Evidence of primordial black holes may be hiding in planets, or even everyday objects here on Earth

Imagine the formation of a black hole and you’ll probably envision a massive star running out of fuel and collapsing in on itself. Yet the chaotic conditions of the early universe may have also allowed many small black holes to form long before the first stars. 

These primordial black holes have been theorized for decades and could even be ever-elusive dark matter, the invisible matter that accounts for 85% of the universe’s total mass. 

Still, no primordial black hole has ever been observed. 

New research co-led by UB proposes thinking both big and small to confirm their existence, suggesting that their signatures could range from very large — hollow planetoids in space — to minute — microscopic tunnels in everyday materials found on Earth, like rocks, metal and glass. 

Set to be published in the December issue of Physics of the Dark Universe and available online now, the theoretical study posits that a primordial black hole trapped within a large rocky object out in the cosmos would consume its liquid core and leave it hollow. Alternatively, a faster primordial black hole might leave behind straight tunnels large enough to be visible by a microscope if passing through solid material, including material right here on Earth. 

“The chances of finding these signatures are small, but searching for them would not require much resources and the potential payoff, the first evidence of a primordial black hole, would be immense,” says the study’s co-author, Dejan Stojkovic, professor of physics, College of Arts and Sciences. “We have to think outside of the box because what has been done to find primordial black holes previously hasn’t worked.”

The study calculated how large a hollow planetoid could be without collapsing in on itself, and the likelihood of a primordial black hole passing through an object on Earth. (If you’re worried about a primordial black hole passing through you, don’t be. The study concluded it would not be fatal.)

“Because of these long odds, we have focused on solid marks that have existed for thousands, millions or even billions of years,” says co-author De-Chang Dai of National Dong Hwa University and Case Western Reserve University.

Stojkovic’s work was supported by the National Science Foundation, while Dai’s work by the National Science and Technology Council (Taiwan).

As the universe rapidly expanded after the Big Bang, areas of space may have been denser than their surroundings, causing them to collapse and form primordial black holes (PBHs).

PBHs would have much less mass than the stellar black holes later formed by dying stars, but they would still be extremely dense, like the mass of a mountain compacted into an area the size of an atom. 

Stojkovic, who has previously proposed where to find theoretical wormholes, wondered if a PBH ever became trapped within a planet, moon or asteroid, either during or after its formation.

“If the object has a liquid central core, then a captured PBH can absorb the liquid core, whose density is higher than the density of the outer solid layer,” Stojkovic says.

The PBH then might escape the object if the object was impacted by an asteroid, leaving nothing but a hollow shell. 

But would such a shell be strong enough to support itself, or would it simply collapse under its own tension? Comparing the strength of natural materials like granite and iron with surface tension and surface density, the researchers calculated that such a hollow object could be no more than one-tenth of Earth’s radius, making it more likely to be a minor planet than a proper planet.

“If it is any bigger than that, it’s going to collapse,” Stojkovic says.

These hollow objects could be detectable with telescopes. Mass, and therefore density, can be determined by studying an object’s orbit.

“If the object’s density is too low for its size, that’s a good indication it’s hollow,” Stojkovic says.

For objects without a liquid core, PBHs might simply pass through and leave behind a straight tunnel, the study proposes. For example, a PBH with a mass of 10²² grams — that’s a 10 with 22 zeros — would leave behind a tunnel 0.1 micron thick.

A large slab of metal or other material could serve as an effective black hole detector by being monitored for the sudden appearance of these tunnels, but Stojovic says you’d have better odds searching for existing tunnels in very old materials — from buildings that are hundreds of years old, to rocks that are billions of years old. 

Still, even assuming that dark matter is indeed made up of PBHs, they calculated that the probability of a PBH passing through a billion-year-old boulder to be 0.000001.

“You have to look at the cost versus the benefit. Does it cost much to do this? No, it doesn’t,” Stojkovic says. 

So the likelihood of a PBH passing through you during your lifetime is small, to say the least. Even if one did, you probably wouldn’t notice it. 

Unlike a rock, human tissue has a small amount of tension, so a PBH would not tear it apart. And while a PBH’s kinetic energy may be huge, it cannot release much of it during a collision because it’s moving so fast. 

“If a projectile is moving through a medium faster than the speed of sound, the medium’s molecular structure doesn’t have time to respond,” Stojkovic says. “Throw a rock through a window, it’s likely going to shatter. Shoot a window with a gun, it’s likely to just leave a hole.”

Theoretical studies such as this are crucial, Stojkovic says, noting that many physical concepts that once seemed implausible are now considered likely. 

The field, Stojkovic adds, is currently facing some serious problems, dark matter among them. Its last major revolutions — quantum mechanics and general relativity — are a century old.

“The smartest people on the planet have been working on these problems for 80 years and have not solved them yet,” he says. “We don’t need a straightforward extension of the existing models. We probably need a completely new framework altogether.”