Quasi-steady radiation of sound from turbulent sonic ergoregions

This project adresses the topic of sonic black holes, where we investigate an acoustic black hole in two spacial dimensions in a dilute BEC. The aim of this work is to shed some light on the subject of Hawking radiation; more precisely, we want to know whether Hawking radiation is a purely astronomical phenomenon, or whether is is instead an effect of “frame dragging”, which can occur in General Relativistic gravity but also in ordinary fluid mechanics. Previous work has all focused on one-dimensional models, but there are important reasons to consider Hawking effects in at least two dimensions, even apart from the fact that real space has three dimensions.

So in the past year we finally bit the computational bullet and did some theoretical simulations of two-dimensional sonic black holes. What we found is an instability that doesn't show up in one dimension: the formation of vortices. Our sonic ergoregion became a turbulent sea of quantized whirlpools. It generated plenty of sound waves, but not the kind of sound predicted by Hawking.

One interpretation of our results would be to say that the appearance of vortices marks the end of the experiment, since these instabilities are only characteristic for fluids, and only everything up to that point counts as a black hole simulation. As long as we only have low-amplitude sound waves moving through a smooth, steady flow, the analogy between fluid and spacetime “frame dragging" still works, and Hawking's theory should still be confirmed.

The problem with this, though, is that the low-amplitude regime in which everything works is not the regime that holds mysteries. The big question is whether black hole thermodynamics survives in fully nonlinear quantum gravity? We cannot simulate quantum gravity—we don’t even know exactly what quantum gravity is—but part of the question about black hole thermodynamics is whether it depends on the precise details of quantum gravity or not. So our results may actually give some useful indications for quantum gravity research. At least superficially, our turbulent sonic black holes do seem to resemble the “fuzzball” models for black holes in string theory.