r)=O(r^2)$ at $r\to 0$, no...
Iguchi, Hideo, Nakao, Ken-ichi, Harada, Tomohiro
The motion of a spherical dust cloud is described by the Lemaitre-Tolman-Bondi solution and is completely specified by initial values of distributions of the rest mass density and specific energy of...
Neutron stars in scalar-tensor theories of gravity and catastrophe theory (1998)
We investigate neutron stars in scalar-tensor theories. We examine their secular stability against spherically symmetric perturbations by use of a turning point method. For some choices of the...
Stability Analysis of Spherically Symmetric Star in Scalar-Tensor Theories of Gravity (1997)
A stability analysis of a spherically symmetric star in scalar-tensor theories of gravity is given in terms of the frequencies of quasi-normal modes. The scalar-tensor theories have a scalar field...
Harada, Tomohiro, Chiba, Takeshi, Nakao, Ken-ichi, Nakamura, Takashi
Unlike general relativity, scalar-tensor theories of gravity predict scalar gravitational waves even from a spherically symmetric gravitational collapse. We solve numerically the generation and...