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Large colliding black holes could be a breeding ground for tiny black holes. If we spot signs of these cosmic lightweights, it could provide proof of the fundamental nature of our universe.

There have been hints in particle physics that our universe may not be in the lowest possible energy state – instead of a true vacuum, it may be in a state called a false or metastable vacuum. If any part of the universe were to collapse into a true vacuum, the laws of physics as we know them would collapse inside that bubble of vacuum, which would expand at the speed of light and eventually swallow up everything.

Some research has suggested that the extreme gravity near a black hole could create a foam of small bubbles of true vacuum. If those bubbles immediately fell into the black hole, though, that process could occur without destroying the universe.

at Manhattan College in New York and his colleagues calculated what might happen if these vacuum bubbles formed in the region between two colliding black holes. “In the region between the colliding horizons of the black holes, you have gravitational pull from both sides balancing out, so maybe for a short time interval the bubble can exist sandwiched between the two black holes,” says Konoplich.

The surface of each vacuum bubble would be expected to form a kind of film similar to a regular soap bubble. Given even a small amount of time to percolate between a pair of black holes, the bubbles could be expected to collide with one another. The researchers calculated that if multiple bubbles collided at once, the intersecting surface could become infinitely dense, forming a micro-black hole.

Because of a process called Hawking radiation, these tiny black holes would emit a random mix of particles and evaporate away extremely quickly. Konoplich and his colleagues calculated that this entire process could take place in just about 10 milliseconds before the larger black holes collided and devoured any bubbles or micro-black holes in their way.

But if bubbles of true vacuum do exist, it isn’t necessarily a certainty that the bubbles will safely fall into the huge black holes that enable their formation, says at King’s College London. “We know that these bubbles, once they’re formed, start to expand quite quickly and rapidly reach the speed of light,” says Gregory. “If they’re outside the horizon, it might be that they would expand instead of falling in.”

This would be a disaster of apocalyptic proportions. “If one of these bubbles of true vacuum escaped, it would destroy the universe – oops,” says Gregory. The fact that the universe is still around suggests that bubbles of true vacuum are rare, if they exist at all, she says.

However, if they do exist and form micro-black holes, we could detect the random radiation from their eventual evaporation. “If we can detect something like this, it would be very important because it would prove that our universe is metastable from an observational result, not just theoretical,” says Konoplich. That would be a major insight into the fundamental nature of our universe, which theoretical physicists are still debating.

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