Primordial black holes, theorized for decades as potentially a form of dark matter, have never been directly observed. New research, co-led by the University at Buffalo, proposes that their existence could be confirmed through both large and small signatures—hollow planetoids in space to minute microscopic tunnels in everyday materials found on Earth, like rocks, metal, and glass.
The study suggests that a primordial black hole trapped within a large rocky object in space could hollow out its core. Alternatively, a faster primordial black hole might create microscopic tunnels in solid materials, such as rocks, metal, and glass, which could be detectable on Earth.
Although the chances of finding these signatures are small, searching for them would require minimal resources. The potential payoff, however, could be immense, offering the first evidence of a primordial black hole.
The study’s co-author, Dejan Stojkovic, PhD, a professor of physics at the UB College of Arts and Sciences, said, “We have to think outside the box because what has been done to find primordial black holes previously hasn’t worked.”
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The study calculated the maximum size a hollow planetoid could reach without collapsing and assessed the likelihood of a primordial black hole passing through an object on Earth. However, there’s no need to worry about such an event being fatal, as the study concluded that a black hole passing through a person would not cause harm.
Co-author De-Chang Dai explained that due to the long odds, the focus has been on looking for solid marks that have existed for thousands, millions, or even billions of years. The National Science Foundation supported Stojkovic’s research, while Dai’s work received support from Taiwan’s National Science and Technology Council.
As the universe expanded rapidly after the Big Bang, regions of space may have been denser than their surroundings, leading to the collapse of these areas and the formation of primordial black holes (PBHs). While PBHs would have much less mass than stellar black holes formed from dying stars, they would still be highly dense, with the mass of a mountain compressed into an area the size of an atom.
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Stojkovic, who has previously proposed potential locations for theoretical wormholes, wondered if a PBH could have become 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.
If an asteroid impacts an object, the primordial black hole (PBH) could potentially escape, leaving behind nothing but a hollow shell.
The researchers considered whether a hollow object created by a primordial black hole would be strong enough to support itself or if it would collapse under its own tension. They calculated that such a hollow object could not exceed one-tenth of Earth’s radius by comparing the strength of materials like granite and iron with surface tension and density.
Larger objects would likely collapse. These hollow objects, however, could be detectable with telescopes. By studying an object’s orbit, scientists can determine its mass and density. If the object’s density is too low for its size, it could indicate that it is hollow.
The study suggests that primordial black holes (PBHs) might pass through objects without a liquid core and leave behind a straight tunnel. For instance, a PBH with a mass of 10²² grams would create a tunnel just 0.1 microns thick.
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A large slab of metal or other material could act as an effective black hole detector by being monitored for the sudden appearance of these tunnels. However, Stojkovic recommends searching for existing tunnels in very old materials—such as buildings hundreds of years old or rocks billions of years old—for better odds of detection.
Even if dark matter is composed of primordial black holes (PBHs), the probability of a PBH passing through a billion-year-old boulder is extremely small, calculated at 0.000001.
Stojkovic emphasizes the cost-benefit balance, noting that searching for such events requires little investment. The likelihood of a PBH passing through a person during their lifetime is very low, and even if it did, it wouldn’t cause noticeable harm.
Human tissue has enough tension to prevent it from being torn apart by a PBH. Although a PBH’s kinetic energy is immense, it would not release much during a collision because it moves so quickly.
Stojkovic says, “If a projectile moves through a medium faster than the speed of sound, the medium’s molecular structure doesn’t have time to respond. Throw a rock through a window; it’s likely going to shatter. Shoot a window with a gun, it’s likely to leave a hole.”
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Stojkovic says theoretical studies like this are essential. He points out that many physical concepts once deemed implausible are now considered likely. He adds that the field is currently grappling with significant challenges, such as the mystery of dark matter.
Stojkovic emphasizes that the last major breakthroughs in physics—quantum mechanics and general relativity—are already a century old, highlighting the need for new advancements to address these unresolved issues.
Stojkovic says, “The smartest people on the planet have been working on these problems for 80 years and have not solved them yet.”
“We don’t need a straightforward extension of the existing models. We probably need a completely new framework altogether.”
Journal Reference:
- De-Chang Dai, Dejan Stojkovic. Searching for small primordial black holes in planets, asteroids, and here on Earth. Physics of the Dark Universe, 2024; 46: 101662 DOI: 10.1016/j.dark.2024.101662