Earth was struck by an extremely powerful cosmic ray and we don’t know where it came from

A cosmic ray known as Amaterasu struck Earth's atmosphere with an energy greater than 240 exa-electron volts, a discovery so revolutionary that it has stunned astronomers.

An artist’s impression of the detection. (Osaka Metropolitan University/L-INSIGHT, Kyoto University/Ryuunosuke Takeshige)

With an energy of 320 exa-electron volts, the discovery of the Oh-My-God particle in 1991 ranked higher than this event. These ultra-high-energy cosmic rays have an unknown origin, which complicates our understanding of astrophysical phenomena today.

The Enigma of Cosmic Rays

Cosmic rays carry more energy than is predicted by theory and travel through the universe at near-light speeds. They are primarily made up of atomic nuclei and sub-nuclear particles like protons and electrons. The sources of these ultra-high-energy cosmic rays are unknown, but it is known that stars and supernovas are the source of lower-energy cosmic rays. Although our atmosphere protects us from direct impact, these rays can still cause a cascade of secondary particles that can be seen by Earthly observatories when they collide with atmospheric particles.

The Telescope Array’s Discovery

On May 27, 2021, an enormous observatory spanning 700 square kilometres, the Telescope Array, discovered the Amaterasu particle. This finding, which was made possible by the use of sophisticated detection methods, demonstrated that these high-energy cosmic events are actual astrophysical phenomena rather than anomalies. The particle's unique characteristics are reflected in its naming, which is after a Shinto sun goddess.

An animation of a cosmic ray particle shower. (Toshihiro Fujii, L-INSIGHT, Kyoto University (Supported by MEXT))

The Challenge of Tracing Origins

The mysterious origins of these cosmic rays are among their most perplexing features. Given their high energy, there ought to be an easy way to return to their source. Still, it appears that Amaterasu and the Oh-My-God particle come from parts of space where there aren't enough energetic phenomena to support them. This disparity implies that there are unidentified processes or origins operating in space.

The Greisen-Zatsepin-Kuzmin Limit

Due to the Greisen-Zatsepin-Kuzmin (GZK) limit, an estimated maximum energy threshold for cosmic rays travelling long distances, ultra-high-energy cosmic rays such as Amaterasu present a significant puzzle. Since Amaterasu's energy is greater than this threshold, its source must be located no more than 160 million light-years away. However, there appears to be no known astrophysical event in this range that could produce such energy.

The Void Conundrum

Following Amaterasu's path returns one to a cosmic void, which is the comparatively empty region that separates galaxy clusters. This discovery adds to the mystery because it is improbable that phenomena able to produce ultra-high-energy cosmic rays could exist in such voids. This knowledge vacuum suggests the possibility of novel astrophysical phenomena or hitherto unknown properties of cosmic magnetic fields.


The Future of Cosmic Ray Research

Amaterasu's discovery opens up new astrophysical possibilities and inspires theories about everything from new cosmic phenomena to unexplained particle acceleration mechanisms. Scientists are thinking of a number of explanations, such as cosmic string collisions or defects in spacetime. This finding emphasises the breadth of the mysteries surrounding our universe and the ongoing development of our knowledge of cosmic occurrences.

In summary, the discovery of the Amaterasu cosmic ray is a critical turning point in astrophysics, casting doubt on accepted notions and igniting a search for fresh explanations. We are on the verge of potentially ground-breaking discoveries that could fundamentally alter our understanding of the universe and its numerous phenomena as scientists continue to explore this mystery.

Reference(s): News Release

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