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Seven billion years ago, two black holes crashed into each other and merged into one enormous black hole with the mass of 142 suns.
The collision reverberated through space and time, and these ripples – a phenomenon called gravitational waves first predicted by Albert Einstein – travelled 16.5 billion light-years through the universe, reaching Earth in May 2019.
For one-tenth of a second, the waves stretched the mile-long arms of two enormous physics observatories: the Laser Interferometer Gravitational-Wave Observatory in the US and its Italian companion, Virgo.
The scientists behind these observatories immediately knew they’d detected something unique.
“This doesn’t look much like a chirp, which is what we typically detect,” Nelson Christensen, a Virgo scientist and a researcher at the French National Centre for Scientific Research, said in a press release. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”
The black-hole merger the observatories detected is the most massive and distant they have ever picked up. But more strikingly, it defies the known laws of physics.
The scientists’ calculations showed that the heavier black hole of the two that crashed was 85 times the mass of the sun – falling within a range that many physicists thought impossible.
“This is exactly what I predicted wasn’t there,” Stan Woosley, an astrophysicist who models the deaths of massive stars (the process that creates black holes), told Business Insider. “A big black hole smack-dab in the middle of the forbidden zone.”
LIGO and Virgo scientists described the new findings in two papers published Wednesday.
“This event opens more questions than it provides answers,” Alan Weinstein, a LIGO scientist and a professor of physics at the California Institute of Technology, said in the release. “From the perspective of discovery and physics, it’s a very exciting thing.”
‘Some of us will owe bottles of wine to others’
Black holes form when heavy stars die and collapse; their gravitational pulls are so strong that not even light can escape.
There are two main types of black hole: stellar-mass (which are tens of solar masses) and supermassive (which have the mass of millions or even billions of suns).
The 142-solar-mass black hole that formed as a result of this 7-billion-year-old collision is the first detected that’s between 100 and 1,000 solar masses. This “intermediate mass” object could reveal a missing link between the two types of black holes. It may also help scientists understand where supermassive black holes come from.
But the 85-solar-mass black hole involved in the collision wasn’t supposed to exist at all.
Though black-hole sizes can range “from microscopic to the size of the universe,” Woosley said, his models suggest that when it comes to pairs of stars orbiting a shared centre of gravity, “it would be very hard to form a black hole with a mass between about 50 and 130 solar masses.”
Instead, physics models suggest that stars in that mass range should die in a unique type of supernova explosion that annihilates the star, leaving behind no material to collapse into a dense black hole.
“But nature finds a way,” Woosley said. “In our defence, they had to scrounge around in a substantial fraction of the visible universe to find one. It’s very far away.”
He added that physicists like him who predicted this mass gap would need to rethink their models. “We and a lot of other people will go back and look hard at our assumptions,” Woosley said.
That may also mean paying up on a lost bet against the gravitational-wave researchers.
“The observers will look for more – just one of anything is not nearly so nice as two. And some of us will owe bottles of wine to others,” Woosley said. “I’m not 100% convinced that they saw an 85 [solar-mass black hole] but am convinced enough to pay out.”
The black hole could have grown from a previous collision
It’s unlikely that this impossible black hole was created directly from a collapsing star, so some researchers think it could have come from a previous merger.
“There are many ideas about how to get around this – merging two stars together, embedding the black hole in a thick disc of material it can swallow, or primordial black holes created in the aftermath of the Big Bang,” Christopher Berry, a gravitational-wave astronomer and LIGO researcher, said in the release. “The idea I really like is a hierarchical merger where we have a black hole formed from the previous merger of two smaller black holes.”
Woosley, too, said the black hole probably got so big because of something that happened after it formed.
“We really just predict the masses of black holes when they are born,” he said.
Another possibility is that the event LIGO and Virgo detected may not have been a black-hole merger at all. A collision, however, is the best fit for the data.
Einstein’s predictions led scientists to violent space collisions and a new realm of physics
Einstein predicted that collisions of massive objects, like black holes and neutron stars, would produce gravitational waves. But he didn’t think anyone would ever detect these ripples in space-time – they seemed too weak to pick up on Earth amid all the noise and vibrations here.
For 100 years, it seemed Einstein was right.
But in the late 1990s, LIGO’s machines in Washington and Louisiana were built in an attempt to pick up the signals. For the first 13 years, they waited in silence.
Finally, in September 2015, LIGO detected its first gravitational waves: signals from the merger of two black holes some 1.3 billion light-years away. The discovery opened a new field of astronomy, and three researchers who helped conceive of the experiment earned a Nobel Prize in physics.
Since then, LIGO and Virgo have identified two other types of collisions. The observatories registered gravitational waves from two neutron stars merging for the first time in October 2017. In August 2019, LIGO and Virgo detected what scientists believe was a black hole swallowing a neutron star.
“After so many gravitational-wave observations since the first detection in 2015, it’s exciting that the universe is still throwing new things at us, and this 85-solar-mass black hole is quite the curveball,” Chase Kimball, an astronomy doctoral student at Northwestern University who works with the LIGO team, said in the release.
Researchers expect to learn more as they delve further into this field of physics. Planned upgrades and new observatories may enable scientists to detect new space collisions every day by the mid-2020s.
“Gravitational-wave observations are revolutionary,” Berry said. “Each new detection refines our understanding of how black holes form. With these gravitational-wave breakthroughs, it won’t be long until we have enough data to uncover the secrets of how black holes are born and how they grow.”
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