On November 24, 2023, the Telescope Array Project in Utah recorded an extraordinary event. For an instant, a single particle struck the desert with an energy equivalent to a brick dropped from waist height.
The collaboration named this particle Amaterasu, after the Japanese sun goddess. Its measured energy reached 244 exa-electronvolts, making it the second most energetic cosmic ray ever observed. This detection presents a significant puzzle for physicists.
The Greisen–Zatsepin–Kuzmin limit, a theoretical boundary proposed in the 1960s, states that cosmic rays above approximately 50 EeV should lose energy while traveling through the cosmic microwave background radiation, the remnant glow of the Big Bang. Amaterasu carries nearly five times that energy threshold.
It should have been degraded during its journey across space, yet it arrived intact. The source of this particle remains unknown. Typical cosmic ray producers such as supernovae, gamma-ray bursts, and active galactic nuclei are powerful, but 244 EeV exceeds what most theoretical models can explain.
The Telescope Array Project team has traced the particle’s arrival direction. That direction points to a void, an empty region of space with no known object capable of generating such an energetic particle.
This empty direction has drawn particular attention from researchers. If no known astronomical object can be the source, alternative explanations must be considered. These include exotic physics, topological defects from the early universe, and decaying superheavy dark matter. None of these possibilities have been proven, but all are now under consideration.
The Telescope Array Project operates a grid of detectors covering 700 square kilometers of Utah desert. When a cosmic ray enters the atmosphere, it produces a cascade of secondary particles that reach the ground.
The detectors capture these particles, and researchers reconstruct the original particle’s energy and direction from the pattern. Amaterasu triggered 23 detectors, providing a clear signal. Other observatories will now search their records for similar events.
The Pierre Auger Observatory in Argentina, the world’s largest cosmic ray detector, has recorded one particle at 244 EeV and one above it. That higher-energy particle, known as the Oh-My-God particle, was detected in 1991 at approximately 320 EeV.
Only two particles at these extreme energies have been found in 33 years. The implications extend beyond astronomy. These particles allow study of physics at energy scales far beyond human-built accelerators.
The Large Hadron Collider operates at teraelectronvolt energies. Amaterasu was a million times more powerful.
Analysis of how it interacted with the atmosphere, how it fragmented, and how it decayed could reveal cracks in the Standard Model of particle physics. No discovery has been claimed yet. The Telescope Array Project will publish its analysis, and other groups will attempt to verify or challenge the findings.
Astronomers plan to scan the void region using radio telescopes, gamma-ray observatories, and neutrino detectors. If the source is exotic, it may leave other detectable traces.
Determining whether the event is a statistical anomaly will take years. Regardless of the outcome, the Amaterasu particle has already demonstrated that the universe contains surprises. It has given the Telescope Array Project a compelling reason to continue their observations.
