arXiv subjects

Naritaka Oshita

Publications and source records attributed to Naritaka Oshita.

At least 37 records · Page 2Linked to original sources

arXiv2024

Greybody Factors Imprinted on Black Hole Ringdowns. II. Merging Binary Black Holes

The spectral amplitude of the merger-ringdown gravitational wave (GW) emitted by a comparable mass-ratio black hole merger is modeled by the greybody factor of the remnant black hole. We also include the post-Newtonian correction to the greybody factor model. Our model includes only a few fitting parameters, which could evade the overfitting issue. We perform the mass-spin inference from the SXS data without tuning the data range of each SXS waveform. Also, we find that the exponential damping in the ringdown spectral amplitude can be modeled well with the exponential damping in the greybody factor at high frequencies. Our findings could be consistent with a conjecture that the light ring of the remnant black hole, which sources the ringdown, forms as early as during the merger stage. We discuss the formation of the light ring in the static binary solution as a first step towards the understanding of how the separation of merging black holes may affect the formation of the light ring.

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arXiv2024

Reconstruction of ringdown with excitation factors

In black hole perturbation formalism, the gravitational waveform is obtained by the convolution of the Green's function and the source term causing radiation emission. Hence, the ringdown properties, namely its start time, depend on both functions. The unknown time-shift encoded in the Green's function introduces a "time-shift problem" for ringdown. We study the ringdown time-shift problem by reconstructing a waveform via the excitation factors of quasi-normal modes (QNMs) of a spinning black hole. For the first time, we reconstruct ringdown with a significant number of QNMs weighted with their excitation factors and confirm its excellent convergence. We then precisely identify the ringdown starting time. We also find (i) that for moderate or large spins and $\ell=m=2$, QNMs should be included up to around the $20$th prograde overtones and around fifth retrograde overtones to reconstruct the ringdown waveform for the delta-function source with a mismatch threshold $ M < O(10^{-3})$. For higher angular modes, a more significant number of QNMs are necessary to reconstruct it; (ii) that the time shift of ringdown caused by the Green's function is the same for different $(\ell, m, n)$ modes but that nontrivial sources can change this conclusion. Finally, we demonstrate (iii) that the greybody factor can be reconstructed with the superposed QNM spectrum in the frequency domain.

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arXiv2024

Black holes, multiple propagation speeds, and energy extraction

The Standard Model of particle physics predicts the speed of light to be a universal speed of propagation of massless carriers. However, other possibilities exist -- including Lorentz-violating theories -- where different fundamental fields travel at different speeds. Black holes are interesting probes of such physics, as distinct fields would probe different horizons. Here, we build an exact spacetime for two interacting scalar fields which have different propagation speeds. One of these fields is able to probe the black hole interior of the other, giving rise to energy extraction from the black hole and a characteristic late-time relaxation. Our results provide further stimulus to the search for extra degrees of freedom, black hole instability, and extra ringdown modes in gravitational-wave events.

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arXiv2024

(In)stability of the black hole greybody factors and ringdowns against a small-bump correction

Recently, it has been proposed that the black hole greybody factors can be important to model ringdown spectral amplitudes. We study the stability of greybody factors against a small-bump correction in the perturbation equation. We find (I) that the greybody factor is stable in the frequency region relevant to ringdown and (II) that it is destabilized at higher frequencies, especially for a sharper bump correction. This behavior is similar to the case of higher overtones, which is also very sensitive to a small correction. We clarify this (in)stability with the WKB analysis. As the greybody factor is stable at the frequency region relevant to the main part of ringdown, we conclude that the greybody factor is suitable to model ringdown amplitude. In order to investigate a bump correction in a self-consistent manner, we consider the small-bump correction that can be realized in the general framework of effective field theory of black hole perturbations.

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arXiv2024

Scalar Quasi-Normal Modes of a Loop Quantum Black Hole

We compute the Quasi-Normal Mode (QNM) frequencies for scalar perturbations for modified Schwarzschild black holes in Loop Quantum Gravity. We study the singularity-free polymerized metric characterised by two parameters encoding loop quantum effects: the minimal area gap $a_0$ and the polymeric deformation parameter $P$. We perform numerical computations using Leaver's continued fraction method and compare our results to other semi-analytical methods and existing literature. We study the effects on the QNM spectrum of variation of both deformation parameters and systematically compare to the standard Schwarzschild case. In particular we find that the scalar fundamental mode is modified from the third decimal for values of $P$ in accordance with the most recent astrophysical constraints. We also show that qualitative differences arise for highly damped modes: on the one hand, a new crossing of the imaginary axis occurs for high values of $a_0$ and, on the other hand, increasing $P$ produces a positive shift of the real part and an increase of the spacing in imaginary part between modes.

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arXiv2024

Slowly decaying ringdown of a rapidly spinning black hole II: Inferring the masses and spins of supermassive black holes with LISA

Electromagnetic observations reveal that almost all galaxies have supermassive black holes (SMBHs) at their centers, but their properties, especially their spins, are not fully understood. Some of the authors have recently shown [Oshita and Tsuna (2023)] that rapid spins of $>0.9$, inferred for masses around $10^7\ M_\odot$ from observations of local SMBHs and cosmological simulations, source {\it long-lived} ringdowns that enhance the precision of black hole spectroscopy to test gravity in the near-extreme Kerr spacetime. In this work, we estimate the statistical errors in the SMBH mass-spin inference in anticipation of the LISA's detection of extreme mass-ratio mergers. We show that for rapidly spinning SMBHs, more precise mass and spin measurements are expected due to the excitations of higher angular modes. For a near-extremal SMBH of mass $10^7M_\odot$ merging with a smaller BH with mass ratio $10^{-3}$ at a luminosity distance of $\lesssim 10\:\mathrm{Gpc}$ (redshift $z \lesssim 1.37$), the measurement errors in the mass and spin of the SMBH would be $\sim 1\:\mathrm{\%}$ and $\sim 10^{-1}\:\mathrm{\%}$ respectively.

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arXiv2023

Slowly Decaying Ringdown of a Rapidly Spinning Black Hole: Probing the No-Hair Theorem by Small Mass-Ratio Mergers with LISA

The measurability of multiple quasinormal (QN) modes, including overtones and higher harmonics, with the Laser Interferometer Space Antenna is investigated by computing the gravitational wave (GW) signal induced by an intermediate or extreme mass ratio merger involving a supermassive black hole (SMBH). We confirm that the ringdown of rapidly spinning black holes are long-lived, and higher harmonics of the ringdown are significantly excited for mergers of small mass ratios. We investigate the measurability and separability of the QN modes for such mergers and demonstrate that the observation of GWs from rapidly rotating SMBHs has an advantage for detecting superposed QN modes and testing the no-hair theorem of black holes.

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arXiv2023

Greybody Factors Imprinted on Black Hole Ringdowns: an alternative to superposed quasi-normal modes

It is shown that the spectral amplitude of gravitational-wave (GW) ringdown of a Kerr black hole sourced by an extreme mass ratio merger can be modeled by the $\textit{greybody factor}$, which quantifies the scattering nature of the black hole geometry. The estimation of the mass and spin of the remnant is demonstrated by fitting the greybody factor to GW data without using black hole quasi-normal modes. We propose that the ringdown modeling with the greybody factor may strengthen the test of gravity as one can avoid the possible overfitting issue and the start time problem in the ringdown modeling with superposed quasi-normal modes.

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arXiv2023

Is the Coleman de Luccia action minimum?: AdS/CFT approach

We use the anti-de Sitter/conformal field theory (AdS/CFT) correspondence to find the least bounce action in an AdS false vacuum state, i.e., the most probable decay process of the metastable AdS vacuum within the Euclidean formalism by Callan and Coleman. It was shown that the $O(4)$ symmetric bounce solution leads to the action minimum in the absence of gravity, but it is non-trivial in the presence of gravity. The AdS/CFT duality is used to evade the difficulties particular to a metastable gravitational system, such as the problems of negative modes and unbounded action. To this end, we show that the Fubini bounce solution in CFT, corresponding to the Coleman de Luccia bounce in AdS, gives the least action among all finite bounce solutions in a conformal scalar field theory. Thus, we prove that the Coleman de Luccia action is the least action when (i) the background is AdS, (ii) the AdS radii, $L_+$ and $L_-$, in the false and true vacua, respectively, satisfy $L_+ / L_- \simeq 1$, and (iii) a metastable potential gives a thin-wall bounce much larger than the AdS radii.

hep-th↗

arXiv2023

dS${}_4$ universe emergent from Kerr-AdS${}_5$ spacetime: bubble nucleation catalyzed by a black hole

The emergence of a four-dimensional de Sitter (dS${}_4$) universe on an expanding bubble in the five-dimensional anti-de Sitter (AdS${}_5$) background has been suggested as a possible cosmological scenario. It is motivated by the difficulties in the realization of a stable de Sitter vacua in string theory. The bubble can be nucleated in a meta-stable pure AdS${}_5$ spacetime, but it is known that a pure AdS spacetime is non-perturbatively unstable. It means that the pure AdS${}_5$ background is an idealized situation, and in realistic situations, non-linear perturbations in AdS may lead to the formation of black holes due to the gravitational turbulent instability. To investigate how the proposed scenario works in a more realistic situation, we here study the nucleation process of a vacuum bubble in the Kerr-AdS${}_5$ spacetime. Especially we investigate conditions sufficient to ensure the nucleation of a vacuum bubble with a rotating black hole and how the black hole affects the transition rate. We find that even in the Kerr-AdS${}_5$ spacetime, a quasi-dS${}_4$ expansion can be realized on the nucleated vacuum bubble without contradicting the de Sitter swampland conjectures.

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arXiv2023

From Entropy to Echoes: Counting the quasi-normal modes and the quantum limit of silence

We estimate the canonical entropy of a quantum black hole by counting its quasi-normal modes. We first show that the partition function of a classical black hole, evaluated by counting the quasi-normal modes with a thermodyanmic Boltzmann weight, leads to a small entropy of order unity due to the small contribution from higher angular modes. We then discuss how this will be modified when taking into account dissipation effects near the horizon due to interaction with the quantum black hole microstates. The structure of quasi-normal modes drastically changes, yielding a fundamental frequency of the inverse of $t_{\rm echo} \sim$ log(Entropy)/Temperature. This is the time-scale for reflection from the microstates (or the quantum time limit of silence, followed by echoes), $\textit{independent of the strength of dissipation}$, and is comparable to the scrambling time proposed by Sekino & Susskind. Setting the dissipation constant to Planck time, we reproduce the Bekenstein-Hawking entropy of $\sim$ (Horizon area)/(Planck area). This result suggests the possibility of simulating black hole entropy in analog horizons realized in condensed matter systems.

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arXiv2023

Polychronic Tunneling: New Tunneling Processes Experiencing Euclidean and Lorentzian Evolution Simultaneously

We discuss new possible tunneling processes in the presence of gravity. We formulate quantum tunneling using the Wheeler-deWitt canonical quantization and the WKB approximation. The distinctive feature of our formulation is that it accommodates the coexistence of Euclidean and Lorentzian evolution. It opens up a new possibility of quantum tunneling; e.g. a bubble wall itself tunnels the potential barrier pulling the field nearby, where the wall region experiences the Euclidean evolution while the other regions experience the Lorentzian evolution simultaneously. We execute numerical analysis and find that such a process can have a much higher tunneling rate than that of the Coleman-De Luccia bounce. We also find that the new tunneling processes exist even in the decoupling regime of gravity and affect low energy phenomenology.

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arXiv2022

Thermal Ringdown of a Kerr Black Hole: Overtone Excitation, Fermi-Dirac Statistics and Greybody Factor

We find a significant destructive interference among Kerr overtones in the early ringdown induced by an extreme mass-ratio merger of a massive black hole and a compact object, and that the ringdown spectrum apparently follows the Fermi-Dirac distribution. We numerically compute the spectral amplitude of gravitational waves induced by a particle plunging into a Kerr black hole and study the excitation of multiple quasi-normal (QN) modes. We find that the start time of ringdown is before the strain peak of the signal and corresponds to the time when the particle passes the photon sphere. When the black hole has the near-extremal rotation, the Kerr QN frequencies are close to the fermionic Matsubara frequencies with the Hawking temperature and the chemical potential of the superradiant frequency. We indeed find that the absolute square of the spectral amplitude apparently follows the Fermi-Dirac distribution with the chemical potential of around the real QN frequency of the fundamental mode. Fitting the Boltzmann distribution to the data in higher frequencies, the best-fit temperature is found out to be close to the Hawking temperature, especially for rapid rotations. In the near-extremal limit, the gravitational-wave spectrum exhibits a would-be Fermi degeneracy with the Fermi surface at the superradiant frequency $ω= μ_{\rm H}$. We show that the greybody factor, i.e., the absorption cross section of a black hole, leads to the Fermi-Dirac distribution. As the greybody factor is another no-hair quantity of black holes, this opens a new possibility that we can test general relativity by observationally searching for the Boltzmann distribution in $ω\gtrsim μ_{\rm H}$ without extracting QN modes from ringdown. We could measure the mass and angular momentum of ringing black holes and could probe the Kerr/CFT by measuring the greybody factor imprinted on the ringdown spectrum.

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arXiv2022

Evaporation of Echoing Black Holes

We compute the graybody factor and evaporation rate of a rotating black hole in the presence of a hypothetical reflective surface slightly outside the outer horizon radius, assuming that it spontaneously emits thermal radiation due to quantum-gravitational effects such as firewalls or stretched horizons. As a result of a resonance caused by a cavity between the reflective surface and angular momentum barrier, the graybody factor is subject to a modulation in the frequency space. By taking into account this effect for multiangular modes of neutrinos, photons, and gravitons, we numerically compute the time development of the mass and angular momentum of the black hole, and show that the excited reflective surface shortens the lifetime of quantum black holes.

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arXiv2022

Global study of the scalar quasi-normal modes of Kerr-AdS${}_5$ black holes: Stability, thermality, and horizon area quantization

We numerically explore the structure of quasi-normal (QN) frequencies of the five-dimensional small and large Kerr-anti de Sitter (Kerr-AdS${}_5$) black hole with equal and unequal rotations. Our investigation also covers low and high Hawking temperatures. We then study the stability of the Kerr-AdS${}_5$ black hole and the structure of highly damped QN modes, which would reflect the thermodynamic property of the Kerr-AdS${}_5$ black hole. We find that the highly damped complex QN frequencies of a nearly maximally spinning Kerr-AdS${}_5$ black hole have the periodic separation of the surface gravity at the horizon in the imaginary part while the real part converges to the superradiant frequency, which may be relevant to the pole structure of the thermal Green's function in the corresponding conformal field theory on the Kerr-AdS${}_5$ boundary. Finally, we discuss a relation between the QN modes of the Kerr-AdS${}_5$ black hole and the Hod's conjecture on the horizon area quantization along with the analysis of the horizon topology of the Kerr-AdS${}_5$ black hole. We show that in general, an ultra-spinning Kerr-AdS${}_5$ black hole, whose spin parameter is infinitesimally close to the AdS curvature radius, has its non-compact horizon, and based on the Hod's conjecture, we argue that the horizon area may be continuous, that is, the unit area of the horizon vanishes in the ultra-spinning regime.

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arXiv2022

Vacuum decay in the Lorentzian path integral

We apply the Lorentzian path integral to the decay of a false vacuum and estimate the false-vacuum decay rate. To make the Lorentzian path integral convergent, the deformation of an integral contour is performed by following the Picard-Lefschetz theory. We show that the nucleation rate of a critical bubble, for which the corresponding bounce action is extremized, has the same exponent as the Euclidean approach. We also extend our computation to the nucleation of a bubble larger or smaller than the critical one to which the Euclidean formalism is not applicable.

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arXiv2021

Black hole evaporation in de Sitter space

We investigate the evaporation process of a Kerr-de Sitter black hole with the Unruh-Hawking-like vacuum state, which is a realistic vacuum state modelling the evaporation process of a black hole originating from gravitational collapse. We also compute the greybody factors for gravitons, photons, and conformal-coupling massless scalar particles by using the analytic solutions of the Teukolsky equation in the Kerr-de Sitter background. It turns out that the cosmological constant quenches the amplification factor and it approaches to zero towards the critical point where the Nariai and extremal limits merge together. We confirm that even near the critical point, the superradiance of gravitons is more significant than that of photons and scalar particles. Angular momentum is carried out by particles several times faster than the mass energy decreases. This means that a Kerr-de Sitter black hole rapidly spins down to a nearly Schwarzschild-de Sitter black hole before it completely evaporates. We also compute the time evolution of the Bekenstein-Hawking entropy. The total entropy of the Kerr-de Sitter black hole and cosmological horizon increases with time, which is consistent with the generalized second law of thermodynamics.

gr-qc↗

arXiv2021

On the ease of excitation of black hole ringing: Quantifying the importance of overtones by the excitation factors

The excitation factors of black hole quasinormal modes quantify the $\textit{ease of excitation}$ of the quasinormal modes and are independent of the source of perturbation. We compute the excitation factors of Kerr black holes up to the 20th overtone and find that the 4th, 5th, and 6th overtones have the first three highest excitation factors for intermediate and high spin parameters. This provides an independent confirmation of the importance of overtones that has been confirmed by the fitting data analysis of numerical relativity waveforms beginning around the strain peak amplitude.

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