Collision of two black holes billions of years ago puzzles scientists today; here’s why

These range from hundreds of thousands of times that of our sun, to billions. (Reuters image)

A collision between two black holes, billions of years ago, sent gravitational waves rippling through the universe. In 2019 the gravitational wave observatory LIGO (US) and the Virgo detector (Italy) detected signals from these waves. But what fascinated scientists is the mass of one of the black holes in the parent, which defies conventional knowledge of how black holes are created.

It was a signal out of a gravitational wave, a relatively new discovery area. Gravitational waves are invisible ripples that form when a star in a supernova explodes; when two major stars circle each other; and when two black holes merge. Gravitational waves, moving at the speed of light, squeeze and stretch everything including space-time in their way.

As mentioned by the LIGO Collaboration, the signal detected at LIGO and Virgo resembled “about four short wiggles,” and lasted less than a tenth of a second. Subsequent research indicated that a fusion of two black holes had most likely formed GW190521. The signal possibly reflected the moment the two merged. It was believed to have originated from a distance of about 17 billion light years, and from a time when the universe was around half of its age.

But those results have given rise to more questions. One of the two merging black holes falls beyond the range of “intermediate-mass” — a misfit that can not be explained by conventional knowledge of how black holes shape.

All the black holes that have been found so far belong to this group. One group varies from a few solar masses (one solar mass is our Sun’s mass), to tens of solar masses. When massive stars die they are believed to be formed.

The other type of black holes is supermassive. These range from hundreds of thousands of times that of our sun, to billions. According to conventional understanding, stars which could give birth to black holes between 65 and 120 solar masses do not do so — stars in this range blow apart when they die, without falling into a black hole.

But the larger black hole in the merger leading to the GW190521 signal was 85 solar masses — well within this unexplained range, known as the pair-instability mass difference. It is the first black hole ever found for “intermediate-mass.”