Hawking Radiation from Acoustic Black Holes, Short Distance and Back-Reaction Effects

TL;DR

This paper explores the analogy between sound propagation in fluids and black hole physics, analyzing Hawking radiation, short-distance dispersive effects, and backreaction phenomena in acoustic black holes.

Contribution

It extends the analogy of acoustic black holes to include quantum effects, short-distance modifications, and backreaction, providing new insights into Hawking radiation in fluid systems.

Findings

Acoustic black holes emit Hawking-like thermal phonons.

Dispersive effects modify Hawking radiation near the atomic scale.

Backreaction of sound waves influences the fluid flow over time.

Abstract

Using the action principle we first review how linear density perturbations (sound waves) in an Eulerian fluid obey a relativistic equation: the d'Alembert equation. This analogy between propagation of sound and that of a massless scalar field in a Lorentzian metric also applies to non-homogeneous flows. In these cases, sound waves effectively propagate in a curved four-dimensional ''acoustic'' metric whose properties are determined by the flow. Using this analogy, we consider regular flows which become supersonic, and show that the acoustic metric behaves like that of a black hole. The analogy is so good that, when considering quantum mechanics, acoustic black holes should produce a thermal flux of Hawking phonons. We then focus on two interesting questions related to Hawking radiation which are not fully understood in the context of gravitational black holes due to the lack of a theory of quantum gravity. The first concerns the calculation of the modifications of Hawking radiation which are induced by dispersive effects at short distances, i.e., approaching the atomic scale when considering sound. We generalize existing treatments and calculate the modifications caused by the propagation near the black hole horizon. The second question concerns backreaction effects. We return to the Eulerian action, compute second order effects, and show that the backreaction of sound waves on the fluid's flow can be expressed in terms of their stress-energy tensor. Using this result in the context of Hawking radiation, we compute the secular effect on the background flow.