Universe Today
Humanity has worked itself into a position where we can detect a single high-energy particle from space and wonder where in Nature it came from. Billions of people likely don't care at all about such matters, but for those that are naturally curious and are fortunate enough to have the time to indulge their curiosity, an extremely energetic neutrino detected in 2023 was a remarkable event, and may even turn out to be an historic one.
The Cubic Kilometre Neutrino Telescope, or KM3NeT, detected the extremely energetic neutrino from its location on the bottom of the Mediterranean Sea. AT 220 PeV, the particle was more energetic than anything produced in our most powerful particle accelerator, the Large Hadron Collider.
The Sun emits an unceasing stream of neutrinos called solar neutrinos, but they're not very energetic. KM3-230213A, the name given to the 100 PeV neutrino, dwarfs the Sun's neutrino output. That event was one billion times more energetic than your average solar neutrino.
There's not a long list of astrophysical phenomena that could potentially juice a neutrino like this. In fact, no currently well-understood object or process can account for it.
When Nature sends us a message like this, something important is encoded in it. It's up to physicists to determine what it means. And in the intervening couple of years since its detection, different physicists have generated different explanations for it.
Explanations include pulsar-powered optical transients, gamma-ray bursts, dark matter decay, active galactic nuclei, black hole mergers, and several explanations based on different types of primordial black holes.
New research in Physical Review Letters has another explanation, and this one is based on primordial black holes, too. The research is titled "Explaining the PeV neutrino fluxes at KM3NeT and IceCube with quasiextremal primordial black holes," and the lead author is Michael Baker. Baker is an assistant professor of physics at the University of Massachusetts, Amherst.
"The KM3NeT experiment has recently observed a neutrino with an energy around 100 PeV, and IceCube has detected five neutrinos with energies above 1 PeV," the authors write. "While there are no known astrophysical sources, exploding primordial black holes could have produced these high-energy neutrinos."
Primordial black holes are entirely hypothetical. Theory says that unlike stellar-mass black holes, PBH didn't need a massive star to explode and collapse in order to form. Instead, they formed immediately after the Big Bang from dense clumps of sub-atomic matter, when the physics underlying the Universe were much different.
*There's a lot of questions about primordial black holes. It's possible that they could've helped the very first stars form, if they exist. Image Credit: NASA and G. Bacon/STSCI*
PBH are much smaller than stellar mass black holes, but they're still incrediby dense and the old adage that "nothing, not even light, can escape a black hole" still applies to them. But PBH share something else with their cousins: Hawking Radiation.
Stephen Hawking developed the idea for Hawking Radiation (HR). Basically, it says that over time HR reduces a black hole's mass, and that eventually a black hole will evaporate, unless it accretes more matter. Unfortunately, HR is normally so weak that it's well below the detection threshold of even our most capable telescopes.
This HR-inspired evaporation could be behind KM3-230213A. While it's undetectable around stellar mass black holes, the situation may be different when it comes to much lighter PBH.
"The lighter a black hole is, the hotter it should be and the more particles it will emit," said co-author Andrea Thamm, an assistant professor of physics at UMass Amherst, in a press release. "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."
As a PBH evaporates via runaway HR, they eventually experience a final burst. In their final second, they become extremely hot and suffer an explosive evaporation. This final act can produce high-energy neutrinos like KM3-230213A.
The researchers think that this could happen every decade, approximately, and that the explosions can produce a cornucopia of sub-atomic particles. They think that these PBH evaporative explosions could produce a catalogue of all of the sub-atomic particles that exist. Not just the ones we know about, like electrons and quarks, but also ones that are only hypothesized at this time, and others that that may be completely unknown.
The research team thinks that KM3-230213A could be the evidence for PBH evaporation. But there's one problem.
The IceCube Neutrino Observatory didn't detect the event, and in fact has never detected any neutrino close to being as energetic as KM3-230213A. If a PBH evaporation explosion happens every decade, shouldn't IceCube have detected at least one? IceCube has been observing for 20 years.
*This is the IceCube Neutrino Observatory in Antarctica. The neutrino detectors are attached to an array of strings that are sunk into the ice. Image Credit: By Christopher Michel - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=128003225*
The researchers say that there's an unusual type of PBH involved.
“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 researchers say that PBHs with a dark charge, which is basically a very heavy, hypothesized version of the electron, a “dark electron,” spend most of their time in a quasi-extremal state. In this state, the PBH is almost at its maximum possible charge-to-mass ratio.
IceCube and KM3NeT are tuned to different energies. IceCube is limited to 10 PeV, and that can explain why it never detected KM3-230213A.
For PBH in the quasi-extremal state, "the neutrino emission at 1 PeV may be more suppressed than at 100 PeV," the authors explain. "The burst rates implied by the KM3NeT and IceCube observations and the indirect constraints can then all be consistent at 1σ."
For study co-author Baker, the added complexity of dark charge PBH adds more veracity to their explanation.
“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.”
To non-physicists, physics can seem like a puzzling, cryptozoological world full of strange particles on strange journeys from strange sources. But these particles all add up to the cosmos we see around us. And answers to some of our biggest questions reside in this strange world.
We know dark matter exists, or we think we know, because there's so much evidence of missing mass. But we simply don't know what it is. There's growing confidence in PBH as a candidate for dark matter, and this research adds to that.
“If our hypothesized dark charge is true,” said 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.”
Will this neutrino be a part of our answer to the dark matter question? Will it help physicists fill in the blanks when it comes to our patchy understanding of the cosmos?
“Observing the high-energy neutrino was an incredible event,” Baker said. “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.”
