by Black holes are monolithic and monolithic regions of space and time that have captured the imagination of humanity. Some are created by the death and collapse of a massive star – but no single star can give birth to the most spectacular black holes. These are the supermassive black holes that lie at the heart of galaxies with masses equivalent to millions, or even billions of suns.
Supermassive black holes like this are believed to have been created when two smaller black holes collided and merged once a day. And now, scientists wonder if we can learn about the black hole’s family tree by working backwards through the generations.
Related: How do some black holes get so big? The James Webb Space Telescope may have an answer
If there is a cosmological object or event designed to be mysterious, it is the black hole. This area of space is marked by a boundary called the event horizon, the barrier between our universe and whatever lies within the void itself. Therefore, it is impossible to receive a signal and, therefore, information from outside this barrier.
In addition to this, black holes have a small identifying characteristic, which theoretical physicist John Wheeler once described by saying, “Black holes have no hair.” That means, unlike looking at a child’s hair color or complexion and making a rough guess about their parents’ hair color and finish, giant “hairless” black holes don’t seem to give any clues about their race – or do they?
Not all black holes are created equal
Imre Bartos is a physicist at the University of Florida who theorizes that the limited characteristics of supermassive black holes – mass, spin and electric charge – may obscure the details of the original black hole assembly line that created them.
The standard picture of black hole birth involving the death and collapse of massive stars, he says, cannot account for the creation of the supermassive black holes we see today. This includes supermassive black holes as well as relatively smaller medium-sized black holes with masses around 100 to 100,000 times the mass of the Sun.
“Stars have a limit to how big they can grow without essentially collapsing on their own. If a star gets really big, it will explode before it has time to form this dense core that can seed its creating a black hole,” Bartos told Space.com. “We expect a natural limit on how massive black holes can be when they form from the death of stars. This mass is like 50 times the mass of our Sun. So it’s a pretty big mass , but nowhere near as massive. We see supermassive or intermediate black holes.”
This means that some other process must exist to create the most monstrous black holes, and observations of tiny ripples in spacetime known as gravitational waves suggest that collisions and subsequent mergers may be a process of the such generations of progressively larger black holes.
“How can we do the detective work to go back in time and see what went down in the environment of merging black holes? Can we say anything about the black hole progenitors?” Bartos wondered.
Bartos thinks that understanding what causes black holes to pull together and merge is key to understanding how the universe has changed—and may even get to the bottom of the laws of fundamental physics.
“We want to learn, and until now, we haven’t fully understood what makes black holes come together and merge completely,” said Bartos. “This cannot be a random process. There must be some mechanism that brings the black holes together and merges them. And this is what we try to understand from the limited properties we have.”
He added that by looking at the mass of a supermassive black hole and gravitational waves from the merger that created it, researchers could estimate the mass of black holes that must have come together to create it in the first place. More than this, Bartos and his colleagues theorize that how fast the black hole is rotating can also show how fast the parent black holes that created it were spinning.
Observing electromagnetic signals from the region where these mergers occurred, according to Bartos, could also show how they were interacting with their environments. For example, were the black holes feeding gas and dust from their surroundings to facilitate their own growth?
Black holes have grandparents, too
Bartos also said that black holes with masses more than 50 times that of the sun could be found in regions where there are many other black holes. This means that, just as we have parents, grandparents, and great-grandparents, black hole mergers are probably not unique events, and the largest black holes come from repeated generations of mergers.
This generational state would not only lead to mass coalescing, but daughter black holes would subsequently gather angular momentum from their progenitors. Bartos said this means that as black holes get bigger and bigger, they should also spin faster and faster.
“If we find a higher spin, that can indicate that there may have been a merger before,” Bartos said. “For some of the heaviest black holes we’ve seen to date, we see evidence of the high accretion we would expect from successive mergers.”
However, while Bartos thinks the idea of black hole “grandparents” is fascinating, and believes we can reveal some details about black hole parents from gravitational observations, the story of black hole grandparents may be lost in time. .
“There is no way we can detect these previous mergers directly from gravitational wave observations. We would have to operate detectors for millions of years, which is unlikely,” he said.
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The chain of mergers that would create a supermassive black hole is believed to take at least 1 billion years, but these cosmic titans have been observed in the universe since just a few hundred million years after the Big Bang. These views are courtesy of the James Webb Space Telescope (JWST). That poses a challenge. Scientists have yet to fully explain how these cosmic titans grew so big, so fast. However, the clues may lie in the ancestral mechanism itself.
“If we see the processes that combine these black holes, we can see what are the starting points for supermassive black holes in the centers of galaxies,” said Bartos. “We can see how fast the process is, how widespread it is, and that can help us understand how big a role this might play in the process of making the black holes more to form a heavy.”
Bartos expects that as gravitational detectors become more sensitive, humanity will see these ripples in space and time from black hole mergers more distantly, and therefore sooner. This means that we should continue to learn more and more about how black hole growth has progressed over the 13.8 billion year history of the cosmos.
“I’m very happy to see things for the first time and find out things as far as we know,” Bartos concluded. “When we look at the extremes, things are very different from what we are used to. And black holes are very large in many ways.”
