Ultra-high-energy cosmic rays defy easy explanations
The Oh-My-God particle, a subatomic nucleus detected in 1991 with energy around 3.2×10^20 eV, remains one of the most extreme cosmic rays ever observed. Since then, at least a dozen–plus similar events have been recorded, confirming ultra-high-energy cosmic rays are real but exceedingly rare: detectors like the Pierre Auger Observatory see roughly one such event per square kilometer per century, while the sources remain unknown. The leading possibilities are extragalactic, with candidates including magnetars and the supermassive black holes at centers of active galaxies, but no definitive origin has been identified.
