arXiv subjects

Hideki Ishihara

Publications and source records attributed to Hideki Ishihara.

At least 19 recordsLinked to original sources

arXiv2026

Geometric deformations of symmetric spacetimes with a string cloud

We establish a deformation framework for highly symmetric solutions to the Einstein equations. In this framework, four-dimensional metrics are constructed from three-dimensional η-Einstein metrics admitting a deformation determined by a single function. Under this deformation, the resulting spacetime solves the Einstein equations with a string-cloud source. Within this framework , a wide range of symmetric spacetimes can be treated in a unified manner. These include FLRW, Kantowski-Sachs, and LRS Bianchi cosmological models (including Taub-NUT-(A)dS solutions), as well as Reissner-Nordström-(A)dS black holes admitting spherical, planar, or hyperbolic symmetry. In the cosmological setting, the deformation leaves the evolution equations for the scale factors unchanged, and hence the expansion history coincides with that of the corresponding undeformed models. For the deformed Reissner-Nordström-(A)dS black holes, the structure of Killing horizons is insensitive to the deformation.

gr-qc↗

arXiv2026

Motion of a charged test particle around a static black hole in a monopole magnetic field

We study the motion of a charged test particle in the spacetime with a spherically symmetric black hole which is immersed in a monopole magnetic field. We show that the radial motion of the charged test particle is governed by completely the same equation as that in the case of no magnetic field. This result implies that the black hole will acquire the electric charge if it is surrounded by the collisionless plasma composed of protons and electrons which obey the Maxwell velocity distribution. The drastically different situation appears in the tangential motions of charged test particles due to the magnetic field. The trajectory of a charged test particle in the black hole with the magnetic field of the order of 10 Gauss near the black hole, is confined on a very thin cone as long as the specific angular momentum of the particle is not much larger than the gravitational radius of the black hole times the speed of light. This result leads to a possibility that a plasma lump can hover over the black hole and is very hot, in the monopole magnetic field.

gr-qc↗

arXiv2025

Vacuum Magnetospheres around Kerr Black Holes with a Thin Disk

We construct a model for stationary and axisymmetric black hole magnetospheres by solving the vacuum Maxwell equations in Kerr backgrounds. The poloidal magnetic field is generated by a toroidal electric current in a thin disk on the equatorial plane with an inner edge, and the poloidal electric field is induced due to the black hole spin. We assume a current distribution whose direction reverses at a radius on the disk. The magnetospheric structure is divided into the inner region and the outer one due to the current reversal. In the inner region, some magnetic field lines connect the black hole and the disk, and others surround the inner edge of the disk. In the outer region, the field lines connect the disk and infinity. We investigate the black hole spin dependence of the magnetospheric structure. In the magnetosphere around the Kerr black hole, the electric fields are generated by the black hole spin. In our model, the charge distribution is induced on the disk and the event horizon. Except near the black hole, the electric field strength is proportional to the black hole spin, while the direction of the electric field remains almost the same regardless of the spin. On the other hand, the electromagnetic field near the event horizon strongly depends on the spin. In the maximally rotating case, the magnetic field lines are expelled from the event horizon and the electric equipotential surface coincides with the horizon.

gr-qc↗

arXiv2024

Spacetime constructed from a contact manifold with a degenerate metric

We construct a four-dimensional spacetime using a three-dimensional contact manifold equipped with a degenerate metric. The degenerate metric is set to be compatible with the contact structure. The compatibility condition is defined in this paper. Our construction yields a Ricci tensor of a particularly simple form, which leads to a solution of the Einstein equation with a null dust and cosmic strings. The solution includes two arbitrary functions: the energy density of the null dust and the number density of the cosmic strings. When there exist the cosmic strings, the spacetime is of Petrov type D. Otherwise, the spacetime is conformally flat. For some simple matter densities, we examine the Einstein equation in detail.

gr-qc↗

arXiv2024

Electrification of a non-rotating black hole

Zajacek et al made an interesting theoretical prediction on the electrification of a non-rotating black hole; if a non-rotating black hole is surrounded by plasma composed of protons and electrons, it will acquire electric charge due to the large difference of the inertial mass of a proton and that of an electron. Furthermore they revealed the effects of the electric charge of the black hole on the surrounding plasma. Since their results mainly rely on non-relativistic analyses, we study the same subject through relativistic analyses in this paper. By investigating a test particle in the Schwarzschild spacetime, we find that if initial velocities of protons and electrons far from a black hole follow the Maxwell distribution, the black hole can acquire electric charge whose value depends on the ratio of temperature of the proton and that of the electron. We also show that if the black hole acquires the electric charge, the radii of the innermost stable circular orbit (ISCO) and the specific energy of a charged particle on ISCO can be very different from those of a neutral particle. In contrast to the result obtained by Zajacek et al, we find that the ISCO radii of a proton and an electron are necessarily larger than that of a neutral test particle as long as the black hole acquires the charge. The large ISCO radius might lead to a larger angular diameter of a black hole shadow and a different estimate of the released energy due to the accretion of plasma from the estimate based on the assumption of electric neutrality of the central black hole.

gr-qc↗

arXiv2024

Gravastars as Nontopological Solitons

We investigate a coupled system consisting of a complex scalar field, a U(1) gauge field, a complex Higgs scalar field, and Einstein gravity. We present nontopological soliton star solutions in which the interior geometry is described by the de Sitter metric, the exterior by the Schwarzschild metric, and these two regions are attached by a spherical surface layer with a finite thickness. This structure is the same as the so-called gravastars, so we refer to these solutions as \lq solitonic gravastars\rq . We demonstrate that compact solitonic gravastars that possessing photon spheres can appear. Additionally, we study the mass and compactness of solitonic gravastars for various sets of parameters that characterize the system.

hep-th↗

arXiv2024

Strings in five-dimensional anti-de Sitter space with a symmetry

The equation of motion of an extended object in spacetime reduces to an ordinary differential equation in the presence of symmetry. By properly defining of the symmetry with notion of cohomogeneity, we discuss the method for classifying all these extended objects. We carry out the classification for the strings in the five-dimensional anti-de Sitter space by the effective use of the local isomorphism between $\SO(4,2)$ and $\SU(2,2)$. We present a general method for solving the trajectory of the Nambu-Goto string and apply to a case obtained by the classification, thereby find a new solution which has properties unique to odd-dimensional anti-de Sitter spaces. The geometry of the solution is analized and found to be a timelike helicoid-like surface.

gr-qc↗

arXiv2024

Collective Excitations of Self-Gravitating Bose-Einstein Condensates: Breathing Mode and Appearance of Anisotropy under Self-Gravity

We investigate the collective mode of a self-gravitating Bose-Einstein condensate (BEC) described by the Gross-Pitaevskii-Poisson (GPP) equations. The self-gravitating BEC has garnered considerable attention in cosmology and astrophysics, being proposed as a plausible candidate for dark matter. Our inquiry delves into the breathing and anisotropic collective modes by numerically solving the GPP equations and using the variational method. The breathing mode demonstrates a reduction in period with increasing total mass due to the density dependence of the self-gravitating BEC, attributed to the density-dependent nature of self-gravitating BECs, aligning quantitatively with our analytical findings. Additionally, we investigate an anisotropic collective mode in which the quadrupole mode intertwines with the breathing mode. The period of the quadrupole mode exhibits similar total mass dependence to that of the breathing mode. The characteristics of these periods differ from those of a conventional BEC confined by an external potential. Despite the differences in density dependence, the ratio of their periods equals that of the BEC confined by an isotropic harmonic potential. Furthermore, an extension of the variational method to a spheroidal configuration enables the isolation of solely the quadrupole mode from the anisotropic collective mode.

cond-mat.quant-gas↗

arXiv2023

Nambu-Goto Strings with a null symmetry and contact structure

We study the classical dynamics of the Nambu-Goto strings with a null symmetry in curved spacetimes admitting a null Killing vector field. The Nambu-Goto equation is reduced to first order ordinary differential equations and is always integrable in contrast to the case of non-null symmetries where integrability requires additional spacetime symmetries. It is found that in the case of null symmetry, an almost contact structure associated with the metric dual 1-form $η$ of the null Killing vector field emerges naturally. This structure determines the allowed class of string worldsheets in such a way that the tangent vector fields of the worldsheet lie in $\ker \mathrm{d}η$. In the special case that the almost contact structure becomes a contact structure, its Reeb vector field completely characterizes the worldsheet. We apply our formulation to the strings in the pp-waves, the Einstein static universe and the Gödel universe. We also study their worldsheet geometry in detail.

gr-qc↗

arXiv2023

Solitonic Gravastars in a U(1) gauge Higgs model

We numerically obtain gravastar solutions as nontopological solitons in a system that consists of a U(1) gauge Higgs model with a complex scalar field and Einstein gravity. The solitonic gravastar solutions are compact enough to have a photon sphere.

hep-th↗

arXiv2022

Relativistic Nontopological Soliton Stars in a U(1) Gauge Higgs Model

We study spherically symmetric nontopological soliton stars (NTS stars) numerically in the coupled system of a complex scalar field, a U(1) gauge field, a complex Higgs scalar field, and Einstein gravity, where the symmetry is broken spontaneously. The gravitational mass of NTS stars is limited by a maximum mass for a fixed breaking scale, and the maximum mass increases steeply as the breaking scale decreases. In the case of the breaking scale is much less than the Planck scale, the maximum mass of NTS stars becomes the astrophysical scale, and such a star is relativistically compact so that it has the innermost stable circular orbit. The first author contributed with a part of numerical calculations. The second did with planning and conducting the research, and the third did with all numerical calculations and finding new properties of the system.

hep-th↗

arXiv2021

Inhomogeneous Generalization of Einstein's Static Universe with Sasakian Space

We construct exact static inhomogeneous solutions to Einstein's equations with counter flow of particle fluid and a positive cosmological constant by using the Sasaki metrics on three-dimensional spaces. The solutions, which admit an arbitrary function that denotes inhomogeneous number density of particles, are a generalization of Einstein's static universe. On some examples of explicit solutions, we discuss non-linear density contrast and deviation of the metric functions.

hep-th↗

arXiv2021

Godel-type Solutions in Einstein-Maxwell-Scalar Field Theories

We show that one-parameter family of Godel-type metrics are exact solutions in a wide class of Einstein-Maxwell-scalar field theories. In these solutions, the gauge field has a 1-form expectation value parallel to the timelike contact form that represents the rotation of the Godel universe.

hep-th↗

arXiv2021

A Variety of Nontopological Solitons in a Spontaneously Broken U(1) Gauge Theory -- Dust Balls, Shell Balls, and Potential Balls

We show, by numerical calculations, that there exist three-types of stationary and spherically symmetric nontopological soliton solutions (NTS-balls) with large sizes in the coupled system consisting of a complex matter scalar field, a U(1) gauge field, and a complex Higgs scalar field that causes spontaneously symmetry breaking. Under the assumption of symmetries, the coupled system reduces to a dynamical system with three degrees of freedoms governed by an effective action. The effective potential in the action has stationary points. The NTS-balls with large sizes are described by bounce solutions that start off an initial stationary point, and traverse to the final stationary point, vacuum stationary point. According to the choice of the initial stationary point, there appear three types of NTS-balls: dust balls, shell balls, and potential balls with respect to their internal structures.

hep-th↗

arXiv2020

Solutions to the Einstein-Maxwell-Current System with Sasakian maifolds

We construct stationary solutions to the Einstein-Maxwell-current system by using the Sasakian manifold for the three-dimensional space. Both the magnetic field and the electric current in the solution are specified by the contact form of the Sasakian manifold. The solutions contain an arbitrary function that describes inhomogeneity of the number density of the charged particles, and the function determines the curvature of the space.

gr-qc↗

arXiv2020

Current status of space gravitational wave antenna DECIGO and B-DECIGO

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 primordial gravitational waves, which could be produced during the inflationary period right after the birth of the universe. There are many other scientific objectives of DECIGO, including the direct measurement of the acceleration of the expansion of the universe, and reliable and accurate predictions of the timing and locations of neutron star/black hole binary coalescences. DECIGO consists of four clusters of observatories placed in the heliocentric orbit. Each cluster consists of three spacecraft, which form three Fabry-Perot Michelson interferometers with an arm length of 1,000 km. Three clusters of DECIGO will be placed far from each other, and the fourth cluster will be placed in the same position as one of the three clusters to obtain the correlation signals for the detection of the primordial gravitational waves. We plan to launch B-DECIGO, which is a scientific pathfinder of DECIGO, before DECIGO in the 2030s to demonstrate the technologies required for DECIGO, as well as to obtain fruitful scientific results to further expand the multi-messenger astronomy.

gr-qc↗

arXiv2019

Observability of spherical photon orbits in near-extremal Kerr black holes

We investigate the spherical photon orbits in near-extremal Kerr spacetimes. We show that the spherical photon orbits with impact parameters in a finite range converge on the event horizon. Furthermore, we demonstrate that the Weyl curvature near the horizon does not generate the shear of a congruence of such light rays. Because of this property, a series of images produced by the light orbiting around a near-extremal Kerr black hole several times can be observable.

gr-qc↗

arXiv2019

Homogeneous Balls in a Spontaneously Broken U(1) Gauge Theory

We study the coupled system consisting of a complex matter scalar field, a U(1) gauge field, and a complex Higgs scalar field that causes spontaneously symmetry breaking. We show by numerical calculations that there are spherically symmetric nontopological soliton solutions. Homogeneous balls solutions, all fields take constant values inside the ball and in the vacuum state outside, appear in this system. It is shown that the homogeneous balls have the following properties: charge density of the matter scalar field is screened by counter charge cloud of the Higgs and gauge field everywhere; an arbitrary large size is allowed; energy density and pressure of the ball behave homogeneous nonrelativistic gas; a large ball is stable against dispersion into free particles and against decay into two smaller balls.

hep-th↗