A Black Hole Might’ve Been Kicked Out of Its Own Galaxy

For the past few days, we’ve all been fawning over the glorious new image of our Milky Way’s black hole. But for a moment, we might want to pause and pay our respects to a lonely chasm across the universe that might’ve been booted from its own galaxy.

On Thursday — the same day the Event Horizon Telescope Collaboration brought us a visceral picture of Sagittarius A* — astronomers announced that somewhere in the cosmos, two black holes could’ve merged with enough force to literally kick the resulting void out of sight. Yes, that means there could be a monstrous abyss plunging through the universe right now.

But don’t panic.  “Space is just incredibly vast. The probability that black hole will run into something else is very low,” said Vijay Varma, a postdoctoral researcher at Albert Einstein Institute and lead author of a study on the forsaken void published May 12 in Physical Review Letters. “Practically speaking, it’s just a free black hole that will not do anything.”

And, according to Varma, there’s a solid chance this merger, dubbed GW200129, only sort of half-blasted the baby black hole from its home. “It’s not known that the black hole necessarily got ejected from its host galaxy,” he said. “What we can say more confidently is that if the black hole was formed in these clusters of stars called globular clusters….it very likely got ejected from the cluster.” 

In other words, the expunged abyss could’ve circumvented complete isolation — but it’s almost definitely on some type of hyper-speed cosmic voyage.

Journey of a lost void

Space is speckled with galaxies like our Milky Way, and galaxies are flecked with stars like our sun.  When one of those stars implodes so all its matter turns to a singular point, a black hole is formed. And sometimes, there are intergalactic regions where stars cluster together, therefore increasing the likelihood of a black hole party.

Those mingling black holes sometimes get caught in a waltz at this dangerous get-together and form what’s known as a binary black hole system, which just means two of them are locked in an orbit. Eventually, those orbits tend to collide and force the dancing bottomless pits to merge.  GW200129 diligently followed all those steps, and thanks to the groundbreaking Laser Interferometer Gravitational-wave Observatory, or LIGO, researchers recently caught the merger’s receipts in the form of gravitational waves. 

But Varma and his team wanted to follow GW200129’s journey beyond even the colossal fusion that rippled through the fabric of space and time. For decades, Varma says, experts theorized such space-shattering black hole unions might create a sort of recoil called a velocity “kick.”  Here’s what that is.

When any two objects smash together, the laws of physics say their momentum has to be conserved. The resulting item must retain the former two’s sum velocity, or speed, and continue moving with a net force. As such, black holes are thought to follow the same rule, thereby “kicking” a merger-derived void outward with some speed, or kick velocity. It’s kind of like how when you fire a gun in a video game, there’s a recoil that makes it harder to hit a target because it jostles your character around.

That subsequent movement happens because of momentum conservation.  If a black hole’s kick velocity meets what’s called a galaxy’s “escape velocity,” simply the velocity required to exit that galaxy, well, it would exit that galaxy. 

And upon calculating the velocity “kick” of GW200129, Varma and his team saw that it indeed meets the escape velocity of its star cluster. “If you want to be more exact,” he noted, “it’s not definitely going to escape. It’s a 99.5% chance of escaping.”

“The idea that these black holes can get kick velocities of thousands of kilometers per second has been known since around 2007,” Varma added. “But this is the first time we’re able to see this from gravitational waves.”  The team says GW200129 probably even meets escape velocity criteria to expel itself from the entire galaxy it lives within. However, “we can’t identify which galaxy or cluster of galaxies it came from,” Varma said, which is why that bit 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 for it to have escaped an elliptical galaxy because such cosmic neighborhoods have really high escape velocities. “We actually tried this two years ago,” he said. “It was kind of disappointing to see that none of the signals showed any measurable kick velocity. To finally see this happen was very pleasing, and for our field in general, it’s been kind of a long time coming.”

Rethinking black hole physics

“If we find large kicks like this are very common, we would expect that black holes will not be retained after the first merger,” Varma said.

And that, he says, would contradict one of the leading theories behind why some black holes are really heavy.  Computer simulations have shown that supernovas shouldn’t be able to create black holes with masses greater than around 45 to 60 times that of our sun. But, Varma explained, “LIGO and Virgo have actually found such black holes.” 

“Where did they come from?” has been an outstanding question for astronomers.  One proposed mechanism is subsequent black hole mergers, because as black holes compound, the resulting one is always bigger. Imagine bubbles combining into bigger bubbles.

Maybe there’s some sort of bubble effect happening with black holes.

Think about how bubbles can combine to make bigger bubbles. Then, those can combine to make even bigger bubbles. Scientists theorize that black holes might do this too.

Getty Images

But, as Varma explained, if a black hole that came from a merger was kicked out of its galaxy, or even star cluster, it probably wouldn’t be able to merge again.

It’d be…kind of lost. “We may have to rethink our astrophysical models,” he said, if super high kick velocities from black hole mergers are ubiquitous.

Rogue supermassive black holes?

So far, regarding kick velocities, we’ve been talking about black holes that aren’t considered supermassive. Supermassive black holes are like the engines that keep our universe going, anchoring together each galactic neighborhood. SgrA*, which anchors our galaxy for instance, is a supermassive void. 

This brings about a stressful question: Can merging galaxies, aka merging supermassive black holes, bring a velocity kick? 

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

EHT Collaboration

“In that situation, the final black hole can actually get ejected from the entire galaxy or get displaced from its center,” Varma said. “This can lead to, for example, galaxies without central supermassive black holes.” However, we still don’t really have concrete evidence of such an incident.  And if there’s still a part of you that’s stuck on the fact that a black hole is barreling through the cosmos because of the probability it blasts into the Milky Way, the new study’s research team offers an extra relief. “I get asked this question a lot,” Varma said, “whether we should expect black holes in the weather forecast. But, I mean, we were also able to measure the direction of the velocity.”

“And in this particular instance, it’s actually pointed away from us.”