arXiv subjects

Hideki Ishihara

Publications and source records attributed to Hideki Ishihara.

88 records · Page 5Linked to original sources

arXiv2005

Classification of Cohomogeneity One Strings

We define the cohomogeneity one string, string with continuous symmetries, as its world surface is tangent to a Killing vector field of a target space. We classify the Killing vector fields by an equivalence relation using isometries of the target space. We find that the equivalence classes of Killing vectors in Minkowski spacetime are partitioned into seven families. It is clarified that there exist seven types of strings with spacelike symmetries and four types of strings with timelike symmetries, stationary strings.

gr-qc↗

arXiv2003

Compact Einstein Spaces based on Quaternionic Kähler Manifolds

We investigate the Einstein equation with a positive cosmological constant for $4n+4$-dimensional metrics on bundles over Quaternionic Kähler base manifolds whose fibers are 4-dimensional Bianchi IX manifolds. The Einstein equations are reduced to a set of non-linear ordinary differential equations. We numerically find inhomogeneous compact Einstein spaces with orbifold singularity.

hep-th↗

arXiv2002

How Does Naked Singularity Look?

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 self-similarity in the region filled with the dust fluid, we study these singular null geodesics in detail. We see the time evolution of the angular diameter of the central naked singularity and show that it might be bounded above by the value corresponding to the circular null geodesic in the Schwarzschild spacetime. We also investigate the angular frequency of a physical field which propagates along the singular null geodesic and find that it depends on the impact parameter. Further, we comment on the non-uniformity of the topology of the central naked singularity.

gr-qc↗

arXiv2001

Big bang of the brane universe

Big bang of the Friedmann-Robertson-Walker (FRW)-brane universe is studied. In contrast to the spacelike initial singularity of the usual FRW universe, the initial singularity of the FRW-brane universe is point-like from the viewpoint of causality including gravitational waves propagating in the bulk. Existence of null singularities (seam singuralities) is also shown in the flat and open FRW-brane universe models.

gr-qc↗

arXiv2001

Dynamics of a string coupled to gravitational waves II - Perturbations propagate along an infinite Nambu-Goto string

The perturbative modes propagating along an infinite string are investigated within the framework of the gauge invariant perturbation formalism on a spacetime containing a self-gravitating straight string with a finite thickness. These modes are not included in our previous analysis. We reconstruct the perturbation formalism to discuss these modes and solve the linearized Einstein equation within the first order with respect to the string oscillation amplitude. In the thin string case, we show that the oscillations of an infinite string must involve the propagation of cosmic string traveling wave.

gr-qc↗

arXiv2000

Causality of the brane universe

Causal structure of the brane universe with respect to null geodesics in the bulk spacetime is studied. It is pointed out that apparent causality violation is possible for the brane universe which contains matter energy. It is also shown that there is no 'horizon problem' in the Friedmann-Robertson-Walker brane universe.

gr-qc↗

arXiv2000

Dynamics of a string coupled to gravitational waves - Gravitational wave scattering by a Nambu-Goto straight string

We study the perturbative dynamics of an infinite gravitating Nambu-Goto string within the general-relativistic perturbation framework. We develop the gauge invariant metric perturbation on a spacetime containing a self-gravitating straight string with a finite thickness and solve the linearized Einstein equation. In the thin string case, we show that the string does not emit gravitational waves by its free oscillation in the first order with respect to its oscillation amplitude, nevertheless the string actually bends when the incidental gravitational waves go through it.

gr-qc↗

arXiv1999

How Do Nonlinear Voids Affect Light Propagation ?

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 by the shell. We study how the angular-diameter distance behaves when such a structure exists. The angular-diameter distance is calculated by integrating the Raychaudhuri equation including the shear. An explicit expression for the junction condition for the massive thin shell is calculated. We apply these results to a dust shell embedded in a Friedmann universe and determine how the distance-redshift relation is modified compared with that in the purely Friedmann universe. We also study the distribution of distances in a universe filled with voids. We show that the void-filled universe gives a larger distance than the FRW universe by $\sim 5%$ at $z \sim 1$ if the size of the void is $\sim 5%$ of the Horizon radius.

astro-ph↗

arXiv1998

Equation of motion for a domain wall coupled to gravitational field

The equation of motion for a domain wall coupled to gravitational field is derived from the Nambu-Goto action. The domain wall is treated as a source of gravitational field. The perturbed equation is also obtained with gravitational back reaction on the wall motion taken into account. For general spherically symmetric background case, the equation is expressed in terms of the gauge-invariant variables.

gr-qc↗

arXiv1997

On the perturbation of domain wall coupled to gravitational waves

The equation of motion for domain wall coupled to gravitational field is derived. The domain wall is treated as a source of gravitational field around the wall. The perturbed equation is also obtained with taking account of the gravitational back reaction on the motion of the domain wall.

gr-qc↗

arXiv1997

Bubble wall perturbations coupled with gravitational waves

We study a coupled system of gravitational waves and a domain wall which is the boundary of a vacuum bubble in de Sitter spacetime. To treat the system, we use the metric junction formalism of Israel. We show that the dynamical degree of the bubble wall is lost and the bubble wall can oscillate only while the gravitational waves go across it. It means that the gravitational backreaction on the motion of the bubble wall can not be ignored.

gr-qc↗

arXiv1996

Onset of inflation in inhomogeneous cosmology

We study how the initial inhomogeneities of the universe affect the onset of inflation in the closed universe. We consider the model of a chaotic inflation which is driven by a massive scalar field. In order to construct an inhomogeneous universe model, we use the long wavelength approximation ( the gradient expansion method ). We show the condition of the inhomogeneities for the universe to enter the inflationary phase.

gr-qc↗

arXiv1996

Perturbing Supersymmetric Black Hole

An investigation of the perturbations of the Reissner-Nordström black hole in the $N=2$ supergravity is presented. We prove in the extremal limit that the black hole responds to the perturbation of each field in the same manner. We conjecture that we can match the modes of the graviton, gravitino and photon because of supersymmetry transformations.

gr-qc↗

arXiv1996

Quasinormal modes of maximally charged black holes

A new algorithm for computing the accurate values of quasinormal frequencies of extremal Reissner-Nordström black holes is presented. The numerically computed values are consistent with the values earlier obtained by Leaver and those obtained through the WKB method. Our results are more precise than other results known to date. We also find a curious fact that the resonant frequencies of gravitational waves with multi-pole index $l$ coincide with those of electromagnetic waves with multi-pole index $l-1$ in the extremal limit.

gr-qc↗

arXiv1995

Hamilton-Jacobi Equation for Brans-Dicke Theory and Its Long-wavelength Solution

Hamilton-Jacobi equation for Brans-Dicke theory is solved by using a long-wavelength approximation. We examine the non-linear evolution of the inhomogeneities in the dust fluid case and the cosmological constant case. In the case of dust fluid, it turns out that the inhomogeneities of space-time grow. In the case of cosmological constant, the inhomogeneities decay, which is consistent with the cosmic no hair conjecture. The inhomogeneities of the density perturbation and the gravitational constant behave similarly with that of space-time.

gr-qc↗

arXiv1994

Does a domain wall emit gravitational waves? -- General-relativistic perturbative treatment

The behavior of gravitational wave perturbations on a locally Minkowskian spacetime background containing a planar domain wall is investigated in the gauge-invariant general relativistic framework. It is shown that for this particular background the domain wall does not emit gravitational waves spontaneously by its free oscillation in the first order, although it scatters incidental gravitational waves.

gr-qc↗