The announcement of two black holes merging was made in 2019. This one was pretty weird, unlike the dozens of such mergers seen before. The signal received indicated that the two component black holes were much larger than the Sun. One idea is that the two were circling each other on elliptical paths before they collided, but that is weird.
How do we explain the strange nature of this event? Some scientists think that the two black holes were not alone. They were surrounded by many, many other black holes, and were in the enormous disk of material surrounding the larger black hole.
A huge black hole in a large group of smaller black holes is a cool idea. It might explain what happened.
The event was detected by the LIGO/Virgo collaboration on May 21, 2019. The ripples in spacetime that are caused by the collision of two black holes are similar to waves moving through a sheet. There is a lot of information about the black holes in those waves. Scientists create models of black holes mergers and try to match the simulation events to the ones actually seen, and then figure out what the black holes from there are.
It is best fit by two black holes with at least 60 times the Sun's mass. Black holes should have less than half their mass. If they were isolated in space it would be nearly impossible.
The Zwicky Transient Facility observed a flash of light in the sky that could have been caused by the merger. Black hole mergers emit no light, but that depends on the environment.
There is a black hole with millions or billions of times the Sun's mass in the center of every big galaxy, surrounded by a huge disk of material that feeds the black hole. There could be a lot of smaller black holes in a typical black hole disk.
What can happen is changed by this. Black holes can grow by eating material around them. The black holes can grow with so many disks colliding.
Two black holes that approach each other in the same disk can fall into a circle around one another. In the empty space between stars, the material in the disk can absorb some of the motion of the two black holes, allowing them to become a system.
The gravity of the third can give a big kick to the pair if they approach another black hole. If they get close enough, they can collide, like the one seen in the movie "Gravity".
A team of scientists created a simulation of what would happen if a lot of stellar-mass black holes were embedded in a huge disk of a black hole. They found that this scenario works well. It's pretty impressive that the odds of an eccentric collision go up by a factor of one hundred.
There is also the issue of spin. The same thing that causes ice skaters to spin more rapidly when they draw their arms in is what causes black holes to spin. In some black hole mergers, the axis of spin of the two individual black holes point in the same direction, and are 90 degrees from the plane of their equator. The black hole spin axes were tipped over more than once.
That could be because of a third black hole. The two black holes were still spinning in the same direction, even though the attacker tipped the binaries.
Being consistent with observations isn't proof, but I want to be clear and say that this may very well explain GW190521, but being consistent with observations isn't proof. We don't know if there is more physics going on here. Simulations can reproduce many characteristics of an event.
The Universe may use this avenue to make most of the mergers we see because having black holes in the disk around a supermassive one is so efficient at merging them. A black hole can take billions of years to be merged, but in a disk that can be sped up considerably. It may be very common.
I was surprised when I first learned of it. It makes me think that pulling even more weird stuff out of a hat is the least surprising thing black holes can do.