In 2023, scientists detected subatomic particles called neutrinos hitting Earth at energy levels that seemed impossible. No known cosmic process can generate so much energy. The particles 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 ideas involve the explosive death of a rare type of black hole known as a “subextremal primordial black hole.”
Clues to the deepest secrets of the universe
In a study published in physical review letterresearchers show how such an event can produce neutrinos with this extraordinary energy. They also suggest that this single particle may provide insight into the fundamental structure of the universe.
What is a primordial black hole?
Scientists already understand how a typical black hole forms. When a giant star runs out of fuel, it goes into a powerful supernova and collapses, leaving behind an object whose gravity is so strong that not even light can escape. These black holes are very 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, just after the Big Bang. These are called primordial black holes (PBHs). Although they have not yet been directly observed, they are predicted by theory. Like regular black holes, they are incredibly dense, but they can have much less mass.
Hawking also showed that black holes are not completely silent. When it gets hot enough, it can emit particles through a process now known as Hawking radiation.
Hawking radiation and black hole explosion
“The lighter the black hole, the hotter it gets, the more particles it should emit,” says Andrea Tam, a co-author of the new study and an assistant professor of physics at the University of Massachusetts Amherst. “As the PBH evaporates, it becomes lighter and lighter, becomes very hot, and releases even more radiation in the runaway process leading up to its explosion. It’s that Hawking radiation that our telescope can detect.”
If scientists could observe one of these explosions, it could reveal all sorts of fundamental particles. This could include not only known particles such as electrons, quarks, and the Higgs boson, but also hypothetical particles such as dark matter particles, and even entirely new forms of matter.
Previous research by a team at the University of Massachusetts Amherst suggests that such explosions may occur more frequently than expected, perhaps once every 10 years. Current equipment may already be able to detect them.
From theory to observation
Until recently, this idea was purely theoretical.
And in 2023, the KM3NeT collaboration detected a very high-energy neutrino. This observation matched the type of signal the researchers expected.
Puzzle between two experiments
But this discovery raises new questions. Another major experiment, IceCube, also designed to detect high-energy neutrinos, did not record anything similar. In fact, we have never observed a neutrino with even a fraction of that energy.
If primordial black holes are common and explode frequently, why don’t we see such events more often? This discrepancy needed an explanation.
The role of “dark charge”
“We think that PBHs with ‘dark charges’, or what we call quasi-extreme PBHs, are the missing link,” says Joaquín Iguazu Juan, a postdoctoral fellow in physics at the University of Massachusetts Amherst and one of the paper’s co-authors.
The proposed “dark charge” behaves similar to the well-known electric force, but contains heavier versions of electrons called “dark electrons.”
“There are other simpler PBH models,” says co-author Michael Baker, an assistant professor of physics at the University of Massachusetts Amherst. “Our dark charge model is more complex, which means it potentially provides a more accurate model of reality. What’s so great is that our model can explain this unexplainable phenomenon.”
“Dark-charged PBHs have unique properties and behavior that differ from other simple PBH models, and we have shown that this can explain all of the seemingly contradictory experimental data,” adds Thamm.
Could this explain dark matter?
The researchers believe their model can 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 form of dark matter exists,” Baker said.
Iguazu-Juan added: “If our hypothesis of dark charge is true, there could be significant amounts of PBH, which is consistent with other astrophysical observations and could account for all the dark matter missing in the universe.”
new window on the universe
“The observation of high-energy neutrinos was an incredible event,” Baker concluded. “It has given us a new window into the universe. But we can now experimentally verify Hawking radiation, have evidence of both primordial black holes and new particles beyond the Standard Model, and be on the brink of explaining the mystery of dark matter.”
