A theoretical analysis indicates that small black holes formed during the early universe might have resulted in vacant planetoids and tiny tunnels, leading researchers to recommend a search within rocks and ancient structures for evidence of these phenomena. The study emphasizes that we should consider both large and small possibilities to verify the existence of primordial black holes, proposing that their evidence could manifest in sizes ranging from substantial hollow planetoids in space to tiny microscopic tunnels in everyday terrestrial materials such as rocks, metal, and glass.
When envisioning a black hole’s formation, many imagine a massive star exhausting its fuel and collapsing. However, the tumultuous conditions of the early universe might have facilitated the creation of numerous small black holes well before the appearance of the first stars.
For decades, scientists have proposed the existence of primordial black holes, which could potentially constitute the elusive dark matter, the invisible substance that makes up 85% of the universe’s mass.
Yet, until now, no primordial black hole has been observed.
A recent study co-led by the University at Buffalo advocates for exploring both large and tiny indicators of their existence, asserting that evidence might be as grand as hollow planetoids in space or as small as minute tunnels left in common Earth materials like rocks, metal, and glass.
The study, set for publication in the December issue of Physics of the Dark Universe and currently available online, suggests that a primordial black hole that becomes ensnared within a substantial rocky body in the cosmos could consume its liquid inner core, resulting in a hollow cavern. Additionally, a swiftly moving primordial black hole could create straight tunnels, visible under a microscope, when passing through solid materials, including those on Earth.
“While the chances of discovering these signs are slim, the search does not require extensive resources and the potential result — the first evidence of a primordial black hole — could be extraordinary,” explains co-author Dejan Stojkovic, PhD, a professor of physics in the UB College of Arts and Sciences. “We need to think innovatively because previous attempts to find primordial black holes have failed.”
The research calculated how large a hollow planetoid could be without collapsing and the likelihood of a primordial black hole moving through Earth-based objects. (If you’re concerned about a primordial black hole passing through you, there’s no need for alarm; the study concluded it wouldn’t be harmful.)
“Given these slim odds, we focused on solid remnants that have existed for thousands, millions, or even billions of years,” adds co-author De-Chang Dai, PhD, from National Dong Hwa University and Case Western Reserve University.
Stojkovic’s research received support from the National Science Foundation, while Dai’s work was backed by the National Science and Technology Council (Taiwan).
Hollow objects could have a maximum size of one-tenth of Earth
In the aftermath of the Big Bang, sections of space may have been denser than their surroundings, creating conditions ripe for the formation of primordial black holes (PBHs).
Though PBHs would possess significantly less mass than the stellar black holes generated by dying stars, they would still exhibit extreme density, akin to compressing the mass of a mountain into the size of an atom.
Stojkovic pondered whether a PBH might become trapped inside a planet, moon, or asteroid during or after its formation.
“If the object has a liquid core, a captured PBH can absorb that liquid, as its density is greater than that of the outer solid layer,” Stojkovic states.
The black hole could potentially exit the object if it were struck by an asteroid, leaving behind merely a hollow shell.
But how structurally sound would this shell be? Would it maintain its integrity or collapse under its weight? By comparing the strength of natural materials like granite and iron to surface tension and density, the researchers concluded that such a hollow formation could not exceed one-tenth of Earth’s radius; a minor planet is thus more likely than a full-fledged planet.
“If it exceeds that size, it will likely collapse,” Stojkovic notes.
These hollow bodies could be detectable using telescopes. An object’s mass and density can be examined through its orbital patterns.
“If an object is less dense than its size would suggest, that could indicate it’s hollow,” Stojkovic clarifies.
Common objects may serve as black hole detectors
In objects lacking a liquid core, primordial black holes could simply pass through, leaving behind straight tunnels, according to the study. For instance, a PBH weighing 1022 grams (a one followed by 22 zeros) could create a tunnel just 0.1 microns in thickness.
A large slab of metal or similar material could act as a useful black hole detector, monitored for the sudden appearance of these tunnels, though Stojkovic suggests targeting existing tunnels in very ancient materials is preferable — from centuries-old buildings to billion-year-old rocks.
Despite this, even if dark matter comprises PBHs, the researchers assessed that the chance of a PBH passing through a billion-year-old rock is approximately 0.000001.
“One must weigh the costs against the benefits. Is it costly to pursue this? Not really,” Stojkovic remarks.
Thus, the likelihood of a PBH crossing your path during your lifetime is very low. Even if it did, you likely wouldn’t be aware of it.
Compared to rocks, human tissue bears minimal tension, meaning a PBH passing through would not tear it apart. Even though a PBH holds immense kinetic energy, it cannot release much during a collision due to its rapid movement.
“If a projectile travels through a medium faster than sound, the medium’s molecular structure doesn’t have time to respond,” Stojkovic points out. “For example, a rock thrown through a window may shatter it, but a bullet likely just leaves a hole.”
New theoretical frameworks are essential
Stojkovic emphasizes the importance of theoretical research like this one, noting that many concepts previously deemed improbable are now considered plausible.
The field, he adds, grapples with significant challenges, including the dark matter enigma. Its most recent significant shifts — quantum mechanics and general relativity — date back a century.
“The brightest minds have been tackling these problems for 80 years with no resolution,” he states. “We need more than an extension of current models; we likely require an entirely new framework.”
