arXiv subjects

Taishi Ikeda

Publications and source records attributed to Taishi Ikeda.

30 records · Page 2Linked to original sources

arXiv2019

The physics of black hole binaries: geodesics, relaxation modes and energy extraction

Black holes are the simplest macroscopic objects, and provide unique tests of General Relativity. They have been compared to the Hydrogen atom in quantum mechanics. Here, we establish a few facts about the simplest systems bound by gravity: black hole binaries. We provide strong evidence for the existence of `global' photosurfaces surrounding the binary, and of binary quasinormal modes leading to exponential decay of massless fields when the binary spacetime is slightly perturbed. These two properties go hand in hand, as they did for isolated black holes. The binary quasinormal modes have high quality factor and may be prone to resonant excitations. Finally, we show that energy extraction from binaries is generic and we find evidence of a new mechanism -- akin to the Fermi acceleration process -- whereby the binary transfers energy to its surroundings in a cascading process. The mechanism is conjectured to work when the individual components spin, or are made of compact stars.

gr-qc↗

arXiv2019

Spontaneous scalarization of black holes in the Horndeski theory

We investigate the possibility of spontaneous scalarization of static, spherically symmetric, and asymptotically flat black holes (BHs) in the Horndeski theory. Spontaneous scalarization of BHs is a phenomenon that the scalar field spontaneously obtains a nontrivial profile in the vicinity of the event horizon via the nonminimal couplings and eventually the BH possesses a scalar charge. In the theory in which spontaneous scalarization takes place, the Schwarzschild solution with a trivial profile of the scalar field exhibits a tachyonic instability in the vicinity of the event horizon, and evolves into a hairy BH solution. Our analysis will extend the previous studies about the Einstein-scalar-Gauss-Bonnet (GB) theory to other classes of the Horndeski theory. First, we clarify the conditions for the existence of the vanishing scalar field solution $ϕ=0$ on top of the Schwarzschild spacetime, and we apply them to each individual generalized galileon coupling. For each coupling, we choose the coupling function with minimal power of $ϕ$ and $X:=-(1/2)g^{μν}\partial_μϕ\partial_νϕ$ that satisfies the above condition, which leaves nonzero and finite imprints in the radial perturbation of the scalar field. Second, we investigate the radial perturbation of the scalar field about the $ϕ=0$ solution on top of the Schwarzschild spacetime. While each individual generalized galileon coupling except for a generalized quartic coupling does not satisfy the hyperbolicity condition or realize a tachyonic instability of the Schwarzschild spacetime by itself, a generalized quartic coupling can realize it in the intermediate length scales outside the event horizon. Finally, we investigate a model with generalized quartic and quintic galileon couplings, which includes the Einstein-scalar-GB theory as the special case.

gr-qc↗

arXiv2019

Electromagnetic emission from axionic clouds and the quenching of superradiant instabilities

The nature of dark matter is one of the longest-standing puzzles in science. Axions or axion-like particles are a key possibility, and arise in mechanisms to solve the strong CP problem but also in low-energy limits of string theory. Extensive experimental and observational efforts are actively looking for `axionic' imprints. Independently on their nature, their abundance, and on their contribution to the dark matter problem, axions form dense clouds around spinning black holes, grown by superradiant mechanisms. It was recently suggested that once couplings to photons are considered, an exponential (quantum) stimulated emission of photons ensues at large enough axion number. Here we solve numerically the classical problem in different setups. We show that laser-like emission from clouds exists at the classical level, and we provide the first quantitative description of the problem.

gr-qc↗

arXiv2019

Scalarized black holes in the presence of the coupling to Gauss-Bonnet gravity

In this paper, we study static and spherically symmetric black hole (BH) solutions in the scalar-tensor theories with the coupling of the scalar field to the Gauss-Bonnet (GB) term $ξ(ϕ) R_{\rm GB}$, where $R_{\rm GB}:=R^2-4R^{αβ}R_{αβ}+R^{αβμν}R_{αβμν}$ is the GB invariant and $ξ(ϕ)$ is a function of the scalar field $ϕ$. Recently, it was shown that in these theories scalarized static and spherically symmetric BH solutions which are different from the Schwarzschild solution and possess the nontrivial profiles of the scalar field can be realized for certain choices of the coupling functions and parameters. These scalarized BH solutions are classified in terms of the number of nodes of the scalar field. It was then pointed out that in the case of the pure quadratic order coupling to the GB term, $ξ(ϕ)=ηϕ^2/8$, scalarized BH solutions with any number of nodes are unstable against the radial perturbation. In order to see how a higher order power of $ϕ$ in the coupling function $ξ(ϕ)$ affects the properties of the scalarized BHs and their stability, we investigate scalarized BH solutions in the presence of the quartic order term in the GB coupling function, $ξ(ϕ)=ηϕ^2 (1+αϕ^2)/8$. We clarify that the existence of the higher order term in the coupling function can realize scalarized BHs with zero nodes of the scalar field which are stable against the radial perturbation.

gr-qc↗

arXiv2019

Axionic instabilities and new black hole solutions

The coupling between scalar and vector fields has a long and interesting history. Axions are one key possibility to solve the strong CP problem and axion-like particles could be one solution to the dark matter puzzle. Given the nature of the coupling, and the universality of free fall, nontrivial important effects are expected in regions where gravity is strong. Here, we show that i. A background EM field induces an axionic instability in flat space, for large enough electric fields. Conversely, a homogeneous harmonic axion field induces an instability in the Maxwell sector. When carried over to curved spacetime, this phenomena translates into generic instabilities of charged black holes (BHs). ii. In the presence of charge, BH uniqueness results are lost. We find solutions which are small deformations of the Kerr-Newman geometry and hairy stationary solutions without angular momentum, which are `dragged' by the axion. Axion fields must exist around spinning BHs if these are immersed in external magnetic fields. The axion profile can be obtained perturbatively from the electro-vacuum solution derived by Wald. iii. Ultralight axions trigger superradiant instabilities of spinning BHs and form an axionic cloud in the exterior geometry. The superradiant growth can be interrupted or suppressed through axionic or scalar couplings to EM. These couplings lead to periodic bursts of light, which occur throughout the history of energy extraction from the BH. We provide numerical and simple analytical estimates for the rates of these processes. iv. Finally, we discuss how plasma effects can affect the evolution of superradiant instabilities.

gr-qc↗

arXiv2019

Gravitational Collapse of a Massless Scalar Field in a Periodic Box

Gravitational collapse of a massless scalar field with the periodic boundary condition in a cubic box is reported. This system can be regarded as a lattice universe model. We construct the initial data for a Gaussian like profile of the scalar field taking the integrability condition associated with the periodic boundary condition into account. For a large initial amplitude, a black hole is formed after a certain period of time. While the scalar field spreads out in the whole region for a small initial amplitude. It is shown that the expansion law in a late time approaches to that of the radiation dominated universe and the matter dominated universe for the small and large initial amplitude cases, respectively. For the large initial amplitude case, the horizon is initially a past outer trapping horizon, whose area decreases with time, and after a certain period of time, it turns to a future outer trapping horizon with the increasing area.

gr-qc↗

arXiv2018

Chameleon Field in a Spherical Shell System

We investigate the validity of screening mechanism of the fifth force for chameleon field in highly inhomogeneous density profile. For simplicity, we consider a static and spherically symmetric system which is composed of concentric infinitely thin shells. We calculate the fifth force profile by using a numerical method for a relatively large Compton wavelength of the chameleon field. An approximate solution is also derived for the small Compton wavelength limit. Our results show that, if the thin-shell condition for the corresponding smoothed density profile is satisfied, the fifth force is safely screened outside the system irrespective of the configuration of the shells inside the system. In contrast to the screening outside the system, we find that the fifth force can be comparable to the Newtonian gravitational force inside the system. If the system is highly inhomogeneous, the chameleon field cannot trace the potential minimum varying with the density and repeats being kicked, climbing up and rolling down the potential even when the effective mass of the chameleon field is sufficiently large in the system on average. One should not feel complacent about the wellbehavedness of the fifth force field with an averaged density distribution when we consider inhomogeneous objects.

gr-qc↗

arXiv2018

Parameter estimation by decoherence in the double-slit experiment

We discuss a parameter estimation problem using quantum decoherece in the double-slit interferometer. We consider a particle coupled to a massive scalar field after the particle passing through the double slit and solve the dynamics non-perturbatively for the coupling by the WKB approximation. This allows us to analyze the estimation problem which cannot be treated by master equation used in the research of quantum probe. In this model, the scalar field reduces the interference fringes of the particle and the fringe pattern depends on the field mass and coupling. To evaluate the contrast and the estimation precision obtained from the pattern, we introduce the interferometric visibility and the Fisher information matrix of the field mass and coupling. For the fringe pattern observed on the distant screen, we derive a simple relation between the visibility and the Fisher matrix. Also, focusing on the estimation precision of the mass, we find that the Fisher information characterizes the wave-particle duality in the double-slit interferometer.

quant-ph↗

arXiv2018

Remarks on the maximum luminosity

The quest for fundamental limitations on physical processes is old and venerable. Here, we investigate the maximum possible power, or luminosity, that any event can produce. We show, via full nonlinear simulations of Einstein's equations, that there exist initial conditions which give rise to arbitrarily large luminosities. However, the requirement that there is no past horizon in the spacetime seems to limit the luminosity to below the Planck value, ${{\cal L}_\textrm{P}\!=\!c^5/G}$. Numerical relativity simulations of critical collapse yield the largest luminosities observed to date, ${\approx \! 0.2 {\cal L}_\textrm{P}}$. We also present an analytic solution to the Einstein equations which seems to give an unboundedly large luminosity; this will guide future numerical efforts to investigate super-Planckian luminosities.

gr-qc↗

arXiv2017

Self-gravitating oscillons and new critical behavior

The dynamical evolution of self-interacting scalars is of paramount importance in cosmological settings, and can teach us about the content of Einstein's equations. In flat space, nonlinear scalar field theories can give rise to localized, non-singular, time-dependent, long-lived solutions called {\it oscillons}. Here, we discuss the effects of gravity on the properties and formation of these structures, described by a scalar field with a double well potential. We show that oscillons continue to exist even when gravity is turned on, and we conjecture that there exists a sequence of critical solutions with infinite lifetime. Our results suggest that a new type of critical behavior appears in this theory, characterized by modulations of the lifetime of the oscillon around the scaling law and the modulations of the amplitude of the critical solutions.

gr-qc↗

arXiv2016

Critical behavior of spherically symmetric domain wall collapse

Critical collapse of a spherically symmetric domain wall is investigated. The domain wall is made of a minimally coupled scalar field with a double well potential. We consider a sequence of the initial data which describe a momentarily static domain wall characterized by its initial radius. The time evolution is performed by a full general relativistic numerical code for spherically symmetric systems. In this paper, we use the maximal slice gauge condition, in which spacelike time slices may penetrate the black hole horizon differently from other widely used procedures. In this paper, we consider two specific shapes of the double well potential, and observe the Type II critical behavior in both cases. The mass scaling, sub-critical curvature scaling, and those fine structures are confirmed. The index of the scaling behavior agrees with the massless scalar case.

gr-qc↗

arXiv2015

Expanding universe with nonlinear gravitational waves

We test the validity of Isaacson's formula which states that high frequency and low amplitude gravitational waves behave as a radiation fluid on average. For this purpose, we numerically construct a solution of the vacuum Einstein equations which contains nonlinear standing gravitational waves. The solution is constructed in a cubic box with periodic boundary conditions. The time evolution is solved in a gauge in which the trace of the extrinsic curvature $K$ of the time slice becomes spatially uniform. Then, the Hubble expansion rate $H$ is defined by $H=-K/3$ and compared with the effective scale factor $L$ defined by the proper volume, area and length of the cubic box. We find that, even when the wave length of the gravitational waves is comparable to the Hubble scale, the deviation from Isaacson's formula $H\propto L^{-2}$ is at most 3\% without taking a temporal average and is below 0.1\% with a temporal average.

gr-qc↗