Accelerated cosmic expansion in a scalar-field universe (2009)
Bhattacharya, Swastik, Joshi, Pankaj S., Nakao, Ken-ichi
We consider here a spherically symmetric but inhomogeneous universe filled with a massless scalar field. The model obeys two constraints. The first one is that the gradient of the scalar field is...
Maximal slicing of D-dimensional spherically-symmetric vacuum spacetime (2009)
Nakao, Ken-ichi, Abe, Hiroyuki, Yoshino, Hirotaka, Shibata, Masaru
We study the foliation of a $D$-dimensional spherically symmetric black-hole spacetime with $D\ge 5$ by two kinds of one-parameter family of maximal hypersurfaces: a reflection-symmetric foliation...
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...
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...
Solving the Inverse Problem with Inhomogeneous Universes (2008)
Yoo, Chul-Moon, Kai, Tomohiro, Nakao, Ken-ichi
We construct the Lema\^itre-Tolman-Bondi (LTB) dust universe whose distance-redshift relation is equivalent to that in the concordance $\Lambda$ cold dark matter ($\Lambda$CDM) cosmological model. In...
Magnification Probability Distribution Functions of Standard Candles in a Clumpy Universe (2007)
Yoo, Chul-Moon, Ishihara, Hideki, Nakao, Ken-ichi, Tagoshi, Hideyuki
Lensing effects on light rays from point light sources, such like Type Ia supernovae, are simulated in a clumpy universe model. In our universe model, it is assumed that all matter in the universe...
New instability in relativistic cylindrically symmetric system (2007)
Nakao, Ken-ichi, Ida, Daisuke, Kurita, Yasunari
We investigate an infinitesimally thin cylindrical shell composed of counter-rotating dust particles. This system was studied by Apostolatos and Thorne in terms of the C-energy for a bounded domain....
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...
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.,...
Can inhomogeneties accelerate the cosmic volume expansion? (2006)
Kai, Tomohiro, Kozaki, Hiroshi, Nakao, Ken-ichi, Nambu, Yasusada, Yoo, Chul-Moon
If expanding and contracting regions coexist in the universe, the speed of the cosmic volume expansion can be accelerated. We construct simple inhomogeneous dust-filled universe models in which the...
Yoo, Chul-Moon, Nakao, Ken-ichi, Kozaki, Hiroshi, Takahashi, Ryuichi
The distance-redshift relation determined by means of gravitational waves in the clumpy universe is simulated numerically by taking into account the effects of gravitational lensing. It is assumed...
The universe out of a monopole in the laboratory? (2006)
Sakai, Nobuyuki, Nakao, Ken-ichi, Ishihara, Hideki, Kobayashi, Makoto
To explore the possibility that an inflationary universe can be created out of a stable particle in the laboratory, we consider the classical and quantum dynamics of a magnetic monopole in the...
Naked singularity resolution in cylindrical collapse (2005)
Kurita, Yasunari, Nakao, Ken-ichi
In this paper, we study the gravitational collapse of null dust in the cylindrically symmetric spacetime. The naked singularity necessarily forms at the symmetry axis. We consider the situation in...
Black Strings in Our World (2005)
Nakano, Hiroyuki, Nakao, Ken-ichi, Yamaguchi, Masahide
The brane world scenario is a new approach to resolve the problem on how to compactify the higher dimensional spacetime to our 4-dimensional world. One of the remarkable features of this scenario is...
Hoop Conjecture in Five-dimensios -Violation of Cosmic Censorship- (2005)
Yoo, Chul-Moon, Nakao, Ken-ichi, Ida, Daisuke
We study the condition of black hole formation in five-dimensional space-time. We analytically solve the constraint equations of five-dimensional Einstein equations for momentarily static and...
Gravitational Radiation from Cylindrical Naked Singularity (2005)
Nakao, Ken-ichi, Morisawa, Yoshiyuki
We construct an approximate solution which describes the gravitational emission from a naked singularity formed by the gravitational collapse of a cylindrical thick shell composed of dust. The...
High-Speed Cylindrical Collapse of Perfect Fluid (2004)
Nakao, Ken-ichi, Morisawa, Yoshiyuki
The gravitational collapse of cylindrically distributed perfect fluid is studied. We assume the collapsing speed of fluid is very large and investigate such a situation by recently proposed...
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...
High Speed Dynamics of Collapsing Cylindrical Dust Fluid (2003)
Nakao, Ken-ichi, Morisawa, Yoshiyuki
We construct approximate solutions that will describe the last stage of cylindrically symmetric gravitational collapse of dust fluid. Just before the spacetime singularity formation, the speed of the...
Hoop Conjecture and Black Holes on a Brane (2003)
Nakamura, Kouji, Nakao, Ken-ichi, Mishima, Takeshi
The initial data of gravity for a cylindrical matter distribution confined to a brane are studied in the framework of the single-brane Randall-Sundrum scenario. In this scenario, the 5-dimensional...
How Does Naked Singularity Look? (2002)
Nakao, Ken-ichi, Kobayashi, Naoki, Ishihara, Hideki
There are non-radial null geodesics emanating from the shell focusing singularity formed at the symmetric center in a spherically symmetric dust collapse. In this article, assuming the...
Thick domain walls around a black hole (2002)
Morisawa, Yoshiyuki, Ida, Daisuke, Ishibashi, Akihiro, Nakao, Ken-ichi
We discuss the gravitationally interacting system of a thick domain wall and a black hole. We numerically solve the scalar field equation in the Schwarzschild spacetime and obtain a sequence of...
Volume Expansion of Swiss-Cheese Universe (2002)
Kozaki, Hiroshi, Nakao, Ken-ichi
In order to investigate the effect of inhomogeneities on the volume expansion of the universe, we study modified Swiss-Cheese universe model. Since this model is an exact solution of Einstein...
Isoperimetric inequality for higher-dimensional black holes (2002)
The initial data sets for the five-dimensional Einstein equation have been examined. The system is designed such that the black hole ($\simeq S^3$) or the black ring ($\simeq S^2\times S^1$) can be...
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...
Hoop Conjecture and Cosmic Censorship in the Brane-World (2001)
Nakao, Ken-ichi, Nakamura, Kouji, Mishima, Takashi
The initial data of gravity for a cylindrical matter distribution confined on the brane is studied in the framework of the single brane Randall-Sundrum scenario. In this scenario, 5-dimensional...
Is dark energy the only solution to the apparent acceleration of the present universe? (2001)
Iguchi, Hideo, Nakamura, Takashi, Nakao, Ken-ichi
Even for the observed luminosity distance D_L(z), which suggests the existence of dark energy, we show that an inhomogeneous dust universe solution without dark energy is possible in general. Future...
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...
Thick Domain Walls Intersecting a Black Hole (2000)
Morisawa, Yoshiyuki, Yamazaki, Ryo, Ida, Daisuke, Ishibashi, Akihiro, Nakao, Ken-ichi
We discuss the gravitationally interacting system of a thick domain wall and a black hole. We numerically solve the scalar field equation in the Schwarzschild spacetime and show that there exist...
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...
How Do Nonlinear Voids Affect Light Propagation ? (1999)
Sugiura, Norimasa, Nakao, Ken-ichi, Ida, Daisuke, Sakai, Nobuyuki, Ishihara, Hideki
Propagation of light in a clumpy universe is examined. As an inhomogeneous matter distribution, we take a spherical void surrounded by a dust shell where the ``lost mass'' in the void is compensated...
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...
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...
Chiba, Takeshi, Maeda, Kei-ichi, Nakao, Ken-ichi, Tsukamoto, Taro
We study the apparent horizon for two boosted black holes in the asymptotically de Sitter space-time by solving the initial data on a space with punctures. We show that the apparent horizon enclosing...
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...
Volume Expansion Rate and The Age of The Universe (1995)
Nakamura, Takashi, Nakao, Ken-ichi, Chiba, Takeshi, Shiromizu, Tetsuya
Under four assumptions such that 1) Einstein's theory of gravity is correct, 2) existence of foliation by geodesic slicing, 3) the trace of the extrinsic curvature, $K$, is negative at the present...
On a Quasi-Local Energy Outside the Cosmological Horizon (1995)
We investigate a quasi-local energy naturally introduced by Kodama's prescription for a spherically symmetric space-time with a positive cosmological constant $\Lambda$. We find that this quasi-local...
Hubble Parameter in Void Universe (1995)
Nakao, Ken-ichi, Gouda, Naoteru, Chiba, Takeshi, Ikeuchi, Satoru, Nakamura, Takashi, Shibata, Masaru
We investigate the distance-redshift relation in the simple void model. As discussed by Moffat and Tatarski, if the observer stays at the center of the void, the observed Hubble parameter is not so...
A Cosmological Constant Limits the Size of Black Holes (1993)
Hayward, Sean A., Shiromizu, Tetsuya, Nakao, Ken-ichi
In a space-time with cosmological constant $\Lambda>0$ and matter satisfying the dominant energy condition, the area of a black or white hole cannot exceed $4\pi/\Lambda$. This applies to event...