The light of a distant star briefly brightens when a massive invisible object passes between it and a telescope. This phenomenon, gravitational microlensing, acts as a natural telescope. Usually, the lens is a black hole, warping the spacetime around it.
But what if the lens is not an ordinary black hole but its acoustic counterpart? An acoustic black hole forms in a fluid when its flow accelerates beyond the speed of sound. Sound waves cannot escape such a region — a horizon emerges, similar to an event horizon. In the new study, the authors tested how such an unusual lens would affect starlight. They took the parameters of real black hole candidates (Cygnus X-1, A0620-00, and GRO J1655-40) and calculated what astronomers would see.
The main result: such events would occur more often and look different. Microlensing statistics collected in sky surveys could reveal the presence of acoustic black holes. This is not just theory: it is a concrete way to find traces of 'fluid' gravity in the dark matter that fills galaxy halos.
Acoustic black holes can be created in the lab — in a water tank where the flow accelerates through a narrow opening. So far, this is the only way to 'touch' an event horizon.
In Peter Watts's novel Blindsight, alien life forms use acoustic effects in a fluid medium — much like the lenses described here.