Could Primordial Black Holes Be Hiding in Plain Sight?

Are Primordial Black Holes real? They could’ve formed in the unusual physics that dominated the Universe shortly after the Big Bang. The idea dates back to the 1960s, but so far, the lack of evidence makes them purely hypothetical.

If they do exist, a new paper suggests they may be hiding in places so unlikely that nobody ever thought to look there.

Black holes form when massive stars reach the end of their lives and suffer gravitational collapse. However, Primordial Black Holes (PBHs) didn’t involve stars. Physicists hypothesize that PBHs formed in the early Universe from extremely dense pockets of sub-atomic matter that collapsed directly into black holes. They could form part or all of what we call dark matter.

However, they remain hypothetical because none have been observed.

New research in Physics of the Dark Universe suggests researchers are not looking in the right places. It’s titled “Searching for small primordial black holes in planets, asteroids and here on Earth.” The co-authors are De-Chang Dai and Dejan Stojkovic, from Case Western Reserve University and the State University of New York, respectively.

The authors claim that evidence for PBHs could be found in objects as large as hollowed out planetoids or asteroids and objects as small as rocks here on Earth.

“Small primordial black holes could be captured by rocky planets or asteroids, consume their liquid cores from inside and leave hollow structures,” the authors write. “Alternatively, a fast black hole can leave a narrow tunnel in a solid object while passing through it. We could look for such micro-tunnels here on Earth in very old rocks,” the authors claim, explaining that the search wouldn’t involve specialized, expensive equipment.

The authors work leans heavily on other research suggesting that PBH masses between 10¹⁶ and 10¹⁰ solar masses could be candidates for dark matter. These PBHs could be captured by stars or trapped in their interiors upon formation. The PBH would slowly consume gas inside the stars.

However, these authors take it in a different direction. “We extend this idea to planets and asteroids, which can also be expected to host PBHs,” they write, explaining that the PBHs could be captured by these objects either during their creation or after their creation. Once inside a rocky body, the PBH would consume the liquid core, hollowing it out and leaving it empty.

“We have to think outside of the box because what has been done to find primordial black holes previously hasn’t worked.”

Dejan Stojkovic, SUNY

“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.

This figure from the research illustrates what could happen when a PBH is inside a rocky body. (A) A planet is formed around a small primordial black hole (or alternatively a planet captures a black hole in its center) (B) The central core gets slowly absorbed by the black hole. If the outer shell has a strong enough compressive strength, then the shell can support itself leading to a hollow object. (C) If the liquid core becomes solid before it is completely eaten by the black hole, there will exist an empty shell between the outer layer and central core. Image Credit: Stojkovic et al. 2024.

If the asteroid or other body suffers an impact, the PBH could escape, leaving nothing but a hollow shell behind, which could be detectable.

“If the object’s density is too low for its size, that’s a good indication it’s hollow,” Stojkovic said. Studying an object’s orbit with a telescope is enough to reveal hollowness.

Another possibility the authors present is fast-moving tiny PBHs that leave microscopic tunnels in objects. “Since the cross-section of a small PHBs is very small, a fast enough PBH will most likely create a straight tunnel after passing through the asteroid,” the authors explain. In that case, a straight tunnel through an asteroid could be evidence of a PBH.

A rapidly moving PBH could leave a straight tunnel the size of its Schwarzschild radius. If the asteroid’s composition is strong, the tunnel wouldn’t collapse immediately. Image Credit: Stojkovic et al. 2024.

PBHs could also leave microscopic tunnels in rocks and other objects on Earth. “The same effect could allow detection of a PBH here on Earth if we look for sudden appearance of narrow tunnels in metal slabs,” the authors write.

What’s different about these hypothesized PBHs is detection. In other scenarios, space telescopes, gravitational wave observatories, or even monitoring distant quasars in microwaves are required to detect them. But in this work, detection is potentially much cheaper and easier.

The James Webb Space Telescope or the Laser Interferometer Space Antenna are proposed ways of detecting PBHs. Image Credit: European Space Agency CC BY-SA 4.0

“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 Stojkovic. “We have to think outside of the box because what has been done to find primordial black holes previously hasn’t worked.”

“While our estimate gives a very small probability of finding such tunnels, looking for them does not require expensive equipment and long preparation, and the payoff might be significant,” the authors explain.

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

This is thinking outside the box, or outside the standard model in any case. Cosmology is kind of at a standstill while we wrestle with the idea of dark matter. Could PBHs be dark matter? Could they behave like the authors suggest, and be detected in this manner?

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

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