Samuel Jarman, SciencePOD

In 2021, the Telescope Array Project in Utah detected a cosmic ray particle with a mysteriously high energy. Named the Amaterasu particle, after the Goddess of the Sun in Japanese mythology, it appeared to originate from the local cosmic void: a space at least 150 million light-years across which contains far fewer galaxies than its surroundings. Yet since its identification, astronomers have been perplexed as to how such an energetic particle could have originated in such a vast, empty region of space.

In a new study published in Physics Letters B, Paul Frampton of the University of Salento, Italy, together with Thomas Kephart of Vanderbilt University, Nashville, Tennessee, propose an alternative theory for the particle’s origins, which appears to validate their earlier theoretical work.

“Based on what we know about the Amaterasu cosmic ray, we think that there is good reason to believe that some of the highest energy cosmic ray primaries are magnetic monopoles,” Kephart says. If the theory is correct, it would make the Ameterasu the first particle to be definitively identified as a magnetic monopole.

In conventional magnets, magnetic fields emerge from a north pole, and re-enter at a south pole. In contrast, magnetic monopoles feature just a single north or south pole from which a magnetic field either emanates outwards or converges inwards, much like the electric field from a charged particle like a proton or electron.

“Magnetic monopoles were predicted by Paul Dirac in 1931, and play a key role in our understanding of particle physics,” Kephart explains. “In particular, they imply that all electric charge in the universe must be quantised. One thing that makes monopoles interesting is their connection with mathematics."

Since the mathematical and topological structures that describe magnetic monopoles are deeply intertwined with the foundations of quantum theory, observing them directly could enable physicists to answer some of their most pressing questions about the nature of quantum mechanics. Since Dirac’s initial proposal, however, direct evidence for these particles has remained elusive.

When the Amaterasu particle was first observed, two main theories emerged to explain its origins: either there could be a source of ultra-high-energy cosmic rays in the local cosmic void itself, or the particle’s path could have been bent by some immense magnetic field, making it appear as if it came from the void. But given its extreme energy, recent studies have agreed that both of these theories are unlikely.

In this paper, Frampton and Kephart suggest an entirely different theory: that the Amaterasu particle was not a proton or atomic nucleus like nearly all cosmic rays observed so far, and didn’t originate in any galaxy. Instead, it was a magnetic monopole produced shortly after the Big Bang, which wandered through the universe until being detected by the Telescope Array Project, and just so happened to approach Earth from the local cosmic void.

If this theory is correct, it could place tighter constraints on proposed models of the fundamental particles and forces of the universe to allow magnetic monopoles appear, unlike the Standard Model currently used by physicists, which has no place for magnetic monopoles. Among these models is the Grand Unified Theory, which aims to unite the electromagnetic, weak, and strong nuclear forces into a single force at high energies.

In their future research, Frampton, Kephart, and his students will now aim to gather further evidence for these as-yet elusive particles. “We hope to do a global analysis of the highest energy cosmic rays and study the hypothesis that they might be magnetic monopoles,” Kephart concludes.

Article details

Frampton, P.H., Kephart, T. W. ‘The Amaterasu cosmic ray as a magnetic monopole and implications for extensions of the standard model’, Physics Letters B (2024).