Black holes: new phenomenon “defies our understanding”

In the past it has had several black holes found. Many of them pose a mystery to the investigation. At least when it came to the emergence of binary systems and their merging, one was pretty sure, until now. A discovery turned common theories upside down, so a new explanation was necessary.

Black Holes: Mystery of GW190521
Not only single black holes have been found, but also pairs that have merged with each other. This happens when two of them orbit very close and attract each other. Previously, two massive stars had to have died, already forming a binary system, allowing the resulting black holes to be close to each other.

But every once in a while there are findings that, according to a new study, “challenge” previous understanding. A merger called GW190521 caused a lot of headaches. The signals received from the event did not fit any existing model. They were also two fairly massive black holes, roughly 50 and 80 times the Sun, respectively.

Black holes formed after supernovae typically have a mass “only” 15 times that of the Sun. The immense sizes of GW190521 suggest that each was the result of earlier mergers in its own right. But that, in turn, makes it virtually impossible for them to orbit each other as parts of a common binary system.

By the way: Sometimes a black hole “shines” by its absence. In any case, the researchers are intrigued by a discovery in which they found something completely different.

Chance Encounters of Black Holes
To find an explanation, the experts modeled various approaches and compared them with signals received from GW190521. This led to an explanation that is believed to be the most likely: in the infinite expanse of the universe, two large black holes crossed paths by chance.

As a result, they were attracted to each other and ended up colliding with each other. This may sound very unlikely at first. But since both specimens are likely to have formed from earlier mergers, there’s a chance this happened in a corner of space with a high density of dying massive stars.

Source: “GW190521 as a dynamic capture of two non-rotating black holes” (Nature Astronomy, 2022)