The same black hole can collide with its kin multiple times, lopsided merger suggests

black holes collision
An artist's depiction of two mismatched black holes colliding. (Image credit: MIT News)

For black holes, a collision doesn't have to be a once-in-a-lifetime experience, new research suggests.

On April 12, 2019, scientists detected a new black-hole merger using a trio of gravitational-wave detectors. Astrophysicists have spotted such events before, but something about the signals was different this time: the two black holes that collided were incredibly unevenly matched, with the larger about three times the size of the smaller. Scientists didn't expect to see such an imbalanced merger between black holes, and now, they think they might understand the unusual event.

"This event is an oddball the universe has thrown at us — it was something we didn't see coming," Salvatore Vitale, a physicist at the Massachusetts Institute of Technology and an author on the new research, said in a statement. "But nothing happens just once in the universe. And something like this, though rare, we will see again, and we'll be able to say more about the universe."

Related: Eureka! Scientists photograph a black hole for the 1st time

Vitale and his colleagues suspect that the strange collision occurred after the larger black hole itself was the product of a black-hole merger. The initial event sent a large black hole bouncing around a neighborhood packed with black holes, this hypothesis goes, enabling the uneven collision.

That's a very different story than scientists' two main scenarios for black-hole mergers, which both encourage fairly even matches. Vitale and his colleagues used two different models to evaluate whether the traditional merger scenarios could create an event like the unbalanced merger. No dice. 

"No matter what we do, we cannot easily produce this event in these more common formation channels," Vitale said.

So the team turned to a process called hierarchical merging, in which the result of a black-hole merger goes on to merge again. And this time, the models seemed to make sense. "You do the math, and it turns out the leftover black hole would have a spin which is very close to the total spin of this merger," Vitale said.

Coincidentally, gravitational wave researchers published other research this week that also points to hierarchical merging. On Wednesday (Sept. 2), the scientists behind the gravitational-wave detectors LIGO (short for "Laser Interferometer Gravitational-Wave Observatory") and Virgo announced that in May 2019, they had seen a black hole that was larger than scientists know how to form by stellar explosions. The suspicion is that this hefty black hole, if not both the original members of the event, was the result of a previous merger.

The scientists behind the new paper analyzing the uneven collision suspect that hierarchical mergers couldn't happen just anywhere, but instead must occur in a relatively dense neighborhood, where black holes can easily interact with each other.

"This merger must have come from an unusual place," Vitale said. "As LIGO and Virgo continue to make new detections, we can use these discoveries to learn new things about the universe."

The research is described in a paper published Sept. 2 in the journal Physical Review Letters.

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Meghan Bartels
Senior Writer

Meghan is a senior writer at and has more than five years' experience as a science journalist based in New York City. She joined in July 2018, with previous writing published in outlets including Newsweek and Audubon. Meghan earned an MA in science journalism from New York University and a BA in classics from Georgetown University, and in her free time she enjoys reading and visiting museums. Follow her on Twitter at @meghanbartels.

  • Mergatroid
    If there are regions of space "packed with black holes", then how can any type of black hole collision be considered unusual?

    I also don't understand why anyone would just assume a black hole couldn't merge more than once. It seems it would happen as often as it happens. It doesn't seem any different than any other two objects colliding.
  • Torbjorn Larsson
    The big news is the big black hole merger involving 70 - 150 solar mass black holes.

    That we can find black holes in these ranges means there is no principle problem of growing super massive black holes as regards size, even if we still don’t know how they grow so large so quickly.

    But there are more clues to hierarchical growth conditions in this accompanying commentary article in Physical Review Letters.

    "“If the object was able to merge again (in this case, to produce GW190412), it would mean the kick that it received was not enough to escape the stellar cluster in which it formed. If GW190412 indeed is a product of hierarchical merging, the team calculated that it would have occurred in an environment with an escape velocity higher than 150 kilometers per second. For perspective, the escape velocity of most globular clusters is about 50 kilometers per second.

    This means that whatever environment GW190412 arose from had an immense gravitational pull, and the team believes that such an environment could have been either the disk of gas around a supermassive black hole, or a “nuclear cluster” — an incredibly dense region of the universe, packed with tens of millions of stars.”"

    The merger radiated away 8 solar masses, which is 2-4 times more than typical earlier mergers, so it is no wonder that they saw twice as far. Even if it was a low frequency chirp with 10-20 Hz main frequency, it seems the upgraded Advanced LIGO could observe that low.

    The event had an expected rate of ~ 0.1 Gpc^-3 yr^-1, so the 100 Gpc^3 volume would see these about every month – we will soon get good statistics on what seems to be happening in the center of galaxies.
  • Torbjorn Larsson
    Mergatroid said:
    If there are regions of space "packed with black holes", then how can any type of black hole collision be considered unusual?

    I also don't understand why anyone would just assume a black hole couldn't merge more than once. It seems it would happen as often as it happens. It doesn't seem any different than any other two objects colliding.

    Well, you would have to read the paper (and its references) for the entire story.

    But besides that we don't know how larger mass black holes got started in the early universe, there has been "mass gaps" that troubled scientists. From star models it wasn't clear if collapsing stars between 2 and 5 solar masses would become neutron stars or black holes. Another problem from such models come in the mass range between 50ish (says some researchers) to 100ish solar masses where x rays released during normal star end life collapse becomes so energetic that they generate electron-positron pairs and had a runaway disintegration instead of collapse. And finally there was no black holes observed between a few solar masses and 10,000ish solar masses (the intermediate range).

    From observations it seems all these mass gaps have been closed, though how we don't know yet. The 2-5 solar mass gap has seen 1-2 merger results that looks like black holes, i.e. no light (but we have to wait a few years in case dust obscures any neutron star). The pair production instability mass gap has now a star inside, likely an earlier merger and the collapse models may survive. And we have a 100+ solar mass merger result, only 1-3 orders of magnitude left on the growth mass gap.

    The problem with hierarchical merger theory is many I think (is it feasible? does it explain what we see?), but my previous comment touch on what I recently learned may have been the largest problem. If a merger happens the resulting black holes gets a kick and tend to disappear from dense clusters. The resolution for late universe black hole growth at intermediate (and super massive) masses may be growth within an active galaxy nucleus. If that is correct, a few years of gravity wave research should be able to elaborate on and test that.

    How to explain early hierarchical growth seems more iffy.
  • david_h
    Is there a model in which two black holes temporarily overlap event horizons but never actually merge? I was picturing what might happen if two black holes are both approaching a given point in space very fast and from opposite directions. It would be wrong to call what results a "collision," since all the actual matter is at the center of the spherical event horizon, and in my scenario, those cores wouldn't hit. I'm imagining the two black holes only grazing past each other, overlapping event horizons temporarily, deflecting each other's trajectories, but still separating. Is that situation even consistent with GR? If so, just how much of an overlap is possible? Could it happen (say, with supermassive black holes approaching each other at speeds close to c) that the center of one of the black holes is temporarily within the event horizon of the other, but then gets out?

    I think I know why a more ordinary object could not pass through the event horizon and emerge: the black hole warps space so much that the only "straight" path for the object is a spiral toward the black hole's center. But when it's two black holes warping the same volume of space, might not the second black hole have an "unwarping" effect? The whole point of this speculation is that it might give us a theoretical handle on extracting information from the interior of the event horizon - i.e., whether there is a "firewall" there or just nothing special. Also: what if some ordinary object C were orbiting just outside the event horizon of black hole A, which grazes the event horizon of black hole B. Could C temporarily cross B's event horizon but still emerge?