Black holes are compact regions of space so densely packed that not even light can escape.
Until now, astronomers only had observed them in two general sizes.
There are “small” ones called stellar black holes that are formed when a star collapses and are about the size of small cities.
And there are supermassive black holes that are millions, maybe billions, of times more massive than our sun and around which entire galaxies revolve.
According to astronomers’ calculations, anything in between didn’t quite make sense, because stars that grew too big before collapse would essentially consume themselves, leaving no black holes.
Star collapses couldn’t create stellar black holes much bigger than 70 times the mass of our sun, scientists thought.
Physicist Nelson Christensen, research director of the French National Centre for Scientific Research explains the new discovery:
“Two very massive black holes merged together and emit gravitational waves as they do so. Now we’ve seen black holes merge and produce gravitational waves already but what’s interesting here is that this system is so massive.”
In May 2019, two detectors picked up a signal that turned out to be the energy from two stellar black holes – each large for a stellar black hole – crashing into each other.
One was 66 times the mass of our sun and the other 85 times the mass of the sun.
The end result: the first ever discovered intermediate black hole, at 142 times the mass of the sun.
Lost in the collision was an enormous amount of energy in the form of a gravitational wave, a ripple in space that travels at the speed of light.
It was that wave that physicists in the United States and Europe, using detectors called LIGO and Virgo, captured last year.
After deciphering the signal and checking their work, scientists published the results Wednesday in Physical Review Letters and Astrophysical Journal Letters.
Because the detectors allow scientists to pick up the gravitational waves as audio signals, scientists actually heard the collision.
For all the violence and drama, the signal lasted only one-tenth of a second and sounded like a ‘thud’.
This crash happened about 7 billion years ago, when the universe was about half its current age, but is only being detected now because it is incredibly far away.
Black hole collisions have been observed before, but the black holes involved were smaller to begin with and even after the merger didn’t grow beyond the size of typical stellar black holes.
Scientists still don’t know how supermassive black holes at the center of galaxies formed, Christensen says, but this new discovery may offer a clue.
“Super massive black holes are probably formed by the merger of other black holes and other objects. But this is like the first real point of data in helping to build that bridge between these two regions where we see black holes. And that’s why the intermediate black holes are so important.”
“We have the great breakthrough today where we’re looking towards the future already with future detectors that will tell us even more about the universe. And I just find that incredible to think that we’ve discovered so much but we’re also working hard to continue this with even more in the future” he adds.
Barnard College astronomer Janna Levin, who wasn’t part of the research, is author of the book “Black Hole Survival Guide”. She says it offers new insight into how black holes form.
“So it opens up all kinds of interesting possibilities about whether it was a supernova explosion that was unexpected and led to an unusual collapse or if they had already merged with other black holes and gotten bigger that way and it might also give us a key to understanding how super massive black holes, the ones that are millions or billions of times the mass of the sun slowly kind of hierarchically grew. So this one that’s 150 times the mass of the sun now, this remnant that was formed maybe 5 billion years ago that we’re only just discovering, by now might have merged with another black hole and gotten even bigger and we won’t find out about it for 5 billion years.”
Scientists still can’t quite explain how merged black holes, flying around the universe, would meet so many others to merge again and grow ever bigger.
It could instead be that supermassive black holes were formed in the immediate aftermath of the Big Bang.