Hawking-like radiation does not require a trapped region

We discuss the issue of quasiparticle production by ‘‘analogue black holes’’ with particular attention paid 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 quasiparticles, 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. This result is generic to curved-space quantum field theory, the ‘‘analogue spacetimes’’ we consider providing a guide to physical intuition, and a possible route to laboratory experiments.