Scientists at the University of Utah and the University of Tokyo have made a groundbreaking discovery in astrophysics; on May 27, 2021, the Telescope Array experiment detected the second-highest extreme-energy cosmic ray ever observed. This cosmic ray, named the “Amaterasu” particle after the sun goddess in Japanese mythology, had an energy level of 2.4 x 10²⁰ electron volts (eV). To put this into perspective, the energy of a single subatomic particle is equivalent to dropping a brick on your toe from waist height.

Cosmic rays are charged particles that travel through space at nearly the speed of light. They are remnants of violent celestial events that strip matter down to its subatomic structures. When cosmic rays hit Earth’s upper atmosphere, they generate secondary particles that scatter to the surface, creating a shower of billions of particles. The Telescope Array, consisting of 507 surface detector stations covering 700 sq. km. (~270 sq. mi.) in Utah’s West Desert, is designed to capture and analyze these cosmic rays.

The recent observation of the Amaterasu particle has left scientists puzzled. Its arrival direction appeared from the Local Void, an empty area of space bordering the Milky Way galaxy. This ultra-high-energy cosmic ray’s trajectory suggests that it originated from a source with immense energy, yet no known astrophysical phenomenon can account for its production. Scientists are considering unconventional explanations, such as defects in the structure of spacetime or colliding cosmic strings.

The Telescope Array experiment, which has observed over 30 ultra-high-energy cosmic rays, including the famous “Oh-My-God” particle, aims to unravel the mysteries surrounding these phenomena. Researchers are analyzing the composition and characteristics of these cosmic rays to gain insights into their origins. The expansion of the Telescope Array, with the addition of 500 new scintillator detectors, will provide a larger footprint for capturing more events and shedding light on the nature of these extreme-energy cosmic rays.

While the discovery of the Amaterasu particle and its predecessor, the Oh-My-God particle, have raised more questions than answers, scientists remain determined to uncover the secrets of these ultra-high-energy cosmic rays. The ongoing research and expansion of the Telescope Array offer hope for a deeper understanding of the universe’s most energetic phenomena.

The research was published in Science.

References

• Bird, D. J., Corbato, S. C., Dai, H. Y., Elbert, J. W., Green, K. D., Huang, M. A., Kieda, D. B., Ko, S., Larsen, C. G., Loh, E. C., Luo, M. Z., Salamon, M. H., Smith, J. D., Sokolsky, P., Sommers, P., Tang, J. K. K., & Thomas, S. B. (1995). Detection of a cosmic ray with measured energy well beyond the expected spectral cutoff due to cosmic microwave radiation. The Astrophysical Journal, 441, 144. https://doi.org/10.1086/175344
• Telescope Array Collaboration*†, Abbasi, R. U., Allen, M. G., Arimura, R., Belz, J. W., Bergman, D. R., Blake, S. A., Shin, B. K., Buckland, I. J., Cheon, B. G., Fujii, T., Fujisue, K., Fujita, K., Fukushima, M., Furlich, G. D., Gerber, Z. R., Globus, N., Hibino, K., Higuchi, R., … Zundel, Z. (2023). An extremely energetic cosmic ray observed by a surface detector array. Science, 382(6673), 903–907. https://doi.org/10.1126/science.abo5095
• University of Utah. (2023, November 23). Telescope Array detects second-highest-energy cosmic ray ever. Phys.Org; University of Utah. https://phys.org/news/2023-11-telescope-array-second-highest-energy-cosmic-ray.html