Scientists saw a very weird particle. Some say it came from an exploding, five-dimensional black hole
A bizarre neutrino that crashed into Earth in February 2023 is gaining no shortage of equally bizarre explanations.
About 35 times more energetic than any neutrino seen before, it was picked up by the under-construction Cubic Kilometer Neutrino Telescope (KM3NeT) detector off the coast of Sicily. Some theorists have guessed it came from a well-known source of extreme astrophysical mayhem, a type of highly active galaxy known as a blazar, among other easy to parse explanations. Another group of scientists, however, has proposed it came from something far stranger: a tiny black hole exploding on the outskirts of the solar system.
This would have been a primordial black hole (PBH), a hypothesized object formed in the universe’s earliest moments. Like all black holes, PBHs would evaporate over time by emitting Hawking radiation, gradually losing mass until finally exploding in a flash of radiation and neutrinos; unlike other black holes, PBHs would be so low-mass that their last-gasp outbursts could be happening now rather than untold eons in the future.
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A new paper leveraging the arcane tenets of string theory has taken this outlandish idea even further. Published in the journal Physical Review D on September 28, it suggests that the KM3NeT event wasn’t caused by just any PBH but one initially formed from light-years-long cracks in spacetime called cosmic strings and cloaked inside a fifth dimension.
Dieter Lüst of the Max Planck Institute for Physics in Munich, a co-author on the paper, says this highly speculative scenario would “completely resolve this puzzle” of why KM3NeT’s neutrino wasn’t accompanied by any photons, which should have been detected by other instruments but weren’t. If the exploding PBH sat inside this fifth dimension, Lüst says, the physics would dictate that only neutrinos could escape into our more familiar universe.
This sort of fifth dimension isn’t a novel idea. It’s sometimes called a “dark dimension” because it could potentially form as a side effect of dark energy, the mysterious force driving the universe’s accelerating expansion. You can imagine it as “an extra circle” about one micron wide, Lüst says, that’s curled up atop every point of our three-dimensional universe, plus time. “The reason we do not see it is that we live on a one-plus-three-dimensional brane, on which all other interactions—like electromagnetism—take place. The only force which can penetrate the fifth dimension is gravity.”
Cumrun Vafa, a string theorist at Harvard University, says the idea of a fifth dimension is “not only plausible; it is derived, in some sense, with minimal assumptions from string theory ideas,” which he described in a paper posted as a preprint in 2022 and published in 2023. “And it turns out when the size of the black holes are smaller than a micron, they can sit inside other points in the extra dimension,” he says.
The idea is understandably controversial. “We don’t know that there is a fifth dimension,” says Dan Hooper, a cosmologist at the University of Wisconsin–Madison, who has studied PBHs. Lüst, Vafa and other dark-dimension-touting string theorists “seem to be making some very strong statements that I wouldn’t be able to defend.”
Hooper and other cosmologists have suggested that PBHs are a natural outcome of conditions in the early universe, such as inflation—a sudden exponential increase in cosmic expansion that is thought to have occurred shortly after the big bang. Such processes could create overdensities—regions so dense that spacetime collapsed in on itself, forming PBHs of a range of masses. The smallest of these would have evaporated quickly, disappearing from the cosmos long ago. But more massive PBHs would last longer, with those roughly the mass of an asteroid enduring to evaporate in our current cosmic epoch.
Last year Alexandra Klipfel and David Kaiser of the Massachusetts Institute of Technology proposed that a PBH expiring within 2,000 astronomical units (2,000 times the Earth-sun distance) of our solar system could have produced the neutrino picked up by KM3NeT. As other ideas have failed to fully explain the event, the wild concept “still looks really beautifully consistent,” Kaiser says, “which doesn’t mean it’s right. But it is using some really off-the-shelf, more typical elements that are woven into many parts of our understanding of black hole physics.”
Lüst and his colleagues’ proposal, meanwhile, requires slightly more stretching of the imagination to be plausible. In their model, the PBH wouldn’t have only been sitting inside this fifth dimension; it also might have initially formed not through inflation-induced overdensities but from collapsing cosmic strings, theoretical cracks in the fabric of spacetime that may have manifested in the very early universe. “Around these cosmic strings, the densities are high enough to form primordial black holes,” Lüst says.
The existence of PBHs alone is “a lot to hang on a single neutrino,” says Ignacio Taboada, an astrophysicist at the Georgia Institute of Technology. “And then you add the fifth dimension, and it gets even more convoluted. Is it plausible? Yes, it is. It’s just that the evidence for that is so tenuous.” Taboada instead favors a more conventional (and somewhat catch-all) scenario in which KM3NeT fortuitously caught an especially energetic “cosmogenic” neutrino produced from high-energy cosmic rays smashing into particles and atoms across the universe. “We know the cosmic ray background exists,” he says, “so the particle physics is not that difficult.”
One reason some theorists prefer the more contrived PBH-sourced scenario is that PBHs might also explain dark matter, the invisible stuff that serves as gravitational glue for most galaxies. PBHs could account for some—or even all—of the dark matter in the universe, replacing or supplementing other candidates such as weakly interacting massive particles (WIMPs) and axions. Remarkably, the existence of a fifth dimension could boost the chances for PBH-based dark matter, Hooper says, because it would allow the microscopic black holes to evaporate more slowly and thus occupy a larger range of masses in the modern-day universe. A fifth dimension gives “more opportunity to make dark matter with black holes,” he says.
PBHs could help resolve other enigmas, too, for instance by acting as seeds for the rapid growth of supermassive black holes in the early universe. “They give very nice explanations for several things that we have evidence for,” says Andrea Thamm, a theoretical physicist at the University of Massachusetts Amherst. So far, however, we “do not know [PBHs] exist” or even exactly the way in which they would have formed.
Another confusing aspect of KM3NeT’s neutrino was that the more capable IceCube Neutrino Observatory detector in Antarctica should have picked up something, too—but it didn’t. “It was really surprising because [KM3NeT’s event] was so much higher in energy than IceCube had ever seen,” says Erin O’Sullivan, a neutrino astrophysicist at Uppsala University in Sweden and spokesperson for IceCube. In early 2023 IceCube’s mesh of detectors in the Antarctic ice spanned an area some 20 times larger than KM3NeT’s submerged detectors and had been operating 10 times longer, since 2011. Work released in September, however, showed no accompanying high-energy neutrinos in IceCube’s data at the same time as KM3NeT, and the Antarctic facility has picked up no neutrinos of comparable energy in its 15 years of operations.
Even so, scientists are fairly confident KM3NeT’s detection was a real neutrino; an instrument error or miscalculation is “very unlikely,” Taboada says.
A nearby PBH, five-dimensional or not, could provide an answer by flinging a neutrino at just the right orientation to hit KM3NeT’s detectors but not IceCube’s. Other searches to find PBHs near and far, however, have so far been unsuccessful. In May, for example, astronomers suggested that a lunar-mass PBH might have been spotted drifting through the halo of our Milky Way, but follow-up work suggested the sighting was a variable star, not a black hole.
Finding additional high-energy neutrinos could provide some answers, and efforts to do so are under way. The Radio Neutrino Observatory is currently under construction in Greenland, which will expand the search by using radio signals instead of optical light, which IceCube utilizes, to look for incoming neutrinos. A proposed second-generation IceCube detector, meanwhile, would similarly use radio and span an area of 500 square kilometers compared with IceCube’s current single cubic kilometer. “You can put the stations much further apart” using radio, O’Sullivan says, and “catch these higher energies.”
For now, KM3NeT’s neutrino has plenty of explanations that are easier to countenance than black holes. “I don’t think there’s any reason it has to be something exotic,” Hooper says. “We know there are high-energy cosmic rays in our universe that will sometimes collide with gas or radiation and make neutrinos,” as well as astrophysical sources, such as blazars, that could produce them, too. Until such ideas can be confirmed, though, five-dimensional primordial black holes formed by cosmic strings remain a possibility, however fantastical they might sound.