Scientists believe that when galaxies collide and merge, the supermassive black holes at their centers should eventually pair up and orbit each other.
These enormous duos, called supermassive black hole binaries, are thought to play a major role in shaping galaxies and producing powerful gravitational waves.
Yet only a handful of widely separated pairs have been clearly observed, and tightly orbiting ones have remained hidden—until now, perhaps.
In a new study published in Physical Review Letters, researchers propose a creative way to detect these elusive systems using a cosmic effect known as gravitational lensing.
This phenomenon happens because massive objects, such as black holes, bend light with their gravity.
When light from a distant star passes near a black hole, the gravity can magnify and brighten that light, acting like a natural telescope.
Supermassive black holes sit at the centers of most galaxies. If two galaxies merge, their central black holes should eventually form a bound pair that spirals closer over time.
As they orbit each other, they release energy in the form of gravitational waves, slowly shrinking the distance between them.
Future space missions will be able to detect those waves directly, but scientists now think we may be able to spot the pairs sooner by watching for telltale flashes of light.
The key difference between a single black hole and a pair lies in how they bend light. A lone black hole can magnify a star’s light only if the alignment is almost perfect.
But a pair of black holes creates a more complex pattern that increases the chances of magnifying background stars. Together, they form a diamond-shaped region in space where stars can briefly appear much brighter.
As the two black holes orbit each other, this magnification pattern moves and changes shape. If a bright star lies behind the pair, it can flare dramatically each time the moving pattern passes over it. The result would be repeating flashes of light that appear at regular intervals. These bursts could serve as a clear signal that a supermassive black hole binary is present.
Researchers say the timing and brightness of these flashes could reveal valuable information. By studying the pattern, astronomers could estimate the masses of the black holes, how fast they are orbiting, and how quickly they are moving toward each other.
Because the orbit gradually shrinks as energy is lost through gravitational waves, the flashes would slowly change over time, offering clues about the system’s evolution.
Although a single pair might produce flashes only every few years, scientists expect that observing many galaxies could reveal multiple systems at different stages of merging. Upcoming wide-field telescopes, such as the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope, will scan huge areas of the sky and may be able to catch these repeating signals.
If successful, this method could allow astronomers to identify supermassive black hole pairs long before future gravitational-wave detectors are launched. It would also open a new way to study how galaxies grow and how gravity behaves under extreme conditions.
By turning faint flashes of starlight into cosmic clues, scientists may soon uncover hidden giants dancing in the hearts of distant galaxies.
