AMHERST, Mass. — In 2023, a subatomic particle called a neutrino crashed into Earth with such a high amount of energy that it should have been impossible. In fact, there are no known sources anywhere in the universe capable of producing such energy—100,000 times more than the highest-energy particle ever produced by the Large Hadron Collider, the world's most powerful particle accelerator. However, a team of physicists at the University of Massachusetts Amherst recently hypothesized that something like this could happen when a special kind of black hole, called a "quasi-extremal primordial black hole," explodes.
In new research published by Physical Review Letters , the team not only accounts for the otherwise impossible neutrino but shows that the elementary particle could reveal the fundamental nature of the universe.
Black holes exist, and we have a good understanding of their life cycle: an old, large star runs out of fuel, implodes in a massively powerful supernova and leaves behind an area of spacetime with such intense gravity that nothing, not even light, can escape. These black holes are incredibly heavy and are essentially stable.
But, as physicist Stephen Hawking pointed out in 1970 , another kind of black hole—a primordial black hole (PBH), could be created not by the collapse of a star, but from the universe's primordial conditions shortly after the Big Bang. PBHs exist only in theory so far, and, like standard black holes, are so massively dense that almost nothing can escape them—which is what makes them "black." However, despite their density, PBHs could be much lighter than the black holes we have so far observed. Furthermore, Hawking showed that PBHs could slowly emit particles via what is now known as "Hawking radiation" if they got hot enough.
"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 such an explosion were to be observed, it would give us a definitive catalog of all the subatomic particles in existence, including the ones we have observed, such as electrons, quarks and Higgs bosons, the ones that we have only hypothesized, like dark matter particles, as well as everything else that is, so far, entirely unknown to science. The UMass Amherst team has previously shown that such explosions could happen with surprising frequency—every decade or so—and if we were to pay attention, our current cosmos-observing instruments could register these explosions.
So far, so theoretical.
Then, in 2023, an experiment called the KM3NeT Collaboration captured that impossible neutrino—exactly the kind of evidence the UMass Amherst team hypothesized we might soon see.
But there was a hitch: A similar experiment, called IceCube, also set up to capture high-energy cosmic neutrinos, not only didn't register the event, it had never clocked anything with even one hundredth of its power. If the universe is relatively thick with PBHs, and they are exploding frequently, shouldn't we be showered in high-energy neutrinos? What can explain the discrepancy?
"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 dark charge is essentially a copy of the usual electric force as we know it, but which includes a very heavy, hypothesized version of the electron, which the team calls 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.
The team is confident that, not only can their dark-charge model PBHs explain the neutrino, it can also answer the mystery of dark matter. "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."
"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."
