Superradiant Instability Research Articles

Superradiant instabilities for a charged black hole in de Rham-Gabadadze-Tolley theory

In this paper, we study the superradiant instabilities for the charged furry black hole in massive gravity. In de Rham-Gabadadze-Tolley (dRGT) theory, the St\uckelberg field may provide St\uckelberg hair for the black hole solutions. To make the furry most obvious in large scale, we only consider to the next order of $1/{r}^{2}$ in the metric function. According to the horizon, we define two new conservation charge as the hair of black hole: one is positive charge ${k}_{+}$; the other is negative charge ${k}_{\ensuremath{-}}$. Both of them are known as St\uckelberg charge. Using a mirrorlike boundary condition, we show that there are superradiant instabilities for both types of black holes. As the St\uckelberg charge increases, the central black hole becomes more stable. The reason for this behavior is caused by the impact of St\uckelberg charge on the horizon. We also plot the parameter space for stabilities of two types black holes. As a matter of fact, the various configuration of St\uckelberg field led to this new feature in the dRGT theory.

Superradiant instability of black resonators and geons

Black resonators and geons in global AdS are rapidly rotating, low-energy solutions with a helical Killing field. We study the linear mode stability of equal angular momenta, five-dimensional black resonators and geons under scalar, electromagnetic, and gravitational perturbations. We find that black resonators are unstable to the superradiant instability, in agreement with previously known results. Perhaps surprisingly, many geons appear linearly stable, despite having an ergoregion. This apparent stability implies that geons are important long-lived, low-energy states in the dual gauge theory. However, we do find that geons are unstable within a certain range of parameter space. We comment on the nature of this instability and to its possible endpoints. We also report on new non-spinning oscillating geons, which we construct within a cohomogeneity two ansatz. Given the existing arguments that suggest our linear stability results may be extended nonlinearly, our findings indicate that most geons are generic and long-lived solutions.

Black holes with synchronised Proca hair: linear clouds and fundamental non-linear solutions

Recent studies have made key progress on the black hole/solitonic solutions of the Einstein-Proca system. Firstly, fully non-linear dynamical evolutions of the Kerr black hole superradiant instability, triggered by a Proca field, have shown the formation of a new equilibrium state, a spinning black hole with synchronised Proca hair. Secondly, non-linear evolutions of spinning Proca stars have established that they are dynamically stable, unlike their scalar cousins. Thirdly, separability of the Proca equation on the Kerr background has been achieved. Motivated by these results, in this paper we reconsider Kerr black holes with synchronised Proca hair. The separability of the Proca equation on the Kerr background allows us to examine the stationary Proca clouds in greater detail, in particular their dependence on the different quantum numbers. These stationary clouds occur at a set of existence lines in the Kerr parameter space, from which the black holes with synchronised Proca hair bifurcate. We construct the domain of existence of these black holes, comparing the fundamental states missed in the original study with the first excited states and with the cousin scalar model, giving illustrative examples of Kerr-like and non- Kerr-like BHs. In the vanishing event horizon limit, these hairy black holes connect to the fundamental states of spinning Proca stars, which include the dynamically stable solutions.

Evolution of black hole shadows from superradiance

Black holes have turned into cosmic laboratories to search for ultra-light scalars by virtue of the superradiant instability. In this paper we present a detailed study of the impact of the superradiant evolution on the black hole shadow and investigate the exciting possibility to explore it with future observations of Very Long Baseline Interferometry. We simulated the superradiant evolution numerically, in the adiabatic regime, and derived analytic approximations modelling the process. Driven by superradiance, we evolve the black hole shadow diameter and (i) find that it can change by a few $\mu$as, just below the current resolution of the Event Horizon Telescope, albeit on timescales that are longer than realistic observation times; (ii) show that the shadow diameter can either shrink or grow; and (iii) explore in detail how the shadow's end state is determined by the initial parameters and coupling.

Superradiant instability and asymptotically AdS hairy black holes in F(R)-charged scalar field theory

We study the phenomena of superradiance for $F(R)$-Maxwell black holes in an AdS space-time. The AdS boundary plays the role of a mirror and provides a natural confining system that makes the superradiant waves bouncing back and forth between the region near the horizon and the reflective boundary, causing a possible superradiant instability. We obtain numerical solutions for static hairy black holes in this scenario and investigate their instability and explicitly address the stability of such solutions for spherical perturbations under specific conditions for the scalar charge and AdS radius. It is shown that for a small scalar charge or AdS radius the static hairy solution is stable under spherical perturbations. We conclude that under such conditions, new hairy black holes emerge as a possible endpoint of superradiant instability of the system.

Black Hole Superradiant Instability from Ultralight Spin-2 Fields.

Ultralight bosonic fields are compelling dark-matter candidates and arise in a variety of beyond standard model scenarios. These fields can tap energy and angular momentum from spinning black holes through superradiant instabilities, during which a macroscopic bosonic condensate develops around the black hole. Striking features of this phenomenon include gaps in the spin-mass distribution of astrophysical black holes and a continuous gravitational-wave (GW) signal emitted by the condensate. So far these processes have been studied in great detail for scalar fields and, more recently, for vector fields. Here we take an important step forward in the black hole superradiance program by computing, analytically, the instability timescale, direct GW emission, and stochastic background, in the case of massive tensor (i.e., spin-2) fields. Our analysis is valid for any black hole spin and for small boson masses. The instability of massive spin-2 fields shares some properties with the scalar and vector cases, but its phenomenology is much richer, for example, there exist multiple modes with comparable instability timescales, and the dominant GW signal is hexadecapolar rather than quadrupolar. Electromagnetic and GW observations of spinning black holes in the mass range M∈(1,10^{10}) M_{⊙} can constrain the mass of a putative spin-2 field in the range 10^{-22}≲m_{b} c^{2}/eV≲10^{-10} . For 10^{-17}≲m_{b} c^{2}/eV≲10^{-15} , the space mission LISA could detect the continuous GW signal for sources at redshift z=20, or even larger.

Superradiance of a global monopole in Reissner\u2013Nordstr\xf6m(\u2013AdS) space-time

In this article, the behavior of a charged and massive scalar field around a global monopole swallowed by a Reissner–Nordström–Anti-de Sitter (RN–AdS) black hole is investigated by considering the Klein–Gordon equation in this geometry. The superradiance phenomenon and instability behavior of the black hole against charged scalar perturbations are studied for both an RN–AdS case and also for an RN black hole surrounded by a reflective mirror, i.e., the black hole bomb case. The effects of the monopole on these cases are discussed analytically and also with the help of several graphs in detail. The monopole charge affects the superradiance threshold frequency and also effects the instability time scale for both cases. The existence of global monopole makes these black holes more stable against superradiance instability.

Thermal phase transition in F(R) -charged AdS4 scalar theory

We investigate instabilities of $F(R)$-charged ${\mathrm{AdS}}_{4}$ black holes by a massive charged scalar field in a linear perturbation regime. We study tachyonic instabilities as the near horizon scalar condensation in a model of $F(R)$ gravity with planar horizon and investigate properties of possible phase transitions. The results show that such transitions are sensitive to the first derivative of $F(R)$ with respect to $R$ in that the larger its value, the higher the critical temperature, thus resulting in a new generation of high-temperature superconductors. Also, for a certain range of parameters, $F(R)$-charged ${\mathrm{AdS}}_{4}$ black holes suffer from superradiant instability. We consider the effects of the scalar mass and charge on such instabilities and conclude that Reissner-Nordstrom (RN) black holes decay into small hairy black holes that have a charged scalar condensate floating near the horizon. It is shown that the existence of phase transition at the critical temperature leading to a hairy black hole solution emerges for $T<{T}_{c}$, while RN black holes exist for $T>{T}_{c}$. The effect of $F(R)$ on the critical temperature is subsequently investigated in the case of superradiant instability, showing that higher critical temperatures would be possible in $F(R)$ gravity. We also check the stability of hairy black holes and show that the resulting hairy solution can be considered as a possible endpoint of superradiant instability of a small charged black hole.

Gravitational collider physics

We study the imprints of new ultralight particles on the gravitational-wave signals emitted by binary black holes. Superradiant instabilities may create large clouds of scalar or vector fields around rotating black holes. The presence of a binary companion then induces transitions between different states of the cloud, which become resonantly enhanced when the orbital frequency matches the energy gap between the states. We find that the time dependence of the orbit significantly impacts the cloud's dynamics during a transition. Following an analogy with particle colliders, we introduce an S-matrix formalism to describe the evolution through multiple resonances. We show that the state of the cloud, as it approaches the merger, carries vital information about its spectrum via time-dependent finite-size effects. Moreover, due to the transfer of energy and angular momentum between the cloud and the orbit, a dephasing of the gravitational-wave signal can occur which is correlated with the positions of the resonances. Notably, for intermediate and extreme mass ratio inspirals, long-lived floating orbits are possible, as well as kicks that yield large eccentricities. Observing these effects, through the precise reconstruction of waveforms, has the potential to unravel the internal structure of the boson clouds, ultimately probing the masses and spins of new particles.

Massive vector fields in Kerr-Newman and Kerr-Sen black hole spacetimes

The superradiant instability modes of ultralight massive vector bosons are studied for weakly charged rotating black holes in Einstein-Maxwell gravity (the Kerr- Newman solution) and low-energy heterotic string theory (the Kerr-Sen black hole). We show that in both these cases, the corresponding massive vector (Proca) equations can be fully separated, exploiting the hidden symmetry present in these spacetimes. The resultant ordinary differential equations are solved numerically to find the most unstable modes of the Proca field in the two backgrounds and compared to the vacuum (Kerr black hole) case.

Resonant instability of axion cloud

The presence of sufficiently light particles in the fundamental Lagrangian could trigger instability in rotating black holes, the so-called superradiance instability. In particular, axion and axion-like-particles (ALPs) are good candidates to prompt such an instability. As a result, a high-density axion cloud forms around the black hole. The system of black holes and the axion cloud surrounding it is called a gravitational atom. Examining the evolution of this gravitational atom could lead to the discovery of an axion or introduce new constraints on their parametric space. The axion cloud becomes unstable under certain conditions when axion-photon interactions and axion self-interactions are considered. The nature of these instabilities is the parametric resonance. In this paper, we obtain an upper bound for the rate of this instability. The results show that for the simplest axion models, this instability occurs at a very low rate because, before the resonance becomes effective, self-interactions cause the axion cloud to collapse. But for some exotic models, the resonance rate could be large enough to introduce observable effects. In addition, we will show that the parametric resonance caused by self-interactions never happens at a significant level.

Localization Driven Superradiant Instability.

The prominent Dicke superradiant phase arises from coupling an ensemble of atoms to a cavity optical field when an external optical pumping exceeds a threshold strength. Here we report a prediction of the superradiant instability driven by Anderson localization, realized with a hybrid system of the Dicke and Aubry-André (DAA) model for bosons trapped in a one-dimensional (1D) quasiperiodic optical lattice and coupled to a cavity. Our central finding is that for bosons condensed in a localized phase given by the DAA model, the resonant superradiant scattering is induced, for which the critical optical pumping of the superradiant phase transition approaches zero, giving an instability driven by the Anderson localization. The superradiant phase for the DAA model with or without a mobility edge is investigated, showing that the localization driven superradiant instability is in sharp contrast to the superradiance as widely observed for a Bose-Einstein condensate in extended states, and should be insensitive to the temperature of the system. This study unveils a novel effect of localization on the Dicke superradiance, and is well accessible based on the current experiments.

Photonic black resonators and photon stars in AdS5

Rapidly rotating Myers–Perry-AdS5 (MPAdS5) black holes are shown to be unstable against rotational superradiance of a Maxwell field. From the onset of the instability, time-periodic neutral black hole solutions equipped with a nontrivial electromagnetic wave are obtained, which we call photonic black resonators. In the horizonless limit, they reduce to geon solutions which may be called photon stars. Specifically, we introduce a cohomogeneity-1 ansatz for the metric and Maxwell field and construct such solutions with an isometry group. We compute thermodynamic quantities and obtain phase diagrams. It turns out that a photonic black resonator has a higher entropy than a MPAdS5 black hole, while it also has a smaller entropy than a black resonator without the Maxwell field. This suggests what is expected for nonlinear dynamics following the Maxwell superradiant instability with the isometry.

A new type of charged black hole bomb

Black hole bombs are usually constructed by surrounding an ergoregion by a mirror. The fields propagating between the event horizon and the mirror are prevented from escaping to infinity and reflected back to the ergoregion, thus undergoing repeated superradiant scattering which leads to a linear instability. We introduce a new construction in which the field is outside the mirror and is therefore prevented from falling into the black hole but is free to escape to infinity. Provided the mirror is inside the ergoregion, it turns out that this still causes linear instabilities. This behaviour is observed on Reissner-Nordstrom and de Sitter-Reissner-Nordstrom backgrounds using numerical simulations, based on a semi-implicit discretisation on a first-order system formulation of the partial differential equations governing the evolution of the scalar field. We also perform simulations for a standard black hole bomb and for another type of contraption: a sandwich black hole bomb.

Aspects of nonlinear effect on black hole superradiance

Under some conditions, light boson fields grow exponentially around a rotating black hole, called the superradiance instability. We discuss effects of nonlinear interactions of the boson on the instability. In particular, we focus on the effect of the particle production and show that the growth of the boson cloud may be saturated much before the black hole spin is extracted by the boson cloud, while the nonlinear interactions also induce the boson emission. For application, we revisit the superradiant instability of the standard model photon, axion and hidden photon.

Charged Dirac perturbations on Reissner-Nordström–anti–de Sitter spacetimes: Quasinormal modes with Robin boundary conditions

We study charged Dirac quasinormal modes (QNMs) on Reissner-Nordstr\"om-Anti-de Sitter (RN-AdS) black holes with generic Robin boundary conditions, by extending our earlier work of neutral Dirac QNMs on Schwarzschild-AdS black holes. We first derive the equations of motion for charged Dirac fields on a RN-AdS background. To solve these equations we impose a requirement on the Dirac field: that its energy flux should vanish at asymptotic infinity. A set of two Robin boundary conditions compatible with QNMs is consequently found. By employing both analytic and numeric methods, we then obtain the quasinormal spectrum for charged Dirac fields, and analyse the impact of various parameters, in particular of electric charge. An analytic calculation shows explicitly that the charge coupling between the black hole and the Dirac field does not trigger superradiant instabilities. Numeric calculations, on the other hand, show quantiatively that Dirac QNMs may change substantially due to the electric charge. Our results illustrate how vanishing energy flux boundary conditions, as a generic principle, are applicable not only to neutral but also to electrically charged fields.

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

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 ∼ 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.

Axion superradiance in rotating neutron stars

It is a well-known fact that compact gravitating objects admit bound state configurations for massive bosonic fields. In this work we describe a new class of superradiant instabilities of axion bound states in neutron star magnetospheres. The instability arises from the mixing of axion and photon modes in the magnetic field of the neutron star which extract energy from the rotating magnetosphere. Unlike for black holes, where the dissipation required for superradiance is provided by an absorptive horizon, the non-hermitian dynamics in this paper come from the resistivity in the stellar magnetosphere arising from a finite bulk conductivity. The axion field mixes with photon modes which superradiantly scatter off the magnetosphere, extracting rotational energy which is then deposited back into the axion sector leading to an instability. We derive the superradiant eigenfrequencies for the axion-photon system using quantum mechanical perturbation theory on the axion boundstate, drawing an analogy with atomic selection rules. We then compare the characteristic time scale of the instability to the spin-down measurements of pulsars which limit the allowed rate of angular momentum extraction from neutron stars.

How an induced Kerr–Newman black hole releases gravitational waves without a mini black hole explosion

Our task is to create circumstances in which the no-hair theorem of black holes no longer applies. First, we must determine if the mini black hole bomb would spontaneously occur. In all, the main end result is to try to avoid the so called black hole bomb effect, where a mini black hole explodes in a laboratory setting within say 10−16 s or so. That is, the idea would be to have a reasonably stable configuration with input laser energy, but a small mass, and to do it over hopefully 10−15, or more, times longer than the 10−16 s preevaporation life of the mini black hole. That is, a duration of say up to 10−1 s which would provide a baseline as to astrophysical modeling of a Kerr–Newman black hole.

Axion boson stars

We study novel solitonic solutions to Einstein-Klein-Gordon theory in the presence of a periodic scalar potential arising in models of axion-like particles. The potential depends on two parameters: the mass of the scalar field ma and the decay constant fa; the standard case of the QCD axion is recovered when ma ∝ 1/fa. When fa → ∞ the solutions reduce to the standard case of “mini” boson stars supported by a massive free scalar field. As the energy scale fa of the scalar self-interactions decreases we unveil several novel features of the solution: new stability branches emerge at high density, giving rise to very compact, radially stable, boson stars. Some of the most compact configurations acquire a photon sphere. When fa is at the GUT scale, a boson star made of QCD axions can have a mass up to ten solar masses and would be more compact than a neutron star. Gravitational-wave searches for these exotic compact objects might provide indirect evidence for ultralight axion-like particles in a region not excluded by the black-hole superradiant instability.