⚡ Flashes That Reveal the Presence of Binary Black Holes

The detection of supermassive black holes orbiting each other could soon move from theory to observation, thanks to a remarkable optical phenomenon. Normally invisible, these cosmic giants could reveal themselves through astonishing bright flashes coming from stars located behind them.

This mechanism relies on gravitational lensing, an effect predicted by Einstein where the gravity of a massive object curves spacetime and deflects light. This effect is usually used to observe distant galaxies, but with a binary system, it becomes more intense.


For a duo of black holes, their rotation around a common center generates a diamond-shaped zone, called a caustic curve, where the lensing effect is amplified. This region sweeps across the spatial background, and when a star aligns with it by chance, its light is briefly but strongly magnified.

These alignments therefore produce periodic bright flashes, visible over several years, that match the orbital period of the black holes. According to scientists, this signature is unique and could allow the identification of such duos, even in the hearts of extremely distant galaxies where individual stars remain too faint to be seen.

Furthermore, the orbit of these black holes is not fixed; it gradually shrinks as they dissipate energy in the form of gravitational waves. This evolution modifies the caustic curve, which alters the frequency and intensity of the bright flashes, thereby encoding data about the objects' mass.

Next-generation observatories, such as the Vera C. Rubin Observatory in Chile and the Nancy Grace Roman Space Telescope scheduled for launch in 2027, will have the required sensitivity to capture these events. In the longer term, the LISA mission, a space-based gravitational wave detector, could collaborate with these telescopes to conduct so-called "multi-messenger" studies.

This approach opens the possibility of observing binary systems well before their ultimate merger, by combining light and gravitational signals. The work that led to this breakthrough is detailed in the journal Physical Review Letters.