The emission of thermal radiation by black holes, discovered by Hawking, is now a broadly
accepted and major prediction of quantum
huge redshift that occurs near the horizon connects the low energy semiclassical physics to
some unknown high energy physics where the assumption of a free field on a smooth manifold can certainly be questioned. Hence, the Hawking process seems to depend on the ultraviolet
structure of gravity to be fully consistent.
In 1981, Unruh showed that sound waves in a moving fluid propagate exactly like the radiation
field in a curved geometry. When the velocity of the
fluid crosses the speed of sound, it behaves much like a horizon. This opens the possibility to mimic black hole geometries in
condensed matter systems.
To describe Hawking radiation in such fluids, one is forced to take into account the departure
from Lorentz invariance at short wavelengths. We establish under which conditions the Hawking
process is recovered in black hole flows, and what replaces the relativistic notion of horizon. In
white hole flows, despite the similarity of the scattering problem, a specific and new physical
phenomenon occurs. The large amplification of low frequencies through the Hawking effect
leads to the emission of a large, classical wave of zero frequency but nite wavelength. We
present the properties of this wave, and its birth, when it is triggered by quantum or thermal fluctuations.