Black hole “billiards” may explain strange aspects of 2019 black hole merger

Enlarge / Illustration of a swarm of smaller black holes in a gas disk rotating around a giant black hole. (credit: J. Samsing/Neils Bohr Institute)

In 2019, the LIGO/VIRGO collaboration picked up a gravitational wave signal from a black hole merger that proved to be one for the record books. Dubbed "GW190521," it was the most massive and most distant yet detected, and it produced the most energetic signal detected thus far, showing up in the data as more of a "bang" than the usual "chirp."

Furthermore, the new black hole resulting from the merger was about 150 times as heavy as our Sun, making GW190521 the first direct observation of an intermediate-mass black hole. Even weirder, the two black holes that merged were locked in an elliptical (rather than circular) orbit, and their axes of spin were tipped far more than usual compared to those orbits.

Physicists love nothing more than to be presented with an intriguing puzzle that doesn't immediately seem to fit established theory, and GW190521 gave them just that. New theoretical simulations suggest that all those bizarre aspects can be explained by the presence of a third single black hole horning in on the binary system's final dance to produce a "chaotic tango," according to a new paper published in the journal Nature.

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