‘Chaotic drama’ of the merging of black holes finally explained by new research

(Independent)

The “chaotic drama” that explains how black holes form and merge has been revealed in new research.

The findings help explain a mystery that has been troubling scientists for years, and could explain how black holes behave.

Scientists remain mystified by black holes, which are difficult to study in large part because they do not emit any light – the main way we are able to explore our universe. In 2015, however, the Laser Interferometer Gravitational Wave Observatory or LIGO detected gravitational waves from the merger of two black holes, finally giving a new way of looking at those enigmatic objects.

But further research using those gravitational waves has thrown up questions as well as answers. In 2019, researchers spotted an event known as GW190521, which did not appear to behave as expected.

“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 co-author Imre Bartos, from the University of Florida.

That suggested that it was relatively common that black holes move around each other in this non-circular way. But scientists did not know how those eccentric mergers actually happened.

One suggestion was that in the middle of galaxies, those merging black holes might not be the only one of their kinds. There are so many that the chaotic movements makes a neat circular orbit impossible.

“In these environments the typical velocity and density of black holes is so high that smaller black holes bounce around as in a giant game of billiards and wide circular binaries cannot exist,” said co-author Bence Kocsis from the University of Oxford.

The trouble may be that scientists were thinking in three dimensions, as is usually the case in other planetary systems. But the breakthrough came when they started considering the black holes’ movements in two dimensions – as if they were taking place on a flat table – and found that the strange behaviour actually made more sense.

That discovery not only explains the bizarre and chaotic game of billiards that happens as black holes merge. It could also offer another way of studying those black holes themselves.

“We have now shown that there can be a huge difference in the signals emitted from black holes that merge in flat, two-dimensional disks, versus those we often consider in three-dimensional stellar systems,  which tells us that we now have an extra tool that we can use to learn about how black holes are created and merge in our Universe, ” said Johan Samsing from the Niels Bohr Institute at the University of Copenhagen, who is lead author of the paper.

A paper describing the findings, ‘AGN as potential factories for eccentric black hole mergers’, is published in Nature today.