282 citations · 2.7k across the 89 of their papers we have counts for
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Radiative gravastar with thermal spectrum; Sudden vacuum condensation without gravitational collapse
Ken-ichi Nakao, Kazumasa Okabayashi, Tomohiro Harada
The gravastar is an exotic compact object proposed as a final product of gravitational collapse of a massive object in order to resolve problems associated with black holes. It is…
Relativistic orbits of S2 star in the presence of scalar field
Parth Bambhaniya, Ashok B. Joshi, Dipanjan Dey +4
The general theory of relativity predicts the relativistic effect in the orbital motions of S-stars which are orbiting around our Milky-way galactic center. The post-Newtonian or h…
Radiative gravastar with Gibbons-Hawking temperature
Ken-ichi Nakao, Kazumasa Okabayashi, Tomohiro Harada
We study the quantum particle creation in a toy model of spherically symmetric gravitational collapse whose final product is not a black hole but a gravastar. Precedent studies rev…
Robustness of particle creation in the formation of a compact object
Kazumasa Okabayashi, Tomohiro Harada, Ken-ichi Nakao
Hawking has predicted that the formation of a black hole by gravitational collapse causes quantum particle creation and the spectrum of the particles is almost thermal. This phenom…
Current status of space gravitational wave antenna DECIGO and B-DECIGO
Seiji Kawamura, Masaki Ando, Naoki Seto +84
Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO) is the future Japanese space mission with a frequency band of 0.1 Hz to 10 Hz. DECIGO aims at the detection of pri…
Do black hole shadows merge?
Kazumasa Okabayashi, Nobuyuki Asaka, Ken-ichi Nakao
The so-called black hole shadow is not a silhouette of a black hole but an image of a collapsing object or a white hole. Hence it is non-trivial whether black hole shadows merge wi…
How does a collapsing star look?
Hirotaka Yoshino, Kazuma Takahashi, Ken-ichi Nakao
Time evolution of an optical image of a pressureless star under gravitational collapse is studied in the geometric optics approximation. The star surface is assumed to emit radiati…
Constant-mean-curvature Slicing of the Swiss-cheese Universe
Chul-Moon Yoo, Ken-ichi Nakao
A sequence of Constant-Mean-Curvature(CMC) slices in the Swiss-Cheese(SC) Universe is investigated. We focus on the CMC slices which smoothly connect to the homogeneous time slices…
On gravastar formation: What can be the evidence of a black hole?
Ken-ichi Nakao, Chul-Moon Yoo, Tomohiro Harada
Any observer outside black holes cannot detect any physical signal produced by the black holes themselves, since, by definition, the black holes are not located in the causal past…
Construction of KAGRA: an Underground Gravitational Wave Observatory
T. Akutsu, M. Ando, S. Araki +222
Major construction and initial-phase operation of a second-generation gravitational-wave detector KAGRA has been completed. The entire 3-km detector is installed underground in a m…
The status of KAGRA underground cryogenic gravitational wave telescope
KAGRA Collaboration, T. Akutsu, M. Ando +146
KAGRA is a 3-km interferometric gravitational wave telescope located in the Kamioka mine in Japan. It is the first km-class gravitational wave telescope constructed underground to…
Non-linear collisional Penrose process: How large energy can a black hole release?
Ken-ichi Nakao, Hirotada Okawa, Kei-ichi Maeda
Energy extraction from a rotating or charged black hole is one of fascinating issues in general relativity. The collisional Penrose process is one of such extraction mechanisms and…
On the stability of a superspinar
Ken-ichi Nakao, Pankaj S. Joshi, Jun-Qi Guo +5
The superspinar proposed by Gimon and Horava is a rapidly rotating compact entity whose exterior is described by the over-spinning Kerr geometry. The compact entity itself is expec…
Spins of primordial black holes formed in the matter-dominated phase of the Universe
Tomohiro Harada, Chul-Moon Yoo, Kazunori Kohri +1
Angular momentum plays very important roles in the formation of PBHs in the matter-dominated phase if it lasts sufficiently long. In fact, most collapsing masses are bounced back d…
Non-linear stability of a brane wormhole
Yumi Akai, Ken-ichi Nakao
We analytically study the non-linear stability of a spherically symmetric wormhole supported by an infinitesimally thin brane of negative tension, which has been devised by Barcelo…
Orbital angular momentum of scalar field generated by gravitational scatterings
Ryusuke Nishikawa, Ken-ichi Nakao, Atsuki Masuda +2
It has been expected that astronomical observations to detect the orbital angular momenta of electromagnetic waves may give us a new insight into astrophysics. Previous works point…
Infinite efficiency of collisional Penrose process: Can over-spinning Kerr geometry be the source of ultra-high-energy cosmic rays and neutrinos ?
Mandar Patil, Tomohiro Harada, Ken-ichi Nakao +2
The origin of the ultra-high-energy particles we receive on the Earth from the outer space such as EeV cosmic rays and PeV neutrinos remains an enigma. All mechanisms known to us c…
Timescale for trans-Planckian collisions in Kerr spacetime
Mandar Patil, Pankaj S. Joshi, Ken-ichi Nakao +2
We make a critical comparison between ultra-high energy particle collisions around an extremal Kerr black hole and that around an over-spinning Kerr singularity, mainly focusing on…
Newtonian self-gravitating system in a relativistic huge void universe model
Ryusuke Nishikawa, Ken-ichi Nakao, Chul-Moon Yoo
We consider a test of the Copernican Principle through observations of the large-scale structures, and for this purpose we study the self-gravitating system in a relativistic huge…
How small can an over-spinning body be in general relativity?
Ken-ichi Nakao, Masashi Kimura, Tomohiro Harada +2
The angular momentum of the Kerr singularity should not be larger than a threshold value so that it is enclosed by an event horizon: The Kerr singularity with the angular momentum…
What wormhole is traversable?: A case of a wormhole supported by a spherical thin shell
Ken-ichi Nakao, Tatsuya Uno, Shunichiro Kinoshita
We analytically explore the effect of falling matter on a spherically symmetric wormhole supported by a spherical shell composed of exotic matter located at its throat. The falling…
Black Hole Universe: Time Evolution
Chul-Moon Yoo, Hirotada Okawa, Ken-ichi Nakao
Time evolution of a black hole lattice universe is simulated. The vacuum Einstein equations in a cubic box with a black hole at the origin are numerically solved with periodic boun…
Ultra-high energy collision with neither black hole nor naked singularity
Ken-ichi Nakao, Masashi Kimura, Mandar Patil +1
We explore the collision between two concentric spherical thin shells. The inner shell is charged, whereas the outer one is either neutral or charged. In the situation we consider,…
Black Hole Universe: Construction and Analysis of Initial Data
Chul-Moon Yoo, Hiroyuki Abe, Yohsuke Takamori +1
We numerically construct an one-parameter family of initial data of an expanding inhomogeneous universe model which is composed of regularly aligned black holes with an identical m…