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.
as written by the authors arXiv:2606.25565
This work explores the application of acoustic black holes as a novel class of lenses in Galactic microlensing, potentially representing dark matter halo objects. While sharing key features like an event horizon, their underlying fluid-dynamical description differs from the vacuum solutions of Einstein's equations, suggesting potentially distinct observational signatures. This work investigates these distinctions by calculating the galactic microlensing predictions for acoustic black holes and comparing them to the standard Schwarzschild case. Using the observed parameters of known black hole candidates (Cygnus X-1, A0620-00, GRO J1655-40) as illustrative lenses, we demonstrate that the acoustic black holes tuning parameter $$\xi $$ significantly alters key observables. Our results show that an increase in $$\xi $$ leads to a larger Einstein ring radius, a longer event duration, and a higher peak magnification in the microlensing Paczy\'{n}ski light curves. Furthermore, we find that the microlensing event rate is enhanced for acoustic black holes compared to their Schwarzschild counterparts, with the probability of detection growing with $$\xi$$. These findings establish galactic microlensing as a promising astrophysical channel for constraining analogue gravity metrics, with the primary effects being potentially detectable in the statistical analysis of current and future microlensing survey data.
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.