Narrow Results

We consider the stationary metrics that have both the black hole and the ergoregion. The class of such metric contains, in particular, the Kerr metric. We study the Cauchy problem with highly oscillatory initial data supported in a neighborhood inside the ergoregion with some initial energy $E_0$. We prove that when the time variable $x_0$ increases this solution splits into two parts: one with the negative energy $-E_1$ ending at the event horizon in a finite time, and the second part, with the energy $E_2=E_0+E_1>E_0$, escaping, under some conditions, to the infinity when $x_0\to +\infty$. Thus we get the superradiance phenomenon. In the case of the Kerr metric the superradiance phenomenon is “short-lived”, since both the solutions with positive and negative energies cross the outer event horizon in a finite time (modulo $O(\frac{1}{k})$) where $k$ is a large parameter. We show that these solutions end on the singularity ring in a finite time. We study also the case of naked singularity.

Motion of massive and massless test particle in equilibrium and nonequilibrium case is discussed in a dyadosphere geometry through Hamilton–Jacobi method. Scalar wave equation for massless particle is analyzed to show the absence of superradiance in the case of dyadosphere geometry.

We show that there is no superradiance for the Dirac field in the rotating BTZ black hole if the field vanishes at infinity. Then we outline the calculation of the expression for the renormalized energy–momentum tensor, the effective action as well as the heat kernel for the Dirac field for the BTZ black hole. Finally, we point out how to construct the Hartle–Hawking–Israel state for the real scalar field in the non-rotating BTZ black hole in two and three dimensions.

Direct detection of gravitational waves from several compact binary coalescences has ushered in a new era of astronomy. It has opened up the possibility of detecting ultralight bosons, predicted by extensions of the Standard Model, from their gravitational signatures. This is of particular interest as some of these hypothetical particles could be components of dark matter that are expected to interact very weakly with Standard Model particles, if at all, but they would gravitate as usual. Ultralight bosons can trigger superradiant instabilities of rotating black holes and form bosonic clouds that would emit gravitational waves. In this paper, we present an overview of such instabilities as gravitational wave sources and assess the ability of current and future detectors to shed light on potential dark matter candidates.

Several physical natures of charged brane-world black holes are investigated. Firstly, the timelike and null geodesics of the charged brane-world black holes are presented. We also analyze all the possible motions by plotting the effective potentials for various parameters for circular and radial geodesics. Secondly, we investigate the motion of test particles in the gravitational field of the charged brane-world black holes using the Hamilton–Jacobi formalism. We consider charged and uncharged test particles and examine their behavior in both static and nonstatic cases. Thirdly, the thermodynamics of the charged brane-world black holes are studied. Finally, it is shown that there is no phenomenon of superradiance for an incident massless scalar field for such a black hole.

In this paper, we solve the Duffin–Kemmer–Petiau (DKP) equation in the presence of hyperbolic tangent potential for spin-one particles. By partitioning the spin-one spinor, we show that the DKP equation is equivalent to the Klein–Gordon equation formalism. The scattering solutions are derived in terms of hypergeometric functions. The reflection $R$ and transmission $T$ coefficients are calculated in terms of the Gamma functions. The results show the presence of the superradiance phenomenon when $R$ for a specific region in the potential becomes greater than one.

We investigate the cooperative effects on optical forces in a system of N two level atoms confined to a volume of dimension less than λ³, where λ is radiation wavelength and driven by a coherent radiation field with a spatial profile like Laguerre–Gaussian or ideal Bessel beam. We show a dramatic enhancement on optical forces as well as on the angular momentum imparted to the atom by a factor of N².

A periodically bunched electron beam is useful for generating high-brightness electron superradiance. This paper studies the generation and acceleration of density-modulated electron beams from a photocathode electron gun driven by a laser beat wave. Computer simulation shows the feasibility of accelerating and preserving the density-modulated electron beam in an accelerator. This paper also details the implementation of a beat-wave laser system with a variable beat frequency for driving a photocathode electron gun.

We propose to use superradiant Rayleigh scattering from degenerate Bose gas to detect unknown frequencies coupled to the pump laser beam. Theoretically we show a measurement of the time evolution of population at the initial momentum state could determine the unknown frequency with respect to a known one at which the pump laser's frequency modulates. We show a range of frequencies from kHz to MHz could be determined with this method at the currently available state-of-the-art technology.

We show that scalar hair can be added to rotating, vacuum black holes (BHs) of general relativity. These hairy black holes (HBHs) clarify a lingering question concerning gravitational solitons: Whether a BH can be added at the centre of a boson star (BS), as it typically can for other solitons. We argue that it can, but only if it is spinning. The existence of such HBHs is related to the Kerr superradiant instability triggered by a massive scalar field. This connection leads to the following conjecture: a (hairless) BH, which is afflicted by the superradiant instability of a given field, must allow hairy generalizations with that field.

Massive bosons in the vicinity of Kerr–Newman black holes can form pure bound states when their phase angular velocity fulfills the synchronization condition, i.e. at the threshold of superradiance. The presence of these stationary clouds at the linear level is intimately linked to the existence of Kerr black holes with synchronized hair at the nonlinear level. These configurations are very similar to the atomic orbitals of the electron in a hydrogen atom. They can be labeled by four quantum numbers: $n$, the number of nodes in the radial direction; $\ell$, the orbital angular momentum; $j$, the total angular momentum; and $m_j$, the azimuthal total angular momentum. These synchronized configurations are solely allowed for particular values of the black holes mass, angular momentum and electric charge. Such quantization results in an existence surface in the three-dimensional parameter space of Kerr–Newman black holes. The phenomenology of stationary scalar clouds has been widely addressed over the last years. However, there is a gap in the literature concerning their vector cousins. Following the separability of the Proca equation in Kerr(–Newman) spacetime, this paper explores and compares scalar and vector stationary clouds around Kerr and Kerr–Newman black holes, extending previous research.

We investigate the dynamics of short linear chains consisting of two-level systems (atoms) coupled by the electromagnetic field. The environment of photon modes acts as a source of noise and leads to the disappearance of the initially present multipartite entanglement. The rate of this process (entanglement degradation) depends on the separation of the atoms, and also on the initial state. With the aid of the appropriate entanglement witnesses we show that this rate is exceptionally low for the so-called subradiant states. Below one resonant wavelength of atomic separation the effect of the environmental noise is weaker than the dipole-dipole interaction and multipartite entanglement can be formed in the initial stage of the time evolution.

A device — referred to as a photonic quantum heat engine — was reported in Nature Photonics [J. Kim, S. Oh, D. Yang, J. Kim, M. Lee and K. An, A photonic quantum engine driven by superradiance, Nat. Photon. 16 (2022) 707–711] with an efficiency of $98\pm 4$%. Moreover, in a related News & Views contribution in the same issue [M. Kim, M. Scully and A. Svidzinsky, A supercharged photonic quantum heat engine, Nat. Photon. 16 (2022) 669–670], this device was reported to exceed the Carnot limit, an extraordinary claim. As Carl Sagan once remarked, “Extraordinary claims require extraordinary evidence.” Here, we outline the fundamental lack of empirical evidence that would be required to support such a claim, show that the actual efficiency of the device is $\sim$ 0% and bring to attention critical aspects of the operating physics of the device.

We investigate the scattering of sound wave perturbations by vortices in trapped Bose–Einstein condensates. Using a variational approach, we show that the energy barrier between the ground state and a state containing an axis-symmetric vortex can be surpassed by scattering with density perturbations. The transfer of angular momentum from the condensate to the perturbation can be made total for suitably chosen density perturbations carrying an energy of about 10% of the ground-state energy.

Greybody factors of rotating cohomogeneity-1 black holes in higher odd dimensions are studied for the cases in which the cosmological constant is zero, positive, or negative. Attention is given to the main superradiant modes. It is shown that the increase of the intensity of the cosmological constant can have diverse effects on the maximum amplification obtained. In the case of de Sitter (dS) black holes, maximum amplification is enhanced for higher values of the cosmological constant. In the case of Anti-de Sitter (AdS) black holes, the increase of the absolute value of the cosmological constant has the effect of suppressing the maximum amplification initially, but eventually this behavior reverses and we observe growth. This phenomenon can be interpreted as contributions from amplification peaks of distinct origin that become dominant in different regimes.

In this paper we investigate the possibility of the analog of a phenomenon like superradiance, that is, amplification of a sound wave by reflection induced by a rotating acoustic black hole in the fluid "draining bathtub" model in the presence of a disclination and analyze the role played by this object in which concerns the amplification or reduction of superradiance with the deficit angle.

The following sections are included:

• Introduction

• Optical Vortex Metric and Superradiance

• Experimental Proposal and Numerical Results

• References

We discuss a method to study linear perturbations of generic rotating spacetimes in the slow-rotation limit. The framework is valid for any perturbation field and it is particularly advantageous when the field equations are not separable. Using this approach, we show that massive vector perturbations in the Kerr metric exibit strong superradiant instabilities, which put competitive constraints on the mass of the photon: m_γ ≲ 10⁻²⁰ eV.