BLACK HOLE BILLIARDS

Around these behemoths, smaller black holes weirdly collide

- Meghan Bartels

Take three black holes and throw them into the disc surrounding a supermassive black hole and things get weird. That’s the conclusion of new research digging into a particularly strange gravitational-wave event that scientists observed in May 2019. Gravitational waves are the ripples in space-time caused by, among other dramatic events, the mergers of black holes. But this particular observation didn’t match other collisions scientists have caught: it resulted in a black hole in the mid-size range that scientists can barely see, much less explain, and some force was stretching the typically circular dance as the behemoths approached each other.

“The gravitational-wave event GW190521 is the most surprising discovery to date. The black holes’ masses and spins were already surprising, but even more surprising was that they appeared not to have a circular orbit leading up to the merger,” said Imre Bartos, a physicist at the University of Florida and coauthor of the new research. Astronomers name gravitational-wave signals with the date they were observed, so GW190521 marks a gravitational wave detected on 21 May 2019.

Even in the earliest analysis of the strange signal, scientists suspected that the merger occurred in a pocket of space rich with black holes. Astronomers know of two types of black holes. Stellar black holes form from dying stars and contain perhaps a dozen times the mass of our Sun. Supermassive black holes, in contrast, hide at the centres of some galaxies, including our Milky Way, and can contain millions of times the mass of their puny counterparts.