The final breath of an ancient black hole might be the origin of the most energetic “phantom particle” discovered thus far, a recent MIT investigation suggests.
In a study published today in Physical Review Letters, MIT scientists present a compelling theoretical argument indicating that a newly detected, highly energetic neutrino could have originated from an ancient black hole detonating outside our solar system.
Neutrinos are often dubbed ghost particles due to their elusive yet ubiquitous nature: They are the most common particle type in the universe, yet they leave minimal evidence. Researchers recently detected hints of a neutrino with the highest energy ever recorded, although the origin of this exceptionally potent particle remains unverified.
The MIT scholars posit that this enigmatic neutrino may have emerged from the inevitable detonation of an ancient black hole. Primordial black holes (PBHs) are hypothetical black holes that are miniature versions of the considerably more substantial black holes found at the heart of most galaxies. PBHs are believed to have formed in the initial moments following the Big Bang. Some researchers assert that primordial black holes could account for a majority or even all of the dark matter present in the universe today.
Like their larger counterparts, PBHs are expected to emit energy and diminish in size over time due to a mechanism known as Hawking radiation, predicted by physicist Stephen Hawking. The more a black hole radiates, the hotter it becomes and the greater number of high-energy particles it releases. This chain reaction is anticipated to result in a violently explosive event of the most energetic particles just prior to a black hole’s complete evaporation.
The MIT scientists calculate that if PBHs constitute a significant portion of the dark matter in the universe, then a small subset of them would be experiencing their final explosions at this very moment throughout the Milky Way galaxy. Moreover, there is a statistically relevant likelihood that such an explosion could have taken place relatively near our solar system. The explosion would have emitted a surge of high-energy particles, including neutrinos, one of which could have had a strong chance of impacting a detector on Earth.
If this scenario indeed unfolded, the recent identification of the highest-energy neutrino would mark the first detection of Hawking radiation, which has long been theorized but never directly observed from any black hole. Furthermore, this event could suggest the existence of primordial black holes and their potential role in comprising a significant fraction of dark matter — a mysterious entity that makes up 85 percent of the universe’s total matter, with its true nature remaining elusive.
“We’ve uncovered a scenario where everything aligns, and we can demonstrate that most of the dark matter [in this context] comprises primordial black holes, while also potentially generating these high-energy neutrinos from a nearby PBH explosion,” states lead author Alexandra Klipfel, a graduate student in MIT’s Department of Physics. “It’s something we can now pursue and validate with ongoing experiments.”
The study’s other co-author is David Kaiser, professor of physics and the Germeshausen Professor of the History of Science at MIT.
High-energy dilemma
In February, scientists at the Cubic Kilometer Neutrino Telescope, or KM3NeT, announced the discovery of the highest-energy neutrino recorded so far. KM3NeT is a large-scale underwater neutrino detector positioned at the bottom of the Mediterranean Sea, where the setting is designed to dampen the effects of any particles except for neutrinos.
The scientists monitoring the detector captured signatures of a passing neutrino with an energy exceeding 100 peta-electron-volts. One peta-electron volt is equal to the energy of 1 quadrillion electron volts.
“This energy is extraordinarily high, quite beyond anything humans can achieve for particle acceleration,” Klipfel mentions. “Consensus on the origin of such high-energy particles is scarce.”
Similarly high-energy neutrinos, although not as significant as what KM3NeT observed, have been recorded by the IceCube Observatory — a neutrino detector buried deep within the ice at the South Pole. IceCube has detected around six such neutrinos, whose unusually elevated energies have also proven difficult to explain. Regardless of their source, the IceCube findings allow scientists to estimate the plausible rate at which neutrinos of those energies generally strike Earth. However, if this estimation is accurate, it would be exceedingly improbable to have witnessed the ultra-high-energy neutrino that KM3NeT recently recorded. Therefore, the discoveries from the two detectors appear to be what scientists term “in conflict.”
Kaiser and Klipfel, who had been pursuing a different project related to primordial black holes, questioned: Could a PBH have generated both the KM3NeT neutrino and the few IceCube neutrinos, assuming PBHs constitute the majority of the dark matter in the galaxy? If they could establish a possibility, it would open up an even more thrilling avenue — suggesting that both observatories recorded not just high-energy neutrinos but also the remnants of Hawking radiation.
“Our best opportunity”
The initial step in their theoretical evaluation was to assess how many particles would be emitted by an exploding black hole. All black holes are expected to gradually radiate energy over time. The larger a black hole, the cooler it remains, and the lesser energy particles it generates as it slowly disintegrates. Hence, particles emitted as Hawking radiation from massive stellar black holes would be nearly impossible to detect. Conversely, considerably smaller primordial black holes would be extremely hot and release high-energy particles at a rate that accelerates as they near complete annihilation.
“We have no realistic expectation of detecting Hawking radiation from astrophysical black holes,” Klipfel remarks. “So if we ever wish to observe it, the smallest primordial black holes provide our best opportunity.”
The researchers estimated the quantity and energies of particles a black hole should emit, based on its temperature and decreasing mass. In its final nanosecond, they predict that once a black hole becomes smaller than an atom, it should release a final burst of particles, including around 1020 neutrinos, or approximately a sextillion of the particles, with energies close to 100 peta-electron-volts (approximately the energy that KM3NeT detected).
They applied this finding to determine the number of PBH explosions that would need to transpire within a galaxy to align with the reported IceCube results. They concluded that, in our portion of the Milky Way galaxy, about 1,000 primordial black holes should be erupting per cubic parsec each year. (A parsec is a unit of distance equating to nearly 3 light years, which is over 10 trillion kilometers.)
They then evaluated the distance from which one such explosion in the Milky Way could have occurred, such that only a handful of the high-energy neutrinos could have reached Earth and contributed to the recent KM3NeT finding. They discovered that a PBH would need to explode relatively near our solar system — approximately 2,000 times greater than the distance between Earth and our sun.
The particles expelled from such a nearby explosion would radiate outward in all directions. However, the team discerned there is a slim, 8 percent chance that an explosion could occur sufficiently close to the solar system every 14 years, enabling enough ultra-high-energy neutrinos to strike Earth.
“An 8 percent chance isn’t exceedingly high, but it falls well within a range we should take seriously — especially since, up to this point, no alternate explanation has emerged that accounts for both the unexplained very-high-energy neutrinos and the even more puzzling ultra-high-energy neutrino event,” Kaiser states.
The team’s scenario appears to hold validity, at least theoretically. To validate their hypothesis will necessitate many more detections of particles, including neutrinos at “extraordinarily high energies.” Following that, scientists can establish improved statistics regarding such rare occurrences.
“In that case, we could leverage all of our combined expertise and tools to try to measure yet-to-be-confirmed Hawking radiation,” Kaiser adds. “That would offer the first substantial evidence for one of the fundamental principles of our understanding of black holes — and could also explain these otherwise exceptional high-energy neutrino events. That’s a very exciting possibility!”
Concurrently, additional initiatives to spot nearby PBHs could further substantiate the theory that these peculiar entities constitute most or all of the dark matter.
This research was supported, in part, by the National Science Foundation, MIT’s Center for Theoretical Physics – A Leinweber Institute, and the U.S. Department of Energy.