arXiv subjects

Zhan-Feng Mai

Publications and source records attributed to Zhan-Feng Mai.

32 records · Page 2Linked to original sources

arXiv2022

Quasi Normal Modes of Black Holes and Detection in Ringdown Process

Quasi-normal modes (QNMs) of a black hole (BH) are the eigen modes describing the dissipative oscillation of various fields in that spacetime, which can be intrinsically produced by the linear perturbation theory. With the discovery of the first gravitational waves (GWs) event, GW150914, a new window into the universe has been opened, allowing for the detection of QNMs associated to the ringdown process, which will enable more accurate measurements of the BHs parameters as well as further testing of general relativity. This article discusses the linear perturbation theory of BHs and provides review of several QNMs calculation methods including the newly developed methods. We will also focus on the connection between QNMs and the detection of GWs as well as some recent advancements in this area.

gr-qc↗

arXiv2022

Extremal Charged Black Holes and Superradiantly Unstable Quasinormal Modes

It was recently shown that the extremal charged black holes in STU supergravity suffer from superradiant instability owing to the existence of the unstable (low-frequency) quasibound states associated with a charged massive scalar field. In this paper, we show numerically that for some charge configurations, these black holes can also excite the (higher-frequency) superradiantly unstable quasinormal modes. We find empirically that the unstable modes are closely related to having a volcano-shaped effective potential in the Schrödinger-like wave equation.

hep-th↗

arXiv2021

Stability analysis on charged black hole with non-linear complex scalar

It has been shown recently that the charged black hole can be scalarized if Maxwell field minimally couples with a complex scalar which has nonnegative nonlinear potential. We firstly prove that such scalarization cannot be a result of continuous phase transition for general scalar potential. Furthermore, we numerically find that it is possible that the RN black hole will be scalarized by a first order phase transition spontaneously and extreme RN black hole is not stable in micro-canonical ensemble. In addition, considering a massless scalar perturbation, we compute the quasi-normal modes of the scalarized charged black hole and the results imply that the spontaneously scalarized charged black hole is not only favored in thermodynamics but also is kinetically stable against scalar perturbation at linear level. Our numerical results also definitely gives negative answer to Penrose-Gibbons conjecture and two new versions of Penrose inequality in charged case are suggested.

gr-qc↗

arXiv2021

Superradiant Instability of Extremal Black Holes in STU Supergravity

We investigate the superradiant (in)stability of the extremal multi-charge static black holes in the STU supergravity model, which reduce to the RN black hole when all the charges are equal. We first show that the frequency of quasi-bound states is necessarily complex and obtain the corresponding superradiant condition. We then study the effective potential of the Schrödinger-like equation associated with the radial function of the charged scalar field. We find that trapping-well configurations can emerge with either a single peak or double peaks. We numerically obtain the corresponding unstable quasi-bound states, organized under the overtone number, as well as how the charged black holes deviate from the RN black hole. We find that the STU black holes are superradiantly unstable as long as not all the charges are equal, indicating that the superradiant stability of the extremal RN black hole is a fine-tuning result in the framework of the STU supergravity model.

hep-th↗

arXiv2021

Dyonic Reissner-Nordstrom Black Holes and Superradiant Stability

Black holes immersed in magnetic fields are believed to be important systems in astrophysics. One interesting topic on these systems is their superradiant stability property. In the present paper, we analytically obtain the superradiantly stable regime for the asymptotically flat dyonic Reissner-Nordstrom black holes with charged massive scalar perturbation. The effective potential experienced by the scalar perturbation in the dyonic black hole background is obtained and analyzed. It is found that the dyonic black hole is superradiantly stable in the regime $0<r_{-}/r_{+}<2/3$, where $r_\pm$ are the event horizons of the dyonic black hole. Compared with the purely electrically charged Reissner-Nordstrom black hole case, our result indicates that the additional coupling of the charged scalar perturbation with the magnetic field makes the black hole and scalar perturbation system more superradiantly unstable, which provides further evidence on the instability induced by magnetic field in black hole superradiance process.

gr-qc↗

arXiv2021

Superradiantly stability analysis on dyonic stringly black hole

The black hole superradiance phenomena state that when a scalar wave perturbation is properly scattering off a charged or rotating black hole, the energy of the reflected scalar wave may be amplified. If this amplification process can occur back and forth through certain confining mechanisms, it will lead to strong instability of the black hole, which is so-called "black hole bomb". In this paper, the superradiant stability is investigated for dyonic black holes in string theory. Although the electric charged black hole in string theory has been proved superrdiantly stable, it is found that the dyonic black hole is more unstable than an RN black hole due to the magnetic charge. Furthermore, by our analysis of the effective potential outside the event horizon, we give the region of the parameter associated with the superradiance stability on dyonic stringly black hole.

gr-qc↗

arXiv2020

Superradiant instability of dyonic black holes in string theory

When a scalar wave perturbation is properly scattering off a charged or rotating black hole, the energy of the reflected scalar wave may be amplified. This is a superradiant process. If this amplification process can occur back and forth through certain confining mechanism, it will lead to strong instability of the black hole. In this paper, the superradiant stability is investigated for a special kind of dyonic black holes in string theory. Although the dynoic black hole has a similar spacetime metric with a electrically charged RN black hole, it is found that the dyonic black hole is more unstable than a RN black hole due to the coupling between magnetic charge of the dyonic black hole and the imping electrically charged scalar wave. We find two superradiantly stable regions in the parameter space for the dyonic black holes and charged massive scalar perturbation.

gr-qc↗

arXiv2019

Superradiant stability of the Kerr black holes

We study the superradiant stability of the system of a Kerr black hole and a massive scalar perturbation. It was proved previously that this system is superradiantly stable when $μ\geq \sqrt{2}mΩ_H$, where $μ$ is the proper mass of the scalar, $m$ is the azimuthal number of the scalar mode, and $Ω_H$ is the angular velocity of the Kerr black hole horizon. Our study is a complementary work of this result. We analytically prove that in the complementary parameter region $μ<\sqrt{2}mΩ_H$, when the parameters of scalar perturbation and Kerr black hole satisfy two simple inequalities, $ω<\fracμ{\sqrt{2}}$,~ $\frac{r_-}{r_+}<0.802$, the system is also superradiantly stable.

gr-qc↗

arXiv2019

Quasi-topological Electromagnetism: Dark Energy, Dyonic Black Holes, Stable Photon Spheres and Hidden Electromagnetic Duality

We introduce the quasi-topological electromagnetism which is defined to be the squared norm of the topological 4-form $F\wedge F$. A salient property is that its energy-momentum tensor is of the isotropic perfect fluid with the pressure being precisely the opposite to its energy density. It can thus provide a model for dark energy. We study its application in both black hole physics and cosmology. The quasi-topological term has no effect on the purely electric or magnetic Reissner-Nordström black holes, the dyonic solution is however completely modified. We find that the dyonic black holes can have four real horizons. For suitable parameters, the black hole can admit as many as three photon spheres, with one being stable. Another intriguing property is that although the quasi-topological term breaks the electromagnetic duality, the symmetry emerges in the on-shell action in the Wheeler-DeWitt patch. In cosmology, we demonstrate that the quasi-topological term alone is equivalent to a cosmological constant, but the model provides a mechanism for the dark energy to couple with other types of matter. We present a concrete example of the quasi-topological electromagnetism coupled to a scalar field that admits the standard FLRW cosmological solutions.

hep-th↗

arXiv2019

Holographic OPE Coefficients from AdS Black Holes with Matters

We study the OPE coefficients $c_{Δ, J}$ for heavy-light scalar four-point functions, which can be obtained holographically from the two-point function of a light scalar of some non-integer conformal dimension $Δ_L$ in an AdS black hole. We verify that the OPE coefficient $c_{d,0}=0$ for pure gravity black holes, consistent with the tracelessness of the holographic energy-momentum tensor. We then study the OPE coefficients from black holes involving matter fields. We first consider general charged AdS black holes and we give some explicit low-lying examples of the OPE coefficients. We also obtain the recursion formula for the lowest-twist OPE coefficients with at most two current operators. For integer $Δ_L$, although the OPE coefficients are not fully determined, we set up a framework to read off the coefficients $γ_{Δ,J}$ of the $\log(z\bar{z})$ terms that are associated with the anomalous dimensions of the exchange operators and obtain a general formula for $γ_{Δ,J}$. We then consider charged AdS black holes in gauged supergravity STU models in $D=5$ and $D=7$, and their higher-dimensional generalizations. The scalar fields in the STU models are conformally massless, dual to light operators with $Δ_L=d-2$. We derive the linear perturbation of such a scalar in the STU charged AdS black holes and obtain the explicit OPE coefficient $c_{d-2,0}$. Finally, we analyse the asymptotic properties of scalar hairy AdS black holes and show how $c_{d,0}$ can be nonzero with exchanging scalar operators in these backgrounds.

hep-th↗

arXiv2018

Universal Structure of Covariant Holographic Two-Point Functions In Massless Higher-Order Gravities

We consider massless higher-order gravities in general $D=d+1$ dimensions, which are Einstein gravity extended with higher-order curvature invariants in such a way that the linearized spectrum around the AdS vacua involves only the massless graviton. We derive the covariant holographic two-point functions and find that they have a universal structure. In particular, the theory-dependent overall coefficient factor $\mathcal{C}_T$ can be universally expressed by $(d-1) \mathcal{C}_T=\ell (\partial a/\partial\ell)$, where $a$ is the holographic $a$-charge and $\ell$ is the AdS radius. We verify this relation in quasi-topological Ricci polynomial, Einstein-Gauss-Bonnet, Einstein-Lovelock and Einstein cubic gravities. In $d=4$, we also find an intriguing relation between the holographic $c$ and $a$ charges, namely $c=\frac{1}{3}\ell (\partial a/\partial \ell)$, which also implies $\mathcal{C}_T=c$.

hep-th↗

arXiv2017

Bounce Universe and Black Holes from Critical Einsteinian Cubic Gravity

We show that there exists a critical point for the coupling constants in Einsteinian cubic gravity where the linearized equations on the maximally-symmetric vacuum vanish identically. We construct an exact isotropic bounce universe in the critical theory in four dimensions. The comoving time runs from minus infinity to plus infinity, yielding a smooth universe bouncing between two de Sitter vacua. In five dimensions we adopt numerical approach to construct a bounce solution, where a singularity occurred before the bounce takes place. We then construct exact anisotropic bounces that connect two isotropic de Sitter spacetimes with flat spatial sections. We further construct exact AdS black holes in the critical theory in four and five dimensions and obtain an exact AdS wormbrane in four dimensions.

hep-th↗

arXiv2017

Black Holes, Dark Wormholes and Solitons in f(T) Gravities

By choosing an appropriate vielbein basis, we obtain a class of spherically-symmetric solutions in $f(T)$ gravities. The solutions are asymptotic to Minkowski spacetimes with leading falloffs the same as those of the Schwarzschild black hole. In general, these solutions have branch-cut singularities in the middle. For appropriately chosen $f(T)$ functions, extremal black holes can also emerge. Furthermore, we obtain wormhole configurations whose spatial section is analogous to an Ellis wormhole, but $-g_{tt}$ runs from 0 to 1 as the proper radial coordinate runs from $-\infty$ to $+\infty$. Thus a signal sent from $-\infty$ to $+\infty$ through the wormhole will be infinitely red-shifted. We call such a spacetime configuration a dark wormhole. By introducing a bare cosmological constant $Λ_0$, we construct smooth solitons that are asymptotic to local AdS with an effective $Λ_{\rm eff}$. In the middle of bulk, the soliton metric behaves like the AdS of bare $Λ_0$ in global coordinates. We also embed AdS planar and Lifshitz black holes in $f(T)$ gravities. Finally we couple the Maxwell field to the $f(T)$ theories and construct electrically-charged solutions.

hep-th↗

arXiv2015

Superradiantly stable non-extremal Reissner-Nordstrom black holes

The superradiant stability is investigated for non-extremal Reissner-Nordstrom black hole. We use an algebraic method to demonstrate that all non-extremal Reissner-Nordstrom black holes are superradiantly stable against a charged massive scalar perturbation. This improves the results obtained before for non-extremal Reissner-Nordstrom black holes.

gr-qc↗