Impossibly powerful 'ghost particle' that slammed into Earth may have come from an exploding black hole — and it could upend both particle physics and cosmology

An impossibly powerful "ghost particle" that recently slammed into Earth may have come from a rare type of exploding black hole, researchers claim.

If true, the extraordinary event may prove a theory that could upend our understanding of both particle physics and dark matter, the team argues. However, this is just one theory, and there is no direct evidence to confirm that this is indeed what happened.

In early 2023, researchers at the Cubic Kilometre Neutrino Telescope (KM3NeT) — a massive, newly constructed array of sensors at the bottom of the Mediterranean Sea — detected a neutrino, a ghostly particle that has almost no mass and does not readily interact with most matter.

In addition to neutrinos' typical weirdness, this specific particle was noteworthy for its unusual intensity. It hit our planet with an estimated energy of up to 220 quadrillion electron volts, which is at least 100 times more powerful than any other neutrino detected to date and around 100,000 times greater than anything observed within human-made particle accelerators, like CERN's Large Hadron Collider.

Explaining the impossible

Researchers were initially unsure what caused this "impossible" neutrino to appear. It may have been birthed when a cosmic ray entered Earth's atmosphere, unleashing a cascade of high-energy particles that rained down on the planet's surface. However, its unprecedented power led experts to assume that it must have originated from some high-energy cosmic event that we don't fully understand.

In the new paper, which has been accepted for publication in the journal Physical Review Letters, one research group believes they have finally identified what really birthed the neutrino: an exploding, primordial black hole (PBH).

PBHs are a hypothetical class of black holes that are extremely small — potentially ranging from the size of an atom to a pinhead — and likely date back to the first moments after the Big Bang. The concept was first popularized by British physicist Stephen Hawking in the early 1970s, who also hinted that these miniature singularities would emit large quantities of high-energy particles, dubbed Hawking radiation, as they slowly evaporated. In theory, this would also mean they have the capacity to explode.

"The lighter a black hole is, the hotter it should be and the more particles it will emit," study co-author Andrea Thamm, a theoretical physicist at the University of Massachusetts Amherst, said in a statement. "As PBHs evaporate, they become ever lighter, and so hotter, emitting even more radiation in a runaway process until explosion."

One of the biggest mysteries surrounding the impossible neutrino, aside from its immense power, is that it was not observed by other neutrino detectors around the world, such as the IceCube Neutrino Observatory buried beneath Antarctica's icy surface. Given that PBHs are supposed to be fairly common throughout the universe, one would reasonably expect that similarly powerful particles also would have been detected before or since this possible discovery, especially as the number of neutrino detectors is quickly increasing.

The researchers said this is because the neutrino was emitted by a special type of PBH, dubbed a quasi-extremal PBH, which has a "dark charge" — a version of regular electric force that includes a very heavy, hypothesized version of the electron dubbed a "dark electron."

The dark properties of this theoretical type of PBH make it less likely that these black holes' explosions would be detected, the researchers suggested. It may also be that some of the less-powerful neutrinos detected to date may be partially incomplete detections of these events, they added.

"A PBH with a dark charge has unique properties and behaves in ways that are different from other, simpler PBH models," Thamm said. "We have shown that this can provide an explanation of all of the seemingly inconsistent experimental data."

Upending cosmic understanding

While the new research hints at the existence of quasi-extremal PBHs, it does not confirm them or prove that they explode as the researchers think. (Regular PBHs have never been directly observed, either, although there is a strong consensus that they exist.)

However, the team is confident that it will not take long to prove these dark explosions are real. The same research group recently predicted that there is a 90% chance we will see the first quasi-extremal PBH blow up by 2035, which would be extremely exciting for two main reasons.

First, these explosions would be so powerful that they would probably emit "a definitive catalog of all the subatomic particles in existence," including known entities, like the Higgs boson; theorized particles, like gravitons or time-traveling tachyons; and "everything else that is, so far, entirely unknown to science," the researchers wrote in the statement.

Second, these black holes could help reveal the mysterious identity of dark matter — the invisible stuff that we cannot see, yet whose gravitational force we can detect within almost every observed galaxy, including the Milky Way. The researchers wrote that quasi-extremal PBHs "could constitute all of the observed dark matter in the universe," so finding one could help put this mystery to bed. (Despite the similar names, dark matter is not directly related to dark charge or dark electrons.)

The researchers, along with several other teams in the fields of physics and cosmology, are now holding their collective breath to see when the first explosion might be detected.

This "incredible event" would provide a "new window on the universe" and help us "explain this otherwise unexplainable phenomenon," study lead author Michael Baker, a theoretical physicist at UMass Amherst, said in the statement.