Researchers have detected one of the most powerful cosmic rays ever seen slamming into Earth — but they have no idea what caused it or where it came from. The extremely energetic particle, which has been named after a Japanese goddess, arrived from the direction of a void in the universe where almost nothing is known to exist, according to new research.

Cosmic rays are highly energetic particles, mainly consisting of protons or helium nuclei, that are constantly raining through every square inch of the universe (including our bodies). But a small subsection of cosmic rays, which hit Earth roughly once per square mile every year, are accelerated to even greater energy levels by some of the universe’s most intense phenomena. 

These extra-energetic particles, known as “ultra-high-energy cosmic rays,” have at least one exa-electron volt (EeV), or 1 quintillion (1 followed by 18 zeros) electron volts, of energy, which is around a million times more energetic than the fastest particles from human-made particle accelerators.

On May 21, 2021, researchers detected one of these supercharged cosmic rays with the Telescope Array project — a detector made of individual substations covering more than 270 square miles (700 square kilometers) in Utah. This particular particle had a whopping 244 EeV of energy, which makes it the most energetic cosmic ray since the “Oh My God” (OMG) particle in 1991 — the most powerful cosmic ray ever detected, which had an energy of 320 EeV and traveled at more than 99.9% the speed of light.

Researchers from Osaka Metropolitan University (OMU) described the recent cosmic ray in a  new study that will be published in the journal Science on Nov. 24. They named the mysterious particle “Amaterasu” after the sun goddess from the Shinto religion who is believed to have helped create Japan.

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 subatomic 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, the Telescope Array consists of 507 surface detector stations arranged in a square grid that covers 700 km² (~270 miles²) outside of Delta, Utah in the state’s West Desert. The event triggered 23 detectors at the north-west region of the Telescope Array, splashing across 48 km² (18.5 mi²). Its arrival direction appeared to be from the Local Void, an empty area of space bordering the Milky Way galaxy.

“The particles are so high energy, they shouldn’t be affected by galactic and extra-galactic 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. “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 of this — what the heck is going on?”

In their observation that published on Nov. 24, 2023, in the journal Science, an international collaboration of 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 the 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, professor at the U and co-author of the study. “It could be defects in the structure of spacetime, colliding cosmic strings. I mean, I’m just spit-balling crazy ideas that people are coming up with because there’s not a conventional explanation.”

Reference
University of Utah. (2023, November 23). Telescope Array detects second highest-energy cosmic ray ever. Science Daily. Retrieved November 26, 2023 from www.sciencedaily.com/releases/2023/11/231123164708.htm