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Open AccessArticle

Superradiant (In)stability, Greybody Radiation, and Quasinormal Modes of Rotating Black Holes in Non-Linear Maxwell f(R) Gravity

by Sara Kanzi, İzzet Sakallı and Behnam Pourhassan

Symmetry 2023, 15(4), 873; https://doi.org/10.3390/sym15040873 - 6 Apr 2023

Cited by 6 | Viewed by 1972

Abstract

This work is dedicated to the investigation of the superradiant stability of a rotating black hole derived from the nonlinear Maxwell theory of gravity,

f (R)

. The evaluation of stability and instability in this study will be based on the [...] Read more.

This work is dedicated to the investigation of the superradiant stability of a rotating black hole derived from the nonlinear Maxwell theory of gravity,

f (R)

. The evaluation of stability and instability in this study will be based on the absence and presence of the magnetic field, respectively, when the magnetic field constant is

c_{4} = 0

and

c_{4} \neq 0

. For the black hole under discussion, analyses of the greybody factors (GFs) and quasi-normal modes (QNMs) are also carried out. To this end, we first consider the Klein–Gordon equation for the scalar waves propagating in the black hole’s geometry. The resulting radial equation is then reduced to a one-dimensional Schrödinger-like wave equation with effective potential energy. The effects of the nonlinear Maxwell

f (R)

gravity theory parameters (q, c, and

c_{4}

) on the effective potential, GFs, and QNMs are examined. The results demonstrate that, although the parameters q, c, and

c_{4}

all influence the effective potential, they do not affect the GFs and QNMs. All results are presented and summarized using appropriate graphics and tables. Full article

(This article belongs to the Section Physics)

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16 pages, 1043 KB

Open AccessArticle

Scalar Perturbations of Black Holes in the

f (R) = R - 2 α \sqrt{R}

Model

by Ping Li, Rui Jiang, Jian Lv and Xianghua Zhai

Universe 2022, 8(1), 47; https://doi.org/10.3390/universe8010047 - 12 Jan 2022

Cited by 2 | Viewed by 2258

Abstract

In this paper, we study the perturbations of the charged static spherically symmetric black holes in the

f (R) = R - 2 α \sqrt{R}

model by a scalar field. We analyze the quasinormal modes spectrum, superradiant modes, and superradiant instability [...] Read more.

In this paper, we study the perturbations of the charged static spherically symmetric black holes in the

f (R) = R - 2 α \sqrt{R}

model by a scalar field. We analyze the quasinormal modes spectrum, superradiant modes, and superradiant instability of the black holes. The frequency of the quasinormal modes is calculated in the frequency domain by the third-order WKB method, and in the time domain by the finite difference method. The results by the two methods are consistent and show that the black hole stabilizes quicker for larger

α

satisfying the horizon condition. We then analyze the superradiant modes when the massive charged scalar field is scattered by the black hole. The frequency of the superradiant wave satisfies

ω \in (\frac{μ}{\sqrt{2}}, ω_{c})

, where

μ

is the mass of the scalar field, and

ω_{c}

is the critical frequency of the superradiance. The amplification factor is also calculated by numerical method. Furthermore, the superradiant instability of the black hole is studied analytically, and the results show that there is no superradiant instability for such a system. Full article

(This article belongs to the Collection Nobel Prize 2020: Selected Articles on Black Hole and General Relativity)

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13 pages, 391 KB

Open AccessEditor’s ChoiceArticle

EHT Constraint on the Ultralight Scalar Hair of the M87 Supermassive Black Hole

by Pedro V. P. Cunha, Carlos A. R. Herdeiro and Eugen Radu

Universe 2019, 5(12), 220; https://doi.org/10.3390/universe5120220 - 27 Nov 2019

Cited by 121 | Viewed by 4384

Abstract

Hypothetical ultralight bosonic fields will spontaneously form macroscopic bosonic halos around Kerr black holes, via superradiance, transferring part of the mass and angular momentum of the black hole into the halo. Such a process, however, is only efficient if resonant—when the Compton wavelength of the field approximately matches the gravitational scale of the black hole. For a complex-valued field, the process can form a stationary, bosonic field black hole equilibrium state—a black hole with synchronised hair. For sufficiently massive black holes, such as the one at the centre of the M87 supergiant elliptic galaxy, the hairy black hole can be robust against its own superradiant instabilities, within a Hubble time. Studying the shadows of such scalar hairy black holes, we constrain the amount of hair which is compatible with the Event Horizon Telescope (EHT) observations of the M87 supermassive black hole, assuming the hair is a condensate of ultralight scalar particles of mass

μ \sim 10^{- 20}

eV, as to be dynamically viable. We show the EHT observations set a weak constraint, in the sense that typical hairy black holes that could develop their hair dynamically, are compatible with the observations, when taking into account the EHT error bars and the black hole mass/distance uncertainty. Full article

(This article belongs to the Special Issue Gravitational Lensing and Optical Geometry: A Centennial Perspective)

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