In 2023, scientists detected a subatomic particle called a neutrino hitting Earth with an energy level so extreme it seemed impossible. No known cosmic process can generate that much energy. The particle carried about 100,000 times more energy than anything ever produced by the Large Hadron Collider, the most powerful particle accelerator on Earth.
Now, physicists at the University of Massachusetts Amherst think they may have found an explanation. Their idea involves the explosive death of a rare type of black hole known as a “quasi-extremal primordial black hole.”
A Clue to the Universe’s Deepest Secrets
In a study published in Physical Review Letters, the researchers show how such an event could produce a neutrino with this extraordinary energy. They also suggest that this single particle could provide insight into the fundamental structure of the universe.
What Are Primordial Black Holes?
Scientists already understand how typical black holes form. When a massive star runs out of fuel, it collapses in a powerful supernova and leaves behind an object with gravity so strong that nothing, not even light, can escape. These black holes are extremely massive and generally stable.
But in 1970, physicist Stephen Hawking proposed another possibility. He suggested that black holes could also form in the early universe, shortly after the Big Bang. These are called primordial black holes (PBHs). They have not yet been directly observed, but they are predicted by theory. Like ordinary black holes, they are incredibly dense, but they could be much smaller in mass.
Hawking also showed that black holes are not completely silent. If they become hot enough, they can emit particles through a process now known as Hawking radiation.
Hawking Radiation and Black Hole Explosions
“The lighter a black hole is, the hotter it should be and the more particles it will emit,” says Andrea Thamm, co-author of the new research and assistant professor of physics at UMass Amherst. “As PBHs evaporate, they become ever lighter, and so hotter, emitting even more radiation in a runaway process until explosion. It’s that Hawking radiation that our telescopes can detect.”
If scientists were able to observe one of these explosions, it could reveal all types of fundamental particles. This would include known particles like electrons, quarks and Higgs bosons, as well as hypothetical ones such as dark matter particles, and possibly entirely new forms of matter.
Previous work by the UMass Amherst team suggests that these explosions might occur more often than expected, perhaps once every decade. With current instruments, it may already be possible to detect them.
From Theory to Observation
Until recently, this idea remained purely theoretical.
Then in 2023, the KM3NeT Collaboration detected the extremely energetic neutrino. The observation matched the kind of signal the researchers had predicted.
A Puzzle Between Two Experiments
However, the discovery raised a new question. Another major experiment, IceCube, which is also designed to detect high-energy neutrinos, did not record anything similar. In fact, it has never observed a neutrino with even a fraction of that energy.
If primordial black holes are common and frequently exploding, why are such events not seen more often? This inconsistency needed an explanation.
The Role of “Dark Charge”
“We think that PBHs with a ‘dark charge’ — what we call quasi-extremal PBHs — are the missing link,” says Joaquim Iguaz Juan, a postdoctoral researcher in physics at UMass Amherst and one of the paper’s co-authors.
The proposed “dark charge” behaves somewhat like the familiar electric force, but it includes a much heavier version of the electron, referred to as a “dark electron.”
“There are other, simpler models of PBHs out there,” says Michael Baker, co-author and an assistant professor of physics at UMass Amherst; “our dark-charge model is more complex, which means it may provide a more accurate model of reality. What’s so cool is to see that our model can explain this otherwise unexplainable phenomenon.”
“A PBH with a dark charge,” adds Thamm, “has unique properties and behaves in ways that are different from other, simpler PBH models. We have shown that this can provide an explanation of all of the seemingly inconsistent experimental data.”
Could This Explain Dark Matter?
The researchers believe their model could do more than explain a single unusual neutrino. It may also help solve one of the biggest mysteries in physics.
“Observations of galaxies and the cosmic microwave background suggest that some kind of dark matter exists,” says Baker.
“If our hypothesized dark charge is true,” adds Iguaz Juan, “then we believe there could be a significant population of PBHs, which would be consistent with other astrophysical observations, and account for all the missing dark matter in the universe.”
A New Window on the Universe
“Observing the high-energy neutrino was an incredible event,” Baker concludes. “It gave us a new window on the universe. But we could now be on the cusp of experimentally verifying Hawking radiation, obtaining evidence for both primordial black holes and new particles beyond the Standard Model, and explaining the mystery of dark matter.”