A black hole may have been kicked out of its own galaxy

We’ve all been there the past few days fawning over the glorious new image of the black hole of our Milky Way. But for a moment, we might want to pause and pay our respects to a lonely chasm in the universe that may have started up from its own galaxy.

On Thursday — same day the Event Horizon Telescope Collaboration brought us a visceral image of Sagittarius A* — astronomers announced that somewhere in the cosmos two black holes could have fused with enough force to literally kick the resulting void out of sight.

Yes, that means a monstrous abyss could be crashing across the universe right now. But don’t panic.

“Space is just unbelievably big. The chances of a black hole encountering anything else are very slim,” said Vijay Varma, a postdoctoral researcher at the Albert Einstein Institute and lead author of a study on the abandoned void. published May 12 in Physical Review Letters. “Practically speaking, it’s just a pretty black hole that won’t do anything.”

And, according to Varma, the chances are that this merger, dubbed GW200129, has only half driven the baby black hole out of its home. “It is not known that the black hole was necessarily ejected from its host galaxy,” he said. “What we can say with more certainty is that if the black hole formed in these clusters of stars called globular clusters … it most likely was ejected from the cluster.”

In other words, the swept abyss could have bypassed complete isolation — but it’s almost certainly on some kind of super-fast cosmic journey.

Journey from a lost void

Space is littered with galaxies like our Milky Way, and galaxies are littered with stars like our sun.

If one of those stars implodes, turning all of its matter into a single point, a black hole is formed. And sometimes there are intergalactic realms where: stars cluster together, increasing the likelihood of a black hole party. Those merging black holes sometimes get tangled up in this dangerous gathering, forming what’s known as a binary black hole system, which simply means two of them are locked in orbit. Ultimately, those orbits tend to collide and force the dancing bottomless pits to coalesce.

GW200129 has diligently followed all those steps, and thanks to the pioneering Laser Interferometer Gravitational Wave Observatoryor LIGO, researchers recently captured the receipts from the fusion in the form of gravitational waves.

But Varma and his team wanted to follow GW200129’s journey, even beyond the colossal fusion that rippled through the fabric of space and time† For decades, Varma says, experts have theorized that such space-expanding black hole unions could create a type of backlash called a “speed kick.”

This is what that is.

When two objects collide, the laws of physics say: their momentum must be maintained. The resulting item must maintain the sum rate or speed of the first two and continue to move with a net force. As such, black holes are believed to follow the same rule, “kicking” out a merging void at some speed, or kick-rate. It’s kind of like when you fire a gun in a video game, there’s a recoil that makes it harder to hit a target as it pushes your character around. That subsequent movement happens because of the conservation of momentum.

If a black hole’s kick speed corresponds to a galaxy’s so-called ‘escape speed’, simply the speed it takes to leave that galaxy, well, then it would leave that galaxy.

And when calculating the velocity “kick” of GW200129, Varma and his team saw that it does indeed reach the escape velocity of its star cluster. “If you’re to be precise,” he noted, “it won’t escape for sure. It’s a 99.5% chance of escaping.”

“The idea that these black holes can gain kick velocities of thousands of kilometers per second has been known since about 2007,” Varma added. “But this is the first time we can see this from gravitational waves.”

The team says GW200129 likely even meets the escape velocity criteria to expel itself from the entire galaxy it inhabits. “However, we can’t identify which galaxy or cluster of galaxies it comes from,” Varma said, which is why that part is still unclear. He gives the merger about an 85% chance of escaping a Milky Way-like realm, for context, but says it’s less likely to have escaped an elliptical galaxy because such cosmic neighborhoods have really high escape rates.

“We actually tried this two years ago,” he said. “It was a little disappointing to see that none of the signals showed a measurable kick speed. To finally see this happen was very pleasing, and for our field in general it has taken a long time.”

A new look at the physics of black holes

“If we find that big stairs like this are very common, we expect that black holes will not be conserved after the first merger,” Varma said. And that, he says, would contradict one of the leading theories as to why some black holes are really massive.

Computer simulations have shown that supernovae should not be able to create black holes with masses greater than about 45 to 60 times that of our Sun. But, Varma explained, “LIGO and Virgo have found such black holes.”

“Where do they come from?” is an excellent question for astronomers.

One proposed mechanism is the merger of black holes, because the resulting merger is always larger. Imagine bubbles merging into bigger bubbles. Maybe there is some kind of bubble effect with black holes.


Think how bubbles can be combined to make bigger bubbles. Those can then be combined to create even bigger bubbles. Scientists theorize that black holes could do this too.

Getty Images

But, as Varma explained, if a black hole resulting from a merger was kicked out of its galaxy, or even a star cluster, it probably wouldn’t be able to merge again. It would be… a little lost. “Maybe we need to rethink our astrophysical models,” he said, if super-high kick rates from black hole mergers are ubiquitous.

Rogue supermassive black holes?

So far, in regards to kick velocities, we’ve talked about black holes that aren’t considered supermassive. Supermassive black holes are like the engines that keep our universe spinning and anchor every galactic neighborhood. SgrA*, which anchors our galaxy, for example, is a super-heavy void.

This raises a stressful question: Can merging galaxies, aka the merging of supermassive black holes, provide a speed boost?


A side by side of the M87* black hole photographed in 2019 and SgrA* this year.

EHT collaboration

“In that situation, the last black hole could actually be ejected from the entire galaxy or moved out of the center,” Varma said. “This could lead to, for example, galaxies without central supermassive black holes.” However, we still don’t really have any concrete evidence of such an incident.

And if there’s still a part of you that’s stuck with the fact that a black hole is hurtling through the cosmos because of the chance of it blasting into the Milky Way, the research team on the new study offers an added relief. “I often get asked this question,” Varma said, “whether we should expect black holes in the weather forecast. But I mean, we were also able to measure the direction of speed.”

“And in this particular case, it’s actually directed away from us.”

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