Hideki Ishihara

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.

Quasinormal modes of maximally charged black holes.

A new algorithm for computing the accurate values of quasinormal frequencies of extremal Reissner-Nordstr{o}m 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. nWe also find a curious fact that the resonant frequencies of gravitational waves with multi-pole index

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

l

How do nonlinear voids affect light propagation

coincide with those of electromagnetic waves with multi-pole index

Onset of inflation in inhomogeneous cosmology

l-1

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

in the extremal limit.

Does the generalized second law hold in the form of time derivative expression

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.

Bubble wall perturbations coupled with gravitational waves

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

Equation of motion for a domain wall coupled to the gravitational field

sim 5%

Perturbing supersymmetric black holes

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