Scientists Mystified by Rare, High-Energy Cosmic Ray, the Most Powerful Since 1991

Scientists in Utah have detected the second-most energetic cosmic ray ever seen. The powerful particle rivals the highest-energy cosmic ray on record, called the Oh-My-God particle, which was spotted in 1991.

Like other cosmic rays, scientists knew this one came from outer space. But when the researchers tried to track down where it originated, they were pointed to a void, per a paper published last week in the journal Science.

Their calculations led them to a spot between galaxies, or “absolutely in the middle of nowhere,” as Vasiliki Pavlidou, an astrophysicist at the University of Crete in Greece who did not contribute to the findings, tells Science News’ Emily Conover.

Cosmic rays are massive amounts of energy packed into tiny particles—mostly protons, though they can also be helium nuclei or heavier nuclei, according to the European Organization for Nuclear Research (CERN). Lower-energy cosmic rays come from the sun, whereas the higher-energy ones, which are less frequent, likely originate in other galaxies.

“If you hold out your hand, one [cosmic ray] goes through the palm of your hand every second, but those are really low-energy things,” John Matthews, a co-author of the study and astroparticle physicist at the University of Utah, tells CNN’s Katie Hunt. “When you get out to these really high-energy [cosmic rays], it’s more like one per square kilometer per century. It’s never going through your hand.”

When cosmic rays reach Earth’s upper atmosphere, they collide with atomic nuclei, which creates more particles, per CERN. These particles fall to the Earth in showers that can be observed by detectors on the ground.

The recent powerful ray was detected on May 27, 2021, by the Telescope Array collaboration in Utah. More than 500 detectors sample the particles when they reach Earth’s surface, and fluorescent telescopes measure light passing through gas in the atmosphere. The array of detectors, arranged in a square grid, surveys nearly 300 square miles of Utah desert.

The instruments clocked the cosmic ray at about 240 exa-electron volts, or about 40 joules of energy. That’s roughly the equivalent of the amount of energy in a brick dropped on your toe from waist height—but carried in one subatomic particle.

“It’s amazing, because you have to think of what could produce such high energy,” Clancy James, an astronomer at Curtin University in Australia who was not involved in the study, tells Nature News’ Gemma Conroy.

Only a few particles above 200 exa-electron volts have ever been discovered. The Oh-My-God particle in 1991 measured 320 exa-electron volts.

Such high-energy cosmic rays contain tens of millions of times the energy of particles in accelerators like the Large Hadron Collider on Earth, Glennys Farrar, a physicist at New York University who did not contribute to the findings, tells CNN.

Evidence suggests that high-energy gamma rays come from supernova explosions and supermassive black holes—and it’s possible that powerful cosmic rays originate with similar sources, per CERN.

Cosmic rays, which carry an electric charge, usually have their trips through space disturbed by electromagnetic fields. The ultra-powerful ones, however, follow a straighter path. Still, the researchers have been unable to find a source galaxy for either of the two most energetic particles.

“If you take the two highest-energy events—the one that we just found, the Oh-My-God particle—those don’t even seem to point to anything,” Matthews tells CNN. “It comes from a region that looks like a local empty space. It’s a void. So, what the heck’s going on?”

One possible explanation is that the magnetic fields in outer space may be stronger than expected and could have significantly deflected the particle.

Or, perhaps some unknown particle physics is at play. Jose Bellido Caceres, an astroparticle physicist at the University of Adelaide in Australia who was not involved with the research, tells Nature News that undiscovered physical processes could allow cosmic rays to travel much farther than previously thought.

“It could be new physics,” he tells the publication.

Will Sullivan | | READ MORE

Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.