In 1991, the University of Utah Fly’s Eye experiment detected the highest-energy cosmic ray ever observed. Dubbed “Oh-My-God particle”, the ray’s energy shocked astrophysicists. Nothing in the Milky Way had the power to produce it, and the particle had more energy than was theoretically possible for cosmic rays travelling to Earth from other galaxies. Simply put, the particle should not exist. The Telescope Array has since observed more than 30 ultra-high-energy cosmic rays, though none approaching the Oh-My-God-level energy. No observations have yet revealed their origin or how they are able to travel to Earth.
On May 27, 2021, the Telescope Array experiment detected the second-highest extreme-energy cosmic ray. At 2.4 x 1020eV, the energy of this single sub-atomic particle is equivalent to dropping a brick on your toe from waist height. Led by the University of Utah (the U) and the University of Tokyo, Telescope Array consists of 507 surface detector stations arranged in a square grid that covers 700 km2 (~270 miles2) outside of Delta, in Utah’s West Desert. The event triggered 23 detectors at the north-west region of Telescope Array, splashing across 48 km2.
Its arrival direction appeared to be from the Local Void, an empty area of space bordering our galaxy.“The particles are so high energy, they shouldn’t be affected by galactic and extragalactic magnetic fields. You should be able to point to where they come from in the sky,” said John Matthews, Telescope Array co-spokesperson at the U and co-author of the study, adding, “But in the case of the Oh-My-God particle and this new particle, you trace its trajectory to its source and there’s nothing high energy enough to have produced it. That’s the mystery.”
In their observations on November 24 this year, in journal Science, researchers describe the ultra-high-energy cosmic ray, evaluate its characteristics, and conclude that the rare phenomena might follow particle physics unknown to science. The researchers named it the Amaterasu particle after the sun goddess in Japanese mythology. The Oh-My-God and Amaterasu particles were detected using different observation techniques, confirming that while rare, these ultra-high energy events are real. “These events seem like they’re coming from completely different places in the sky. It’s not like there’s one mysterious source,” said John Belz, co-author of the study. “It could be defects in the structure of spacetime, colliding cosmic strings,” he suggested.
Natural particle accelerators
Cosmic rays are echoes of violent celestial events that have stripped matter to its sub-atomic structures and hurled it through the universe at near speed of light. Essentially, cosmic rays are charged particles with a wide range of energies consisting of protons, electrons, or entire atomic nuclei that travel through space and rain down to Earth nearly constantly.
Cosmic rays hit Earth’s upper atmosphere and blast apart nuclei of oxygen and nitrogen gas, generating many secondary particles, which travel a short distance in the atmosphere and repeat the process, building a shower of billions of secondary particles that scatter to the surface. The footprint of this secondary shower is massive and requires that detectors cover an area as large as Telescope Array. The surface detectors utilise a suite of instrumentation that gives researchers information about each ray; the timing of the signal shows its trajectory and the amount of charged particles hitting each detector reveals the primary particle’s energy.
Because particles have a charge, their flight path resembles a ball in a pinball machine as they zigzag against the electromagnetic fields. It’s nearly impossible to trace the trajectory of most cosmic rays, which lie on the low- to middle-end of the energy spectrum. Even high-energy cosmic rays are distorted by the microwave background. Particles with Oh-My-God and Amaterasu energy blast through intergalactic space relatively unbent. Only the most powerful of celestial events can produce them.
Source: University of Utah
Ultra-high-energy cosmic rays
According to scientists, ultra-high-energy cosmic rays are the highest energy messengers of the universe as we know it, with energies up to 1020 eV (electronvolts). Studies of astrophysical particles (nuclei, electrons, neutrinos and photons) at their highest observed energies have implications for fundamental physics, as well as astrophysics. The primary particles interact in the atmosphere, after reaching Earth from outer space, and generate extensive air showers.