The last gasp of a primordial black hole may hold the key to understanding the highest-energy ghost particle detected to date, according to a groundbreaking study by physicists at MIT. In a paper published in Physical Review Letters, the researchers present a compelling theoretical argument suggesting that a recently observed, highly energetic neutrino might have originated from an exploding primordial black hole located outside our solar system.
Neutrinos are often referred to as ghost particles due to their elusive and pervasive nature. They are the most abundant type of particle in the universe, yet they interact so weakly with matter that they leave barely a trace. Recently, scientists detected a neutrino with the highest energy ever recorded, but its source remains a mystery. The MIT researchers propose that this extraordinary neutrino could have been produced by the explosion of a primordial black hole.
Primordial black holes (PBHs) are hypothetical black holes that are much smaller than the massive black holes found at the centers of galaxies. They are theorized to have formed during the first moments following the Big Bang. Some scientists speculate that these primordial black holes could account for a significant portion, if not all, of the dark matter in the universe today. Like their massive counterparts, PBHs are expected to emit energy and diminish in size over time through a process known as Hawking radiation, a phenomenon predicted by physicist Stephen Hawking.
As a primordial black hole radiates energy, it becomes hotter and emits increasingly high-energy particles. This process culminates in a dramatic explosion just before the black hole completely evaporates. The MIT physicists estimate that if PBHs are indeed the predominant form of dark matter, a small subset of them would be nearing their explosive end throughout the Milky Way galaxy. Statistically, this suggests a possibility that one such explosion could occur relatively close to our solar system, releasing a burst of high-energy particles, including neutrinos.
If the recent detection of the highest-energy neutrino were indeed linked to a primordial black hole explosion, it could represent the first direct observation of Hawking radiation. This would not only provide evidence for the existence of primordial black holes but could also confirm that they constitute a major portion of dark matter—a mysterious substance that makes up about 85% of the universe's total mass, the nature of which remains largely unknown.
Study lead author Alexandra Klipfel, a graduate student in MIT's Department of Physics, notes, "There’s this scenario where everything seems to line up... We can also produce these high-energy neutrinos from a fluke nearby PBH explosion. It’s something we can now try to look for and confirm with various experiments."
In February, scientists from the Cubic Kilometer Neutrino Telescope (KM3NeT) reported the detection of the highest-energy neutrino recorded to date. Located at the bottom of the Mediterranean Sea, KM3NeT is designed to filter out interference from other particles, allowing for clearer neutrino detection. This particular neutrino was found to have an energy exceeding 100 peta-electron-volts, a level of energy far beyond human capabilities for particle acceleration.
The origins of such high-energy particles are still hotly debated among scientists. While the IceCube Observatory in Antarctica has detected several other high-energy neutrinos, the sources of these particles remain elusive. The discrepancy between the detection rates of KM3NeT and IceCube has created a scientific tension, prompting Kaiser and Klipfel to explore whether a primordial black hole could account for both sets of observations.
The researchers began their analysis by calculating the number of particles emitted by an exploding black hole. They determined that while larger black holes emit lower-energy particles, smaller primordial black holes would emit high-energy particles as they near complete evaporation. According to their estimates, a PBH smaller than an atom could produce a final burst of approximately 1020 neutrinos with energies around 100 peta-electron-volts, similar to what was detected by KM3NeT.
They further calculated that in our region of the Milky Way, about 1,000 primordial black holes should be exploding per cubic parsec each year—one parsec being approximately three light-years. The researchers found that a PBH explosion occurring about 2,000 times the distance from the Earth to the sun could produce enough ultra-high-energy neutrinos to reach Earth and match the recent KM3NeT event.
The team found that there is an 8% chance of such an explosion occurring close enough to our solar system roughly every 14 years, which is significant enough to warrant serious consideration. Kaiser remarked, "An 8% chance is not terribly high, but it’s well within the range for which we should take such chances seriously." This scenario could potentially explain both the unexplained very-high-energy neutrinos and the ultra-high-energy neutrino event.
While the theoretical framework proposed by the MIT researchers appears strong, confirming their hypothesis will require further detections of particles, including high-energy neutrinos. Successful observations could lead to a better understanding of Hawking radiation and its implications for our knowledge of black holes. As Kaiser notes, "That would provide the first-of-its-kind evidence for one of the pillars of our understanding of black holes—and could account for these otherwise anomalous high-energy neutrino events as well. That’s a very exciting prospect!"
In parallel, ongoing efforts to detect nearby primordial black holes could further support the idea that these enigmatic objects comprise a significant portion of dark matter in the universe.
