The spiral dance of a pair of colliding black holes should take billions of years. Nevertheless, since 2016 we have caught about 10 collisions with black holes – far more than we would expect. Some processes need to be active to speed up the collision process and speed up the creation of black holes.

The problem begins before black holes form. Black holes are essentially dead relics of massive stars. As these predecessor stars age, they go through a phase in which they expand into oversized stars that are many times their original size. If two stars circle each other at this point, one is subsumed into the other, and the pair collides before it ever becomes black holes.

This suggests that each pair of large black holes must exist extremely far apart – so far apart that collisions are extremely rare. And yet such collisions are quite frequent. "We theorists like it a lot when a new puzzle arises," said Smadar Naoz, a theoretical astrophysicist at the University of California, Los Angeles. "Everyone jumps with new ideas."

So how do you get close-knit black-hole pairs that were never close-meshed oversized star pairs? One possible explanation is that two massive stars could start far apart and grow closer together if they fall into black holes. Or maybe some stars collapse without ever bloating to oversized stars, or individual black holes collide and mate.

In recent years, another idea has emerged. Under the right conditions, a third object can trigger a process that brings an object pair closer together. This three-body effect allows huge stars that are far apart to collapse first into black holes and then approach so far that they collide. And since massive stars are often present in triple systems, it is important, according to the researchers, to consider this three-body effect.

To understand how this process might work out, imagine the Earth and the Moon spinning around each other. These two move almost indefinitely around their common center of gravity – unless something bothers them.

A third object would not necessarily affect the stability of the Earth-Moon system as long as the three objects rotate in the same plane (as almost every object in the solar system).

However, objects in space are generally not limited to a single flat surface. Imagine the third object rotating at an angle around the Earth-Moon system so that the orbits are not aligned. When the angle between the orbits is large enough, gravitational effects of the third object can disturb the orbits of the earth and the moon. Their paths extend to long ellipses that pull the objects further apart before swinging much closer together. If they are closest, other effects can be used to further reduce orbit. Eventually, Earth and Moon could collide, with catastrophic consequences for both.

In the world of black holes there is this three-body process or "channel" in different flavors. The third object could be a star-shaped black hole or a massive star that has not yet collapsed. It could even be one of the most supermassive black holes found in the centers of most galaxies. In this case, two massive stars in the galactic center collapse into black holes. This pair of smaller black holes and the supermassive black hole then form a three-body system. The supermassive black hole can even trigger special effects of general relativity that increase the likelihood that the pair of smaller black holes will fuse.

"The nice thing about this channel is that the merging of the black holes is fraught with very few uncertainties," said Fabio Antonini, astrophysicist at the University of Surrey, who has published several articles on this idea. "It's just gravity, it's just momentum."

But like any of the other proposed formation channels for black hole mergers, the triple process contains pieces that the researchers still need to figure out. For example, it is unclear how often the orbits in three-star systems are angled enough to trigger the effect.

A key advantage of this idea is that it can be tested. Black holes that merge through the triple process should have orbits that are less circular or more eccentric than those of black holes that merge from an undistorted binary system. Scientists may be able to measure the eccentricity of black hole tracks in the near future, said Daniel Holz, astrophysicist at the University of Chicago and member of the LIGO collaboration, looking for gravitational waves resulting from collisions with black holes.

"Part of what makes this exciting is that you may have systems with, for example, high eccentricity," said Holz, who does not investigate the triple-system process. "And if that's something that you could measure, it would be a kind of smoking weapon with something special in it."

The rotation of black holes can also tell scientists whether a black hole fusion has occurred due to a triple-system process. If a black hole binary system has formed through the evolution of two stars without the influence of other bodies, they should rotate and revolve approximately in the same direction – like two skaters turning clockwise as they move clockwise around each other. According to the work of astrophysicists such as Dong Lai and Bin Liu of Cornell University, interference from other objects, such as a third body in a triple system, can tilt the black hole's orbits so that their orbital and rotational axes are at an angle to each other. The effect is difficult to directly measure with current technology, but researchers hope to find clever new ways to cope with these rotational orientations.

While it's too early to say exactly how close black holes are to fusion, researchers see the problem as an example of why gravitational wave detection is so important. "They do not just want to do gravitational wave observations," said Ilya Mandel, astrophysicist at Monash University in Australia. "They want to use them as probes to study things that are otherwise difficult to understand and directly measure."

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