The KM3NeT Collaboration, a network of neutrino detectors based in the Mediterranean, announced in February that they had found the highest-energy neutrino detected to date. In a recent study, researchers from MIT proposed that this “ghost particle” could be the product of energetic Hawking Radiation emitted by a Primordial Black Holes (PBH) after it decayed outside our Solar System. If true, these findings could be the first evidence of the theoretical radiation Hawking proposed in 1974 by combining quantum field theory and General Relativity.
The study was conducted by Alexandra Klipfel, a fourth-year undergraduate physics major and researcher with the Kirschvink Biomagnetics Group, and David Kaiser, a Professor of Physics and the Germeshausen Professor of the History of Science (STS) at Caltech (respectively). The paper detailing their findings, “Ultrahigh-Energy Neutrinos from Primordial Black Holes,” appeared on September 18th in the journal Physical Review Letters. Neutrinos, so-named because they are electrically neutral, are the most common particle in the Universe.
Despite their commonality, neutrinos are incredibly difficult to detect because they interact with normal matter through the weak nuclear force and gravity. Their invisible nature and the way they leave barely a trace of their interactions earned them the nickname, “ghost particles.” Meanwhile, PBHs are hypothetical objects that are basically microscopic versions of their much larger stellar-mass and supermassive counterparts and are considered a candidate for Dark Matter. PBHs are theorized to have formed in the first seconds after the Big Bang when pockets of hot material came together that may have been dense enough to form black holes.
If these objects do exist, they should be subject to the same laws of entropy as their larger counterparts, as theorized by Stephen Hawking. As these black holes radiate, they grow hotter and lose mass, releasing more high-energy particles in the process. In theory, this process culminates in a violent explosion that releases the most energetic particles just before the black hole evaporates. The larger the black hole, the colder it is, which means the particles it emits would be low-energy by comparison and nearly impossible to detect. In contrast, PBHs would be extremely hot and emit high-energy particles, especially as they evaporated.
“We don’t have any hope of detecting Hawking radiation from astrophysical black holes,” Klipfel said in an MIT News release. “So if we ever want to see it, the smallest primordial black holes are our best chance.” Klipfel and his colleagues argue that, if PBHs make up most of the Dark Matter in the Universe, then a small subset of them would be reaching the end of their lifespans today. If one exploded outside our Solar System, it would have released a burst of high-energy particles (including neutrinos) that would be detectable to instruments on Earth.
The first step in testing this theory was to calculate the number of particles that would be emitted by an exploding black hole based on its theoretical temperature and shrinking mass. They estimate that when the PBH reached the end of its life cycle, its explosion would release about a sextillion (1020) neutrinos with energies of about 100 peta-electron-volts (1016 eV) – around the same energy as the one observed by KM3NeT. Said Klipfel:
It turns out there’s this scenario where everything seems to line up, and not only can we show that most of the dark matter [in this scenario] is made of primordial black holes, but 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. This is an incredibly high energy, far beyond anything humans are capable of accelerating particles up to. There’s not much consensus on the origin of such high-energy particles.
In addition, the IceCube Observatory, located near the South Pole, has detected similarly high-energy neutrinos on a few occasions, which defied explanation. Based on these events, scientists were able to calculate a plausible rate at which high-energy neutrinos reach Earth. However, these results were in tension with the detection made by the KM3NeT detector. However, Kaiser and Klipfel’s research offers a possible resolution to this tension by proposing that a PBH could have produced the KM3NeT detection and the handful detected by IceCube.
This image shows a visual representation of one of the highest-energy neutrino detections superimposed on a view of the IceCube Lab at the South Pole. Credit: Penn State ICDS/IceCube.
Using the results of their study, they calculated the rate of PBH explosions that would need to occur in the Milky Way to explain the reported IceCube results. They also calculated the distance beyond the Solar System at which such an explosion would need to occur for Earth-based neutrino experiments to detect the small number of neutrinos observed. They found that in our region of the galaxy, approximately 1,000 PBHs should be exploding per cubic parsec per year (~35 cubic light-years) and that one would have had to have exploded about 2,000 AU from Earth, placing it within the Oort Cloud.
Their calculations further showed that there is an 8% chance that an explosion of this kind could happen once every 14 years, producing enough ultra-high-energy neutrinos that could reach Earth. If this scenario is true, it would represent the first observation of Hawking radiation and could also provide indirect evidence that PBHs exist and comprise Dark Matter. Confirming their results, however, will require many more neutrino detections at extremely high energies. Meanwhile, parallel efforts to detect nearby PBHs could bolster the hypothesis that these objects constitute Dark Matter. Said Kaiser:
An 8 percent chance is not terribly high, but it’s well within the range for which we should take such chances seriously — all the more so because so far, no other explanation has been found that can account for both the unexplained very-high-energy neutrinos and the even more surprising ultra-high-energy neutrino event. In that case, we could use all of our combined experience and instrumentation to try to measure still-hypothetical Hawking radiation. 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!
Further Reading: MIT News, Physical Review Letters