When an incident wave scatters o of an obstacle, it is partially reflected and partially transmitted. In theory, if the obstacle is rotating, waves can be amplified in the process, extracting energy from the scatterer. Here we describe in detail the first laboratory detection of this phenomenon, known as superradiance [1][2][3][4] . We observed that waves propagating on the surface of water can be amplified after being scattered by a draining vortex. The maximum amplification measured was 14% ± 8%, obtained for 3.70 Hz waves, in a 6.25-cm-deep fluid, consistent with the superradiant scattering caused by rapid rotation. We expect our experimental findings to be relevant to black-hole physics, since shallow water waves scattering on a draining fluid constitute an analogue of a black hole [5][6][7][8][9][10] , as well as to hydrodynamics, due to the close relation to over-reflection instabilities [11][12][13] . In water, perturbations of the free surface manifest themselves by a small change ξ(t, x) of the water height. On a flat bottom, and in the absence of flow, linear perturbations are well described by superpositions of plane waves of definite frequency f (Hz) and wavevector k (rad m −1 ). When surface waves propagate on a changing flow, the surface elevation is generally described by the sum of two contributions ξ = ξ I + ξ S , where ξ I is the incident wave produced by a source, for example, a wave generator, while ξ S is the scattered wave, generated by the interaction between the incident wave and the background flow. In this work, we are interested on the properties of this scattering on a draining vortex flow which is assumed to be axisymmetric and stationary. At the free surface, the velocity field is given in cylindrical coordinates by v = v r e r + v θ e θ + v z e z .Due to the symmetry, it is appropriate to describe ξ I and ξ S using polar coordinates (r, θ). Any wave ξ(t, r, θ ) can be decomposed into partial waves 10,14 ,where m ∈ Z is the azimuthal wavenumber and ϕ f ,m (r) denotes the radial part of the wave. Each component of this decomposition has a fixed angular momentum proportional to m, instead of a fixed wavevector k. (To simplify notation, we drop the indices f ,m in the following.) Since the background is stationary and axisymmetric, waves with different f and m propagate independently. Far from the centre of the vortex, the flow is very slow, and the radial part ϕ(r) becomes a sum of oscillatory solutions,where k = ||k|| 2 is the wavevector norm. This describes the superposition of an inward wave of (complex) amplitude A in propagating towards the vortex, and an outward wave propagating away from it with amplitude A out . These coefficients are not independent. The A in values, one for each f and m component, are fixed by the incident part ξ I . If the incident wave is a plane wave ξ = ξ 0 e −2iπft+ik·x , then the partial amplitudes are given by A in = ξ 0 e imπ+iπ/4 / √ 2πk. In other words, a plane wave is a superposition containing all azimuthal waves, something that we have exploited...

Quasinormal modes are a set of damped resonances that describe how an excited open system is driven back to equilibrium. In gravitational physics these modes characterize the ringdown of a perturbed black hole, e.g., following a binary black hole merger. A careful analysis of the ringdown spectrum reveals the properties of the black hole, such as its angular momentum and mass. In more complex gravitational systems, the spectrum might depend on more parameters and hence allows us to search for new physics. We present a hydrodynamic analog of a rotating black hole that illustrates how the presence of extra structure affects the quasinormal mode spectrum. The analogy is obtained by considering wave scattering on a draining bathtub vortex flow. We show that due to vorticity of the background flow, the resulting field theory corresponds to a scalar field on an effective curved spacetime which acquires a local mass in the vortex core. The obtained quasinormal mode spectrum exhibits long-lived trapped modes, commonly known as quasibound states. Our findings can be tested in future experiments building upon recent successful implementations of analog rotating black holes.

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Black holes are like bells; once perturbed they will relax through the emission of characteristic waves. The frequency spectrum of these waves is independent of the initial perturbation and, hence, can be thought of as a 'fingerprint' of the black hole. Since the 1970s scientists have considered the possibility of using these characteristic modes of oscillation to identify astrophysical black holes. Inspired by the black hole-fluid analogy, we demonstrate the universality of the black-hole relaxation process through the observation of characteristic modes emitted by a hydrodynamical vortex flow. The characteristic frequency spectrum is measured and agrees with theoretical predictions obtained using techniques developed for astrophysical black holes. Our findings allow for the first identification of a hydrodynamical vortex flow through its characteristic waves. The flow velocities inferred from the observed spectrum agree with a direct flow measurement. Our approach establishes a noninvasive method, applicable to vortex flows in fluids and superfluids alike, to identify the wavecurrent interactions and hence the effective field theories describing such systems.

Spectroscopy is a fundamental tool in science which consists in studying the response of a system as a function of frequency. Among its many applications in Physics, Biology, Chemistry and other fields, the possibility of identifying objects and structures through their emission spectra is remarkable and incredibly useful. In this paper we apply the spectroscopy idea to a numerically simulated hydrodynamical flow, with the goal of developing a new, non-invasive flow measurement technique. Our focus lies on an irrotational draining vortex, which can be seen, under specific conditions, as the analogue of a rotating black hole (historically named a dumb hole). This paper is a development of a recent experiment that suggests that irrotational vortices and rotating black holes share a common relaxation process, known as the ringdown phase. We apply techniques borrowed from black hole physics to identify vortex flows from their characteristic spectrum emitted during this ringdown phase. We believe that this technique is a new facet of the fluid-gravity analogy and constitutes a promising way to investigate experimentally vortex flows in fluids and superfluids alike. arXiv:1905.00356v1 [gr-qc] 1 May 2019Analogue Black Hole Spectroscopy; or, how to listen to dumb holes ‡ Note that we use the term sound as a generic term to describe the frequency of generic radiation such as scalar, electromagnetic or gravitational waves.

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We extend the concept of Hawking-Moss, or up-tunnelling, transitions in the early universe to include black hole seeds. The black hole greatly enhances the decay amplitude, however, order to have physically consistent results, we need to impose a new condition (automatically satisfied for the original Hawking-Moss instanton) that the cosmological horizon area should not increase during tunnelling. We motivate this conjecture physically in two ways. First, we look at the energetics of the process, using the formalism of extended black hole thermodynamics; secondly, we extend the stochastic inflationary formalism to include primordial black holes. Both of these methods give a physical substantiation of our conjecture.

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.