In 2021, scientists detected the most powerful cosmic ray seen in over three decades, a discovery that has both excited and puzzled the astrophysics community. This cosmic ray, named “Amaterasu” after a Japanese sun goddess, carried an energy of 240 exa-electronvolts (EeV), rivaling the record-holding “Oh-My-God” particle detected in 1991, which had an energy of about 320 EeV. This event represents a significant milestone in cosmic ray research, given the rarity of such high-energy particles. Fewer than one particle with energies exceeding 100 EeV strikes each square kilometer of Earth’s surface each century.
The detection of the Amaterasu particle was made by the Telescope Array experiment, a cosmic-ray detector in Utah, USA. This facility, which includes 507 surface detector stations, is uniquely positioned to capture such ultra-high-energy cosmic rays. The event triggered 23 detectors, covering an area of approximately 48 square kilometers. Interestingly, the particle seemed to originate from the Local Void, an empty area of space bordering the Milky Way galaxy. This finding is intriguing because ultra-high-energy cosmic rays like Amaterasu are believed to travel through space relatively undisturbed by magnetic fields, which should theoretically allow scientists to pinpoint their origins. Yet, in this case, tracing the particle’s trajectory led to a seemingly empty region of space, deepening the mystery.
Cosmic rays are essentially subatomic particles – often protons, electrons, or entire atomic nuclei – that move through space at nearly the speed of light. They are thought to be remnants of violent celestial events that strip matter down to its subatomic components. Upon reaching Earth’s atmosphere, cosmic rays collide with atomic nuclei, creating a cascade of secondary particles that scatter to the surface. Detecting and analyzing these secondary particles allows researchers to infer the properties of the original cosmic ray. However, tracing the trajectories of most cosmic rays is challenging due to their interactions with electromagnetic fields in space. In contrast, particles like Amaterasu, with their extreme energies, are less affected by these fields and should theoretically provide clearer clues about their origins.
The Amaterasu particle’s energy level is so high that it exceeds what is known as the Greisen-Zatsepin-Kuzmin (GZK) cutoff. This theoretical limit suggests the maximum energy a proton can retain over long distances in space before losing energy due to interactions with cosmic microwave background radiation. Known sources capable of generating such high-energy particles, like active galactic nuclei or black holes, are typically located over 160 million light-years away from Earth. Yet, back-tracing the trajectory of the Amaterasu particle leads to an empty space, which contradicts the current understanding of cosmic ray physics.
The Telescope Array experiment is undergoing an expansion to improve its detection capabilities. This expansion includes the addition of 500 new scintillator detectors, increasing the array’s coverage to an area nearly the size of Rhode Island. This enhancement aims to capture more ultra-high-energy cosmic rays and hopefully provide further insights into their origins and the physical processes that generate them.
In summary, the detection of the Amaterasu particle not only challenges our current understanding of cosmic rays but also hints at the possibility of unknown physical processes at play. It raises fundamental questions about the nature of these high-energy particles and their origins, potentially leading to new discoveries in particle physics and astrophysics.