The biggest. The farthest. The most energetic. A new detection of gravitational waves from two colliding black holes has caused a bit of a stir with boffins this week
What’s more, it also marks the first definitive sighting of an medium sized black hole, one with a mass between 100 and 100,000 times the sun’s mass. That midsize black hole was formed when the two smaller black holes merged to form a larger one with about 142 solar masses. It significantly outweighs all black holes previously detected using gravitational waves; the tell tale ripples that wrinkle spacetime in the aftermath of extreme events.
“This is the big guy we’ve been waiting for, for the longest time,” says Emanuele Berti, a physicist at Johns Hopkins University who was not involved with the research. One of the behemoth’s two progenitors was itself so massive that scientists are pondering how to explain its existence.
Detected on May 21, 2019, the gravitational waves originated from a source about 17 billion light-years from Earth, making this the most distant detection confirmed so far. It’s also the most energetic event yet seen, radiating about eight times the equivalent of the sun’s total energy, says astrophysicist Karan Jani of Vanderbilt University in Nashville, a member of the LIGO Scientific Collaboration. “I hope it deserves its own entry in the record book.”
The new event dethrones the previous record-holder, a collision that occurred about 9 billion light-years away that radiated about five solar masses worth of energy, and created a black hole of 80 solar masses.
Researchers with LIGO, or the Advanced Laser Interferometer Gravitational-Wave Observatory, in the United States and Advanced Virgo in Italy reported the new detection on September 2 in two papers in Physical Review Letters and the Astrophysical Journal Letters.
While scientists know of black holes with tens of solar masses and others with millions or billions of solar masses, the intermediate sized ones have remained elusive. Previous purported sightings of intermediate mass black holes have been questioned.
But, for the new event, “there’s no doubt,” says astrophysicist Cole Miller of University of Maryland at College Park, who was not involved with the study. “This demonstrates that there is now at least one intermediate mass black hole in the universe.”
The black hole’s two parents were themselves heftier than any seen colliding before — at about 85 and 66 times the mass of the sun. That has scientists puzzling over how this smashup came to be.
Kicking it up a notch
On May 21, 2019, the LIGO and Virgo gravitational wave observatories detected two merging black holes. The duo combined to create a new black hole much larger than those found in previous mergers. Here, coloured circles represent the relative sizes of black holes, with bluer colours representing bigger black holes.
Normally, physicists expect that the black holes involved in these mergers would each have formed in the collapse of a dying star. But in the new event, the larger of the pair is so big that it couldn’t have formed that way. The known processes that go on within a star’s core mean that stars that are the right mass to form such a big black hole would blow themselves apart completely, rather than leaving behind a corpse.
Instead, it might be that one or both of the colliding black holes formed from an earlier round of black hole mergers, within a crowded cluster of stars and black holes (SN: 1/30/17). That would make for a family tree that began with black holes lightweight enough to form from collapsing stars.
But there’s a problem with the multiple-merger explanation. Each time black holes merge, that coalescence provides a kick to their velocity, which would normally launch the resulting black hole out of the cluster, preventing further mergers.
However, mergers as massive as the new event seem to be very rare, given that LIGO and Virgo have detected only one. That means, Miller says, “my gosh, you’re allowed to invoke a tooth fairy,” a relatively unlikely process. Perhaps, he says, the kick might sometimes be small enough that the black holes could stay within their cluster and merge again.
The May 21 gravitational wave event had previously been publicly reported as an unconfirmed candidate, to allow astronomers to look for flashes of light in the sky that might have resulted from the collision. Some researchers had suggested that the waves might have been associated with a flare of light from the center of a distant galaxy (SN: 6/25/20). But that galaxy is significantly closer than the distance now pinpointed in the new papers, at about 8 billion light-years from Earth rather than 17 billion, making the explanation less plausible.
The longer LIGO and Virgo observe the heavens, the more the bounty of unusual events can be expected to grow, Miller says. “We are going to have a set of ‘gosh, didn’t expect that’ type of events, which are thrilling to think about and extremely informative about the universe.
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