Did the Local Void shoot a monster extreme-energy cosmic ray our way?

The Earth’s atmosphere has been blasted by the second strongest ultrahigh-energy cosmic ray (UHECR) to ever be observed. While its exact origins remain unknown, astrophysicists are pointing the finger at the Local Void, a fundamentally empty area of the cosmos next to the Milky Way.

The subatomic particle that the ray beamed to Earth has been dubbed the “Amaterasu” particle by scientists in an homage to the mythological Japanese sun goddess. It arrived on May 27, 2021 and has just been described by researchers in the journal Science today. The study was led by researchers from the University of Utah (the U) and the University of Tokyo.

The ray was detected by the Telescope Array, a field of 507 detectors spread out over 700 square km (about 270 square miles) in Utah’s West Desert. The arrays operate at an elevation of about 1,200 m (4,000 ft) in the dry, light-pollution-free air that allows them the exceptional ability to detect particles from cosmic rays. According to researchers, arrivals of the particles that are the signatures of the most robust UHECRs are extremely rare – about one particle per century per square kilometer – which is why the arrays that detect them must cover a large area. There have even been plans to put a cosmic-ray detector array on the Moon.


The energy contained in the Amaterasu particle is estimated to be second only to the Oh-My-God particle, which was also picked up in Utah using an observatory known as the Fly’s Eye in 1991. In a statement about the discovery, the U says the force of the Amaterasu ray would be akin to dropping a brick on your toe from waist height, and they liken its kinetic energy to that of a pitcher’s fastball traveling millions of light years.

To achieve UHECR status, the ray-produced particle must have energy exceeding 240 exa-electron volts (EeVs). The Oh-My-God particle contained 320 EeVs and the Amaterasu particle has clocked in around 244 EeVs. To put that in perspective, one EeV is about a million times higher than the energy our particle accelerators can achieve here on Earth.

Despite knowing that UHECRs do occasionally slam into Earth, knowing what produces them has remained a mystery to scientists. The energy contained in the Oh-My-God particle should not have been able to be created by anything observable in our galaxy. Yet because both that particle and Amaterasu had so much kinetic energy, they are likely not affected by magnetic fields and scientists should be able to trace them back to their sources.

“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,” said John Matthews, Telescope Array co-spokesperson at the U and co-author of the study. “That’s the mystery of this – what the heck is going on?”

“Things that people think of as energetic, like supernova, are nowhere near energetic enough for this,” added Matthews. “You need huge amounts of energy, really high magnetic fields to confine the particle while it gets accelerated.”

The researchers’ best guess is that the Amaterasu particle zoomed to Earth from the Local Void, an origin that is especially puzzling as that cosmic space bordering the Milky Way seems to be fundamentally empty. That zone is also different from the predicted source of the Oh-My-God particle.

“These events seem like they’re coming from completely different places in the sky,” said John Belz, professor at the U and co-author of the study. “It’s not like there’s one mysterious source. 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.” Among those ideas, write the authors, could be a need to revise the current thinking about high-energy particle physics.

Researchers are hoping that an upcoming expansion of the Telescope Array will help them unravel the mystery of UHECRs. It is due to receive 500 new detectors and spread to an area nearly the size of Rhode Island at 2,900 square km (1,100 square mi).

Sources: AAAS via EurekAlert; University of Utah

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