In 2023, the KM3NeT neutrino detector picked up an impossibly powerful signal—tens of thousands of times more energetic than anything produced by humanity’s best particle accelerators. Since then, scientists have tried to pinpoint its source, dipping into some creative—and arguably wild—explanations.
But a new theory may just take the cake for the weirdest. Some scientists are now arguing that, simply, the signal came from an ancient black hole made of cosmic strings, hiding inside a fifth dimension. So, actually, not very simple—but according to a recent paper published in Physical Review D, the rules of quantum gravity “inevitably” lead to five-dimensional primordial black holes, or super-dense, hypothetical black holes from the cosmic dawn. To be clear, the study focused on the possibility of primordial black holes emerging from this string-based framework.
However, if this interpretation is validated, it would very nicely explain what we’ve yet to understand about the 2023 signal. For instance, this neutrino—a tiny, neutral particle—wasn’t accompanied by other photons, which is why detectors as capable as KM3NeT never caught it, explained the team, which includes Dieter Lüst, a physicist at the Max Planck Institute for Physics in Germany.
The cosmic pitcher
Neutrinos are sometimes described as “ghostly” because they rarely interact with other matter, making them difficult to detect and study despite their extreme abundance. Although many things about the neutrino—including things as simple as its mass—remain undefined, astrophysicists have found that they’re incredibly useful for studying cosmic phenomena. Specifically, the energy levels of a neutrino clue scientists into the identity and characteristics of whatever flung it to Earth.
However, it’s not always easy to identify a neutrino’s cosmic pitcher, so to speak. This was precisely the case for the 2023 KM3NeT signal. This tiny particle slammed into Earth at an energy level roughly 30,000 times higher than anything that CERN’s Large Hadron Collider is capable of producing. Since then, scientists have tried to find a decent-fitting candidate for the signal’s origins. Some explanations were more common, like blazars, whereas others ventured into more theoretical (arguably strange) territory, such as the explosion of primordial black holes leaking dark electrons.
Enter the dark dimension
Unless you’re a string theorist, the latest work definitely falls into the latter category. As Harvard physicist Cumrun Vafa previously explained to Gizmodo, the “dark” dimension is a hypothetical extra dimension in string theory. According to the paper, only gravity rules over this fifth dimension, which can be as tiny as a thousandth of a millimeter and emerges as a consequence of the universe’s expansion.
The idea is that primordial black holes come from the collapse of cosmic strings in this hypothetical dimension—as opposed to the more popular interpretation that they formed via rapid inflation during the cosmic dawn. As the team admitted in the paper, to deviate from this account may seem “exotic,” but studying quantum gravity has continuously challenged what “natural” means in gravitational theories. At the very least, previous analyses have suggested that the inflationary mechanism is “in tension” with expectations of quantum gravity, the researchers wrote.
Extreme measures
So it’s worthwhile to consider alternative scenarios—like the dark dimension, which the researchers wrote “naturally explain the absence of an associated high-energy photon” in the 2023 neutrino signal. At the cosmic dawn, strings—seen here as thin cracks in spacetime—would have collected into “loops” that collapse into relatively small black holes.
According to the team’s calculations, the rate of evaporation for such objects would be “comparable” to the age of the universe. As a result, these ancient black holes may have been leaking neutrinos to the observable section of the universe, the researchers added.
The jury is still out
Experts expressed concerns about the new work to Scientific American, given its highly theoretical nature. However, physicists who also previously attributed the 2023 signal to primordial black holes were more sympathetic. For example, Andrea Thamm at the University of Massachusetts said it gave “very nice explanations for several things that we have evidence for,” whereas David Kaiser at the Massachusetts Institute of Technology noted it “still looks beautifully consistent.”
“Which doesn’t mean it’s right,” Kaiser added. “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.”
Then again, it’s not like scientists have proven beyond a reasonable doubt that the signal came from easier-to-explain astrophysical sources. Not yet. So for now, many things could be fair game—and we know the universe loves surprises.
