Primordial black holes have been theorized for decades and could even be ever-elusive dark matter. Still, no primordial black hole has ever been observed. According to a duo of astrophysicists from the University at Buffalo, Case Western Reserve University and National Dong Hwa University, these small black holes could be captured by rocky planets or asteroids, consume their liquid cores from inside and leave hollow structures; alternatively, they can leave micro-tunnels here on Earth in very old rocks, or even glass or other solid structures in very old buildings.
An artist’s impression of primordial black holes. Image credit: NASA.
Small primordial black holes are perhaps the most interesting and intriguing relics from the early Universe.
They can serve as dark matter candidates, source primordial gravitational waves, help in resolving cosmological problems like the domain wall and magnetic monopole problems, etc.
However, no convincing primordial black hole candidate has ever been observed so far.
“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,” said University at Buffalo’s Professor Dejan Stojkovic.
“We have to think outside of the box because what has been done to find primordial black holes previously hasn’t worked.”
Professor Stojkovic and his colleague, Dr. De-Chang Dai from Case Western Reserve University and National Dong Hwa University, 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.
“Because of these long odds, we have focused on solid marks that have existed for thousands, millions or even billions of years,” Dr. Dai said.
“If the object has a liquid central core, then a captured primordial black hole can absorb the liquid core, whose density is higher than the density of the outer solid layer,” Professor Stojkovic added.
“The primordial black hole 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,” Professor Stojkovic said.
“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.”
For objects without a liquid core, primordial black holes might simply pass through and leave behind a straight micro-tunnel.
For example, a primordial black hole with a mass of 1022 grams 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 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 primordial black holes, they calculated that the probability of a primordial black hole 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,” Professor Stojkovic said.
“So the likelihood of a primordial black hole 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 primordial black hole would not tear it apart.”
“And while a primordial black hole’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.”
“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.”
The team’s paper was published in the journal Physics of the Dark Universe.
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De-Chang Dai & Dejan Stojkovic. 2024. Searching for small primordial black holes in planets, asteroids and here on Earth. Physics of the Dark Universe 46: 101662; doi: 10.1016/j.dark.2024.101662
