In astrophysics, extreme events may call for extreme interpretations. Sometimes, that means weighing every possible option for what something could have been—or what it could explain.
In 2023, a detector buried off the Mediterranean Sea spotted an impossibly powerful neutrino signal—tens of thousands of times more energetic than anything produced by humanity’s most powerful particle accelerators. But the signal raised more questions than answers, particularly with regard to its origins. Now, one team offers an ambitious solution: the explosion of primordial black holes leaking dark electrons.
A Physical Review Letters paper on the proposal, set for publication on February 10, is currently available as a preprint on arXiv.
“At the moment, no one knows what actually caused this neutrino—our proposal is one possibility,” Andrea Thamm, study senior author and a particle physicist at the University of Massachusetts Amherst, told Gizmodo. “With time we may observe more highly energetic particles—or not—and this will inform whether our proposal is right.”
The most powerful “ghost particle”
Trillions of neutrinos—nearly massless, neutrally charged particles—pass through us every second, but we only acknowledge their existence when these so-called “ghost particles” slam against the many giant neutrino detectors on Earth.
In February 2023, a neutrino of undefined origins entered the detection range of the European neutrino facility KM3NeT, located off the coast of Malta in the Mediterranean. The tiny particle’s energy level was unfathomably large—roughly 30,000 times higher than any particle produced by CERN’s Large Hadron Collider, the world’s most powerful accelerator.
“It was not expected that such a high-energy neutrino would be seen, and there were no known astrophysical sources,” Thamm noted.
Indeed, the signal was an enigma to physicists for a variety of reasons. For one, the neutrino only appeared to KM3NeT, but not to experiments like IceCube. In fact, the similarly capable detector “not only didn’t register the event [but] had never clocked anything with even one hundredth of its power,” the researchers explained in a statement.
To see the invisible, try the impossible
Thamm and her colleagues believe the answer could lie in the quirky characteristics of primordial black holes—hypothetical black holes born from the Big Bang as opposed to a dying star. Astronomers have yet to actually spot any, although they suspect such ancient black holes would be “featherweight” entities with masses similar to Earth’s.
“As Stephen Hawking pointed out in the 1970s, black holes radiate [a phenomenon referred to as Hawking radiation] and thereby lose mass,” Thamm explained to Gizmodo. The mass of a black hole is inversely proportional to its temperature, so lighter primordial black holes would heat up and radiate even more and lose mass faster than standard black holes, she added.
But the new research isn’t interested in any primordial black hole. Instead, it considers the feasibility of a “quasi-extremal primordial black hole.” According to the paper, this special type of black hole has its Hawking radiation suppressed by the unseen mass of “dark electrons,” a much heavier—yet hypothetical—counterpart of regular electrons.
Eventually, however, the dark electric field around the black hole grows so powerful that even the heavier dark electrons start leaking from the black hole. When that happens, the black hole loses its (dark) charge very rapidly, leading to an enormous explosion lasting mere seconds, Thamm explained to Gizmodo.
What this means is that the explosion only emits neutrinos within a specific range of energy levels. And if that coincided with energy levels typically captured by IceCube, it could explain why the 2023 signal only showed up in KM3NeT’s radars, the paper explained.
The truth remains in the dark
The model entertains an interesting bunch of ideas but admittedly relies on many hypothetical assumptions. As Thamm notes, this is just one of many competing accounts for the origin of the ultra-powerful neutrino. For the foreseeable future, physicists will be comparing notes to arrive at an acceptable conclusion.
“While we are very excited about the physics in our paper, this doesn’t mean that it is definitely the correct explanation of the origin of the neutrino,” she said. “More theoretical analysis and experimental data will be needed to tell which one is correct.”