Acoustic black-hole bombs and scalar clouds in a photon-fluid model

Abstract

Massive bosonic fields in the background of a Kerr black hole can either trigger superradiant instabilities (black-hole bombs) or form equilibrium configurations corresponding to pure bound states, known as stationary scalar clouds. Here, similar phenomena are shown to emerge in the fluctuation dynamics of a rotating photon-fluid model. In the presence of suitable vortex flows, the density fluctuations are governed by the massive Klein-Gordon equation on a (2+1) curved spacetime, possessing an ergoregion and an event horizon. We report on superradiant instabilities originating from quasi-bound phonon states trapped by the vortex background and, remarkably, on the existence of stationary modes in synchronous rotation with the horizon. These represent the acoustic counterpart of astrophysical scalar clouds. Our system offers a promising platform for analogue gravity experiments on superradiant instabilities of massive bosons and black-hole-field equilibrium configurations.

Stationary scalar clouds supported by rapidly-rotating acoustic black holes in a photon-fluid model

“…The relation (2) implies that the co-rotating massive scalar field configurations are spatially bounded to the central black hole and cannot radiate their energy and angular momentum to infinity. Intriguingly, an analogous physical phenomenon in a rotating photon-fluid system has recently been revealed in the highly important work [9]. Photon-fluids are nonlinear optical systems whose physical and mathematical properties can be described by the hydrodynamic equations of an interacting Bose gas [9][10][11][12].…”

“…Photon-fluids are nonlinear optical systems whose physical and mathematical properties can be described by the hydrodynamic equations of an interacting Bose gas [9][10][11][12]. In particular, it has been shown [9,13] that photon-fluid systems are characterized by the presence of long-wavelength elementary excitations (phonons) that behave as massive scalar fields in an effective acoustic curved spacetime.…”

“…The dynamics of massive phonons over a draining vortex flow in the photon-fluid model has been investigated recently in [9] as the acoustic analogue of the (more familiar) dynamics of massive scalar fields in rotating curved blackhole spacetimes. In particular, it has been explicitly proved [9] that the dynamics of linearized acoustic excitations in the photon-fluid model with a draining vortex flow are governed by the familiar Klein-Gordon equation of an effective scalar field of proper mass Ω 0 that propagates in an acoustic (2 + 1)-dimensional spinning black-hole spacetime which, like the familiar Kerr black-hole spacetime, possesses an ergoregion. Intriguingly, using direct numerical techniques, it has been explicitly demonstrated in [9] that the acoustic spinning black-hole spacetime may support stationary linearized density fluctuations (acoustic scalar 'clouds') in its exterior regions.…”

“…In particular, it has been explicitly proved [9] that the dynamics of linearized acoustic excitations in the photon-fluid model with a draining vortex flow are governed by the familiar Klein-Gordon equation of an effective scalar field of proper mass Ω 0 that propagates in an acoustic (2 + 1)-dimensional spinning black-hole spacetime which, like the familiar Kerr black-hole spacetime, possesses an ergoregion. Intriguingly, using direct numerical techniques, it has been explicitly demonstrated in [9] that the acoustic spinning black-hole spacetime may support stationary linearized density fluctuations (acoustic scalar 'clouds') in its exterior regions. In particular, it has been revealed [9] that the composed acoustic-black-hole-stationary-massive-scalar-field configurations of the photon-fluid model are characterized by the narrow regime of existence [9,14]…”

“…Intriguingly, using direct numerical techniques, it has been explicitly demonstrated in [9] that the acoustic spinning black-hole spacetime may support stationary linearized density fluctuations (acoustic scalar 'clouds') in its exterior regions. In particular, it has been revealed [9] that the composed acoustic-black-hole-stationary-massive-scalar-field configurations of the photon-fluid model are characterized by the narrow regime of existence [9,14]…”