Supermassive black holes are thought to exist at the centers of nearly all large galaxies. These enormous objects can weigh millions or even billions of times as much as the Sun, but scientists still do not fully understand how they grow to such extraordinary sizes. Pulling in (accreting) gas from their surroundings is one way they gain mass, although this process alone may be too slow to explain the largest examples. Merging with other massive black holes could provide the missing explanation.

Galaxy collisions are common throughout the universe, making it likely that their central black holes also eventually come together. As two galaxies merge, their black holes are expected to move closer, begin orbiting one another, and gradually spiral inward until they combine into a single object.

Despite this expectation, scientists still struggle to model the final stages of these mergers.…