  Quantum effects of fields on curved spacetimes may be studied in the
laboratory thanks to quantum fluids. Here we use a polariton fluid to study the
Hawking effect, the correlated emission from the quantum vacuum at the acoustic
horizon. We show how out-of-equilibrium physics affects the dispersion
relation, and hence the emission and propagation of correlated waves: the fluid
properties on either side of the horizon are critical to observing the Hawking
effect. We find that emission may be optimised by supporting the phase and
density of the fluid upstream of the horizon in a regime of optical
bistability. This opens new avenues for the observation of the Hawking effect
in out-of-equilibrium systems as well as for the study of new phenomenology of
fields on curved spacetimes.
