Black hole collision could have sent an invisible monster into space

For a few days, we are all flattering the glorious new image of the black hole of our Milky Way. But for a moment, we might want to pause and pay homage to a lonely chasm across the universe that may have been started from its own galaxy.

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

Yes, that means there could be a monstrous abyss plunging into the universe right now. But don’t panic.

“Space is just incredibly vast. The likelihood of a black hole colliding with anything else is very small,” said Vijay Varma, postdoctoral researcher at the Albert Einstein Institute and lead author of a vacuum study. abandoned. published May 12 in Physical Review Letters. “In practice, it’s just a free black hole that won’t do anything.”

And, according to Varma, there’s a good chance that this merger, dubbed GW200129, only half-exploded the baby black hole from home. “It is unclear whether the black hole was necessarily ejected from its host galaxy,” he said. “What we can say with more confidence is that if the black hole formed in these clusters of stars called globular clusters…it was most likely ejected from the cluster.”

In other words, the cleared abyss could have circumvented complete isolation – but this is almost certainly some type of high-speed cosmic travel.

Journey of a lost void

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

When one of these stars implodes, all of its matter turns to a singular point, a black hole is formed. And sometimes there are intergalactic regions where the stars come together, thus increasing the probability of a black hole evening. These intermingling black holes sometimes get caught up in a waltz during this dangerous encounter and form what is called a binary black hole system, which simply means that two of them are locked in an orbit. Eventually, these orbits tend to collide and force the dancing bottomless pits to merge.

GW200129 diligently followed all these steps, and thanks to the revolutionary Laser interferometer Observatory of gravitational wavesor LIGO, researchers have recently captured the recipes of fusion in the form of gravitational waves.

But Varma and his team wanted to follow GW200129’s journey beyond even the colossal fusion that wavy through the fabric of space and time. For decades, Varma says, experts have speculated that such space-shattering unions of black holes could create a kind of recoil called a speed “kick”.

Here is what it is.

When two objects collide, the laws of physics say their momentum must be kept. The resulting element should keep the speed or velocity of the sum of the first two and keep moving with a net force. As such, black holes are thought to follow the same rule, thereby “pushing” a fusion-derived vacuum outward with a certain velocity, or kicking velocity. It’s kind of like when you shoot a gun in a video game, there’s recoil that makes it harder to hit a target because it jostles your character. This subsequent motion occurs due to the conservation of momentum.

If the ejection velocity of a black hole is what is called the “escape velocity” of a galaxy, simply the velocity required to exit that galaxy, well, it would exit that galaxy.

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

“The idea that these black holes can reach trigger speeds 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 probably even meets the escape velocity criteria to kick itself out of the entire galaxy it lives in. However, “we can’t identify which galaxy or cluster of galaxies it came from,” Varma said, so 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 that it escaped an elliptical galaxy because those cosmic quarters have velocities very high exhaust.

“We actually tried this two years ago,” he said. “It was a little disappointing to see that none of the signals were showing any measurable hitting speed. To finally see that happen was very nice, and for our field in general, it’s been a while in coming.”

Rethinking black hole physics

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

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 actually found such black holes.”

“Where do they come from?” has been a lingering question for astronomers.

One of the proposed mechanisms is the subsequent merger of black holes, because as compounds of black holes, the resulting one is always larger. Imagine bubbles combining into bigger bubbles. Maybe there’s some sort of bubble effect that happens with black holes.


Think about how bubbles can combine to form bigger bubbles. Then these can combine to form even bigger bubbles. Scientists believe that black holes could also do this.

Getty Images

But, as Varma explained, if a merged black hole were kicked out of its galaxy, or even its star cluster, it probably couldn’t merge again. It would be… a little lost. “We may have to rethink our astrophysical models,” he said, if the very high kicking velocities from black hole mergers are ubiquitous.

Rogue supermassive black holes?

So far, when it comes to kicking velocities, we’ve talked about black holes that aren’t considered supermassive. Supermassive black holes are like the engines that run our universe, anchoring every galactic neighborhood together. SgrA*, which anchors our galaxy for example, is a supermassive void.

This raises a stressful question: can merging galaxies, i.e. merging supermassive black holes, lead to an increase in speed?

black holes side by side

Next to the black hole M87* imaged in 2019 and SgrA* imaged this year.

EHT cooperation

“In this situation, the final black hole may actually be ejected from the entire galaxy or be displaced from its center,” Varma said. “This may lead, for example, to 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 about a black hole traversing the cosmos due to the likelihood of it exploding in the Milky Way, the new study’s research team offers a additional 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 velocity.”

“And in this particular case, he’s actually headed away from us.”

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