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Kei-ichi Maeda

Publications and source records attributed to Kei-ichi Maeda.

142 records · Page 8Linked to original sources
arXiv1996

Dynamics of Quiet Universes

We study the stability of a contracting {\it silent universe}, which is a spacetime with irrotational dust and vanishing magnetic part of the Weyl tensor, $H_{ab}=0$. Two general relativistic backgrounds are analyzed; one is an attractor of {\it silent universes}, i.e., a locally Kasner spacetime, and the other is a particular class of inhomogeneous Szekeres solutions. In both cases their stabilities against perturbations with non-zero magnetic part depend on a contraction configuration; a spindle-like collapse is unstable, while a pancake-like collapse is still stable. We also find that a similar instability exists in spindle collapse for a Newtonian case. We conclude that $H_{ab}=0$ is not a generic ansatz even in general relativistic dust collapse.

astro-ph↗
arXiv1996

Dilatonic Black Holes with Gauss-Bonnet Term

We discuss black holes in an effective theory derived from a superstring model, which includes a dilaton field, a gauge field and the Gauss-Bonnet term. Assuming U(1) or SU(2) symmetry for the gauge field, we find four types of spherically symmetric solutions, i.e., a neutral, an electrically charged, a magnetically charged and a ``colored'' black hole, and discuss their thermodynamical properties and fate via the Hawking evaporation process. For neutral and electrically charged black holes, we find critical point and a singular end point. Below the mass corresponding to the critical point, nosolution exists, while the curvature on the horizon diverges and anaked singularity appears at the singular point. A cusp structure in the mass-entropy diagram is found at the critical point and black holes on the branch between the critical and singular points become unstable. For magnetically charged and ``colored" black holes, the solution becomes singular just at the end point with a finite mass. Because the black hole temperature is always finite even at the critical point or the singular point, we may conclude that the evaporation process will not be stopped even at the critical point or the singular point, and the black hole will move to a dynamical evaporation phase or a naked singularity will appear.

gr-qc↗
arXiv1996

Monopole-Bubble in Early Universe

The nucleation and evolution of bubbles are investigated in the model of an $O(3)$-symmetric scalar field coupled to gravity in the high temperature limit. It is shown that, in addition to the well-known bubble of which the inside region is true vacuum, there exists another decay channel at high temperature which is described by a new solution such that a false vacuum region like a global monopole remains at the center of a bubble. The value of the Euclidean action of this bubble is higher than that of the ordinary bubble; however, the production rate of it can be considerable for a certain range of scalar potentials.

gr-qc↗
arXiv1996

Chaos in Schwarzschild Spacetime : The Motion of a Spinning Particle

We study the motion of a spinning test particle in Schwarzschild spacetime, analyzing the Poincaré map and the Lyapunov exponent. We find chaotic behavior for a particle with spin higher than some critical value (e.g. $S_{cr} \sim 0.64 μM$ for the total angular momentum $J=4 μM$), where $μ$ and $M$ are the masses of a particle and of a black hole, respectively. The inverse of the Lyapunov exponent in the most chaotic case is about three orbital periods, which suggests that chaos of a spinning particle may become important in some relativistic astrophysical phenomena. The ``effective potential" analysis enables us to classify the particle orbits into four types as follows. When the total angular momentum $J$ is large, some orbits are bounded and the ``effective potential"s are classified into two types: (B1) one saddle point (unstable circular orbit) and one minimal point (stable circular orbit) on the equatorial plane exist for small spin; and (B2) two saddle points bifurcate from the equatorial plane and one minimal point remains on the equatorial plane for large spin. When $J$ is small, no bound orbits exist and the potentials are classified into another two types: (U1) no extremal point is found for small spin; and (U2) one saddle point appears on the equatorial plane, which is unstable in the direction perpendicular to the equatorial plane, for large spin. The types (B1) and (U1) are the same as those for a spinless particle, but the potentials (B2) and (U2) are new types caused by spin-orbit coupling. The chaotic behavior is found only in the type (B2) potential. The ``heteroclinic orbit'', which could cause chaos, is also observed in type (B2).

gr-qc↗
arXiv1996

Solitonic Island in the Bubble

We review how the continuous symmetry can support a soliton inside a high-temperature bubble at the time of its nucleation. This solitonic island in disoriented phase remains stable during the growth of bubbles before their collision.

hep-th↗
arXiv1995

Dynamics of Topological Defects and Inflation

We study the dynamics of topological defects in the context of ``topological inflation" proposed by Vilenkin and Linde independently. Analysing the time evolution of planar domain walls and of global monopoles, we find that the defects undergo inflationary expansion if $η\stackrel{>}{\sim}0.33m_{Pl}$, where $η$ is the vacuum expectation value of the Higgs field and $m_{Pl}$ is the Planck mass. This result confirms the estimates by Vilenkin and Linde. The critical value of $η$ is independent of the coupling constant $λ$ and the initial size of the defect. Even for defects with an initial size much greater than the horizon scale, inflation does not occur at all if $η$ is smaller than the critical value. We also examine the effect of gauge fields for static monopole solutions and find that the spacetime with a gauge monopole has an attractive nature, contrary to the spacetime with a global monopole. It suggests that gauge fields affect the onset of inflation.

gr-qc↗
arXiv1995

Tree-level String Cosmology

In this paper we examine the classical evolution of a cosmological model derived from the low-energy tree-level limit of a generic string theory. The action contains the metric, dilaton, central charge and an antisymmetric tensor field. We show that with a homogeneous and isotropic metric, allowing spatial curvature, there is a formal equivalence between this system and a scalar field minimally coupled to Einstein gravity in a spatially flat metric. We refer to this system as the shifted frame and using it we describe the full range of cosmological evolution that this model can exhibit. We show that generic solutions begin (or end) with a singularity. As the system approaches a singularity the dilaton becomes becomes large and loop corrections will become important.

hep-th↗
arXiv1995

Evaporation and Fate of Dilatonic Black Holes

We study both spherically symmetric and rotating black holes with dilaton coupling and discuss the evaporation of these black holes via Hawking's quantum radiation and their fates. We find that the dilaton coupling constant $α$ drastically affects the emission rates, and therefore the fates of the black holes. When the charge is conserved, the emission rate from the non-rotating hole is drastically changed beyond $α= 1$ (a superstring theory) and diverges in the extreme limit. In the rotating cases, we analyze the slowly rotating black hole solution with arbitrary $α$ as well as three exact solutions, the Kerr--Newman ($α= 0$), and Kaluza--Klein ($α= \sqrt{3}$), and Sen black hole ($α= 1$ and with axion field). Beyond the same critical value of $α\sim 1$, the emission rate becomes very large near the maximally charged limit, while for $α<1$ it remains finite. The black hole with $α> 1$ may evolve into a naked singularity due to its large emission rate. We also consider the effects of a discharge process by investigating superradiance for the non-rotating dilatonic black hole.

hep-th↗
arXiv1995

Cosmic Colored Black Holes

We present spherically symmetric static solutions (a particle-like solution and a black hole solution) in the Einstein-Yang-Mills system with a cosmological constant.Although their gravitational structures are locally similar to those of the Bartnik-McKinnon particles or the colored black holes, the asymptotic behavior becomes quite different because of the existence of a cosmological horizon. We also discuss their stability by means of a catastrophe theory as well as a linear perturbation analysis and find the number of unstable modes.

gr-qc↗
arXiv1994

Superradiance around Rotating Dilatonic Black Hole

We consider a superradiance effect around rotating dilatonic black holes. We analyze two cases: one is an exact solution with the coupling constant $α=\sqrt{3}$, which effective action is reduced from the 5-dimensional Kaluza-Klein theory, and the other is a slowly rotating dilatonic black holes with arbitrary coupling constant. We find that there exists a critical value ($α\sim 1$), which is predicted from a superstring model, and the superradiant emission rate with coupling larger than the critical value becomes much higher than the Kerr-Newman case ($α=0$) in the maximally charged limit. Consequently, 4-dimensional primordial black holes in higher dimensional unified theories are either rotating but almost neutral or charged but effectively non-rotating.

hep-th↗
arXiv1994

Junction Conditions of Friedmann-Robertson-Walker Space-Times

We complete a classification of junctions of two Friedmann-Robertson-Walker space-times bounded by a spherical thin wall. Our analysis covers super-horizon bubbles and thus complements the previous work of Berezin, Kuzumin and Tkachev. Contrary to sub-horizon bubbles, various topology types for super-horizon bubbles are possible, regardless of the sign of the extrinsic curvature. We also derive a formula for the peculiar velocity of a domain wall for all types of junction.

gr-qc↗
arXiv1994

Non-Abelian Black Holes and Catastrophe Theory I : Neutral Type

We re-analyze the globally neutral non-Abelian black holes and present a unified picture, classifying them into two types; Type I (black holes with massless non-Abelian field) and Type II (black holes with ``massive" non-Abelian field). For the Type II, there are two branches: The black hole in the high-entropy branch is ``stable" and almost neutral, while that in the low entropy branch, which is similar to the Type I, is unstable and locally charged. To analyze their stabilities, we adopt the catastrophe theoretic method, which reveals us a universal picture of stability of the black holes. It is shown that the isolated Type II black hole has a fold catastrophe structure. In a heat bath system, the Type I black hole shows a cusp catastrophe, while the Type II has both fold and cusp catastrophe.

gr-qc↗
arXiv1994

Finding Principal Null Direction for Numerical Relativists

We present a new method for finding principal null directions (PNDs). Because our method assumes as input the intrinsic metric and extrinsic curvature of a spacelike hypersurface, it should be particularly useful to numerical relativists. We illustrate our method by finding the PNDs of the Kastor-Traschen spacetimes, which contain arbitrarily many $Q=M$ black holes in a de Sitter back-ground.

gr-qc↗
arXiv1994

Gauge-Invariant Cosmological Perturbations in Generalized Einstein Theories

Using the covariant approach and conformal transformations, we present a gauge-invariant formalism for cosmological perturbations in generalized Einstein theories (GETs), including the Brans-Dicke theory, theories with a non-minimally coupled scalar field and certain curvature-squared theories. We find an enhancement in the growth rate of density perturbations in the Brans-Dicke theory, and discuss attractive features of GETs in the structure formation process.

astro-ph↗