Acoustic Hawking radiation from an evolving horizon in a dynamical analogue spacetime

Abstract

Our knowledge of dynamical black holes suffers from a lack of observational insight. In an analogue model of gravity, we can design a longitudinally symmetric dynamical acoustic black hole with a moving horizon. In this symmetric spacetime, the marginally outer trapped surface and the so called evolving horizon are degenerate. Interestingly, there are many ways of assigning a surface gravity to the horizon in the absence of time translation invariance. Here, we present two of them that are distinguished by whether they are defined only locally or take into account the global properties of spacetime outside the black hole. It is expected that a dynamical black hole would emit spontaneous thermal radiation and its temperature would be proportional to a surface gravity of the moving horizon but there is no consensus on which of the surface gravities would give the Hawking temperature. We propose that a non-stationary analogue spacetime when realised experimentally can possibly help resolve these mysteries and provide the first observational signatures of dynamical Hawking radiation.