Hawking-like radiation does not require a trapped region

TL;DR

This paper demonstrates that analogue Hawking radiation can occur without a trapped region, requiring only a rapidly changing flow approaching sonic conditions, challenging previous assumptions about the necessity of trapping horizons.

Contribution

It introduces simple geometric acoustic models showing that stationary Hawking-like radiation does not require a trapped region, expanding the understanding of analogue black hole experiments.

Findings

Hawking-like radiation can be produced without a trapped horizon.

Rapidly changing flows approaching sonic regimes suffice for radiation.

Stationary and Planckian emission is achievable without trapping regions.

Abstract

We discuss the issue of quasi-particle production by ``analogue black holes'' with particular attention to the possibility of reproducing Hawking radiation in a laboratory. By constructing simple geometric acoustic models, we obtain a somewhat unexpected result: We show that in order to obtain a stationary and Planckian emission of quasi-particles, it is not necessary to create a trapped region in the acoustic spacetime (corresponding to a supersonic regime in the fluid flow). It is sufficient to set up a dynamically changing flow asymptotically approaching a sonic regime with sufficient rapidity in laboratory time.