Black hole ‘carnivals’ could produce the signals seen by gravitational wave detectors

Since 2015, the LIGO-Virgo-KAGRA collaboration has detected around 85 pairs of colliding black holes. We now know that Einstein was right: gravitational waves are generated by these systems as they wind around each other, warping spacetime with their colossal masses as they go. We also know that these cosmic collisions occur frequently: as the detector’s sensitivity improves, we expect to detect these events on an almost daily basis during the next observing campaign, starting in 2023. we don’t know yet, that’s what causes them. collisions occur.

Black holes form when massive stars die. Typically, this death is violent, an extreme burst of energy that would destroy or knock back nearby objects. It is therefore difficult to form two black holes close enough to merge at the age of the universe. One way to merge them is to push them together in densely populated environments, like the centers of star clusters.

In star clusters, black holes that start very far apart can be brought closer together via two mechanisms. First, there is mass segregation, which causes the most massive objects to sink toward the middle of the gravitational potential well. This means that all black holes scattered throughout the cluster should end up in the middle, forming an invisible “black core”. Second, there are dynamic interactions. If two black holes pair up in the cluster, their interactions may be influenced by the gravitational influence of nearby objects. These influences can pull orbital energy out of the binary and bring it closer together.

The mass segregation and dynamical interactions that can take place in star clusters can leave their imprint on the properties of merged binaries. A key property is the shape of the binary’s orbit just before it merges. Since mergers in star clusters can happen very quickly, orbital shapes can be quite elongated – less like the quiet, calm circle that Earth traces around the sun, and more like the squashed ellipse that the comet de Halley travels during his visits. of the solar system. When two black holes are in such an elongated orbit, their gravitational wave signal exhibits characteristic modulations and can be studied for clues as to where the two objects met.

A team of researchers and OzGrav alumni are working together to study the orbital shapes of black hole binaries. The group, led by Dr Isobel Romero-Shaw (formerly of Monash University, now based at the University of Cambridge) as well as Professors Paul Lasky and Eric Thrane of Monash University, found that some of the binaries observed by the LIGO-Virgo-KAGRA collaboration is indeed likely to have elongated orbits, indicating that they may have collided in a densely populated star cluster. Their findings indicate that a large proportion of observed binary black hole collisions – at least 35% – may have been forged in star clusters.

“I like to think of black hole binaries as dance partners,” says Dr. Romero-Shaw. “When a pair of black holes move together in isolation, they’re like a couple performing a slow waltz alone in the ballroom. It’s very controlled and careful; beautiful, but nothing unexpected. Contrasting this, the carnival-style atmosphere inside a star band, where you could have lots of different dances going on simultaneously; small and big dance groups, freestyle and lots of surprises.” Although the study results cannot tell us exactly where the observed black hole binaries are merging, they do suggest that black hole carnivals at the center of star clusters could be an important contribution.