A new study by researchers from the University at Buffalo suggests that small black holes formed in the early universe might have left behind hollow planetoids and microscopic tunnels.
An illustration of small primordial black holes. In reality, such tiny black holes would have a difficult time forming the accretion disks that make them visible here. Image Credit: NASA
Imagine the process of a black hole forming, and you likely picture a massive star exhausting its fuel and collapsing under its own gravity. However, the chaotic conditions of the early universe might have allowed many smaller black holes to form long before stars even existed.
The concept of primordial black holes has been theorized for decades. These black holes are even speculated to be dark matter, the mysterious substance that makes up 85 % of the universe’s mass. Yet, no primordial black hole has ever been observed.
New research, co-led by the University at Buffalo, proposes a dual approach to uncovering their existence, suggesting their signatures could range from large-scale phenomena, like hollow planetoids in space, to minute traces, such as microscopic tunnels in materials like rocks, metal, and glass found on Earth.
This theoretical study, set to be published in the December issue of Physics of the Dark Universe and currently available online, suggests that a primordial black hole trapped inside a large rocky object in space could consume the object’s liquid core, leaving it hollow. Alternatively, a faster-moving primordial black hole might pass through solid material, leaving behind straight microscopic tunnels that could be detected in everyday materials.
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. We have to think outside of the box because what has been done to find primordial black holes previously has not worked.
Dejan Stojkovic, Professor and Study Co-Author, College of Arts and Sciences, University at Buffalo
The study explores how large a hollow planetoid could be without collapsing under its own weight and calculates the likelihood of a primordial black hole passing through a solid object on Earth. For those concerned, the researchers concluded that such an event would not be harmful to humans.
Because of these long odds, we have focused on solid marks that have existed for thousands, millions, and even billions of years.
De-Chang Dai, PhD, Study Co-Author, National Dong Hwa University
Hollow Objects Could Be No Bigger Than 1/10 of Earth
As the universe expanded rapidly following the Big Bang, certain dense regions of space may have collapsed into primordial black holes (PBHs). These black holes would have far less mass than those formed later by collapsing stars but would still be extraordinarily dense, packing the mass of a mountain into an atom-sized area.
Stojkovic speculated whether a PBH could become trapped inside a planet, moon, or asteroid 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 said.
If an asteroid struck the object and only a hollow shell remained, the PBH might then escape.
If such a PBH escaped, perhaps due to an asteroid impact, it would leave behind a hollow shell. The researchers calculated that this shell could only sustain itself if it were no larger than one-tenth the radius of Earth, making it more likely to be a small planetary body rather than a full-sized planet.
“If it is any bigger than that, it is going to collapse,” Stojkovic said.
These hollow objects could potentially be detected using telescopes by analyzing their mass and density.
“If the object’s density is too low for its size, that is a good indication it is hollow,” Stojkovic added.
Everyday Objects Could Be Black Hole Detectors
For objects lacking a liquid core, the study suggests that PBHs might pass straight through, leaving behind a linear tunnel. For instance, a PBH with a mass of 1022 grams — equivalent to one followed by 22 zeros—could create a tunnel approximately 0.1 microns in diameter.
A large slab of metal or similar material could potentially function as an effective detector for these black holes by being observed for the sudden formation of such tunnels. However, according to Stojkovic, the chances of success are higher when examining pre-existing tunnels in extremely old materials, such as centuries-old buildings or rocks that date back billions of years.
Even so, the researchers calculated that, if dark matter is indeed composed of PBHs, the likelihood of a PBH passing through a billion-year-old boulder is extraordinarily low, approximately 0.000001.
“You have to look at the cost versus the benefit. Does it cost much to do this? No, it does not,” Stojkovic said.
The likelihood of a PBH passing through you during your lifetime is exceedingly small. Even in the rare event that it did, you likely would not notice it.
Unlike solid materials such as rock, human tissue has minimal tension, meaning a PBH would not cause significant damage or tear it apart. Additionally, while PBHs possess immense kinetic energy, their incredibly high speed prevents them from releasing much of that energy during a collision.
If a projectile is moving through a medium faster than the speed of sound, the medium’s molecular structure does not have time to respond. Throw a rock through a window, it is likely going to shatter. Shoot a window with a gun, it is likely to just leave a hole.
Dejan Stojkovic, Professor and Study Co-Author, College of Arts and Sciences, University at Buffalo
New Theoretical Frameworks Needed
Theoretical studies like this are essential, Stojkovic emphasizes, pointing out that many physical concepts once deemed implausible are now widely accepted.
Stojkovic also notes that the field is grappling with significant challenges, including the mystery of dark matter. The last groundbreaking advancements — quantum mechanics and general relativity — date back over a century.
Stojkovic concluded, “The smartest people on the planet have been working on these problems for 80 years and have not solved them yet. We do not need a straightforward extension of the existing models. We probably need a completely new framework altogether.”
Journal Reference:
Dai, D.-C., et al. (2024) Searching for small primordial black holes in planets, asteroids and here on Earth. Physics of the Dark Universe. doi.org/10.1016/j.dark.2024.101662.