These black holes shouldn’t exist, but there they are
By Dennis Overbye
Well, that was some clash of the heavyweights.
Astronomers reported last week that they had detected the loudest, most massive and most violent collision yet between a pair of black holes. Two Goliaths of darkness crashed into each other 7 billion years ago, vibrating space-time and producing a loud, sharp chirp — almost a bang, one astronomer said — lasting just one-tenth of a second in the antennas of the Laser Interferometer Gravitational-Wave Observatory and the Virgo interferometer observatory.
That short signal from a galaxy far, far away has left astrophysicists with new questions about how black holes form and grow.
Daniel Holz, a theorist at the University of Chicago and a member of the LIGO team, called the new discovery “the first LIGO/Virgo detection that’s truly surprising. All the other binary systems that we’ve detected fit reasonably well within expectations. But the black holes in this event aren’t supposed to exist!”
One and perhaps both of the colliding holes were too massive to have been produced by the collapse of a star, according to conventional theories. Moreover, the merger created an even larger black hole, 142 times as massive as the sun, belonging to a whole new category of intermediate-mass, or “missing link,” black holes never reliably seen before.
“Another discovery from the worldwide gravitational-wave detector network that rewrites what we know about our universe,” Zsuzsanna Marka, an astrophysicist at Columbia University who works on LIGO, wrote in an email.
Janna Levin, a cosmologist at Barnard College who is not part of the LIGO group, added: “Yes! I’ve been waiting for something like this since I first became interested in gravitational waves.”
The event unfolded at an almost unimaginable distance from Earth — 17 billion light-years away according to standard cosmological calculations that describe an expanding universe. One black hole with 85 times the mass of the sun, and a second with 66 solar masses, collided, creating a black hole 142 times as massive as the sun.
Another eight or so suns’ worth of mass and energy disappeared into gravitational waves, ripples of the space-time fabric, in a split second of frenzy, ringing the universe like a bell on the morning of May 21, 2019.
An international team of scientists who compose the LIGO Scientific Collaboration and the Virgo Collaboration reported their findings in two papers published Wednesday in Physical Review Letters and The Astrophysical Journal Letters.
Their papers largely affirm a preliminary analysis of the event, known as GW190521 (after the date when it was recorded), made by a group outside the collaborations. In June, a team led by Matthew Graham of the California Institute of Technology, going on publicly available data, ran a preliminary analysis, hoping to beat the LIGO and Virgo groups to the answer.
Using a telescope in California called the Zwicky Transient Facility, or ZTF, Graham’s team detected a flash of light that could have been caused by the newly formed black hole racing through a disk of dense gas surrounding the center of a faraway galaxy.
They predicted that a final analysis would show that the combined masses of the colliding black holes would exceed 100 solar masses, and that the resulting black hole would spin wildly and have a large recoil velocity.
“This is exactly what LIGO is now reporting,” Graham wrote in an email. “This is a great discovery from LIGO and provides strong evidence in support of the merger model and environment that we have been promoting.”
Most known black holes are the corpses of massive stars that have died and collapsed catastrophically into nothing: dark things a few times as massive as the sun. But galaxies harbor black holes millions or billions of times more massive than that. How these objects can grow so big is an abiding mystery of astronomy.
Until recently there had been scant evidence of black holes of intermediate sizes, with 100 to 100,000 solar masses. The black hole created in the GW190521 merger is the first solid example of this missing link.
“I was searching for heavy black holes for 15 years and here it is!” Sergey Klimenko, a physicist at the University of Florida, wrote in an email. “This discovery is a milestone in gravitational wave astronomy.”
As a result, he said, astronomers may have glimpsed the process by which the universe builds black holes, transforming pipsqueaks into leviathans like the one in the galaxy M87 that was the first ever imaged.
“This is the first and only firm/secure mass measurement of an intermediate mass black hole at the time of its birth,” Vicky Kalogera, an astrophysicist at Northwestern University, wrote in an email. “Now we know reliably at least one way” these objects can form, “through the merger of other black holes.”
This merger process could be an important clue to the origin of the heavier of the two black holes that collided in June. That black hole had a mass of 85 suns, and it should not have been existed, according to standard astrophysical logic. Black holes with masses between about 50 and 120 suns cannot be formed, at least from a dying star, so the story and the calculations go.
In stars massive enough to make such a beastly hole, the interior grows so hot when collapsing that light spontaneously creates pairs of electrons and positrons. This makes the star even hotter, which produces more particles, in a runaway reaction that results in a particularly violent explosion called a pair-instability supernova. Such a conflagration leaves nothing behind.
“No neutron star,” Holz said. “No black hole. Nothing.”
He mentioned the black hole in GW190521 with 85 solar masses: “The bigger black hole is right smack in the middle of the region where black holes don’t belong. Nature seems to have ignored all of our careful theoretical calculations arguing that black holes of this mass don’t exist.”
He added: “A discovery like this is simultaneously disheartening and exhilarating. On the one hand, one of our cherished beliefs has been proven wrong. On the other hand, here’s something new and unexpected, and now the race is on to try to figure out what is going on.”