Telescope Array Discovers Second-Highest-Energy Cosmic Ray on Record


A recent discovery made by the Telescope Array experiment has astrophysicists intrigued once again. On May 27, 2021, the experiment detected the second-highest-energy cosmic ray ever observed, baffling scientists and presenting a conundrum as to its origin and journey to Earth.

Back in 1991, the University of Utah Fly’s Eye experiment detected the highest-energy cosmic ray ever recorded, commonly referred to as the Oh-My-God particle due to its incredible energy levels. This particle surprised scientists because it exceeded anything that could be produced within our own galaxy. Furthermore, it possessed more energy than what was theoretically possible for cosmic rays traveling to Earth from other galaxies. Essentially, it defied all known laws of physics.

Since then, the Telescope Array has documented over 30 ultra-high-energy cosmic rays, but none have come close to the energy of the Oh-My-God particle. These observations have not yet revealed the source or the mechanisms that allow these cosmic rays to make their way to Earth.

The recent discovery by the Telescope Array experiment unveiled a second-highest-energy cosmic ray with an energy level equivalent to dropping a brick from waist height onto your toe. This single subatomic particle registered an energy of 2.4 x 1020eV. The experiment, led by the University of Utah and the University of Tokyo, utilized the Telescope Array, which consists of 507 surface detector stations covering an area of 700 km2 (~270 miles2) in Utah’s West Desert.

The event triggered 23 detectors in the north-west region of the Telescope Array, spanning an area of ​​48 km2 (18.5 mi2). The particle appeared to come from the Local Void, an empty region of space bordering our Milky Way galaxy. These high-energy particles are typically unaffected by galactic and extra-galactic magnetic fields, allowing scientists to determine their source in the sky. However, both the Oh-My-God particle and this newly discovered particle have trajectories that lead back to the empty void, leaving scientists puzzled as to what could have generated them.

In a study published in the journal Science, an international collaboration of researchers described the ultra-high-energy cosmic ray, assessing its characteristics and suggesting that it may follow particle physics principles that are currently unknown to us. The researchers named the particle the Amaterasu particle, after the sun goddess in Japanese mythology. Importantly, the different observation techniques used to detect the Oh-My-God particle and the Amaterasu particle confirmed that these ultra-high-energy events are indeed genuine, albeit rare.

These events appear to originate from different locations in the sky, indicating that there isn’t a single mysterious source. Some scientists have proposed various theories, such as defects in the structure of spacetime or colliding cosmic strings, to explain these phenomena. However, there is currently no conventional explanation for the origin and nature of these high-energy cosmic rays.

Cosmic rays are remnants of violent celestial events that strip matter down to its subatomic particles and propel them through the universe at speeds close to that of light. These rays consist of charged particles, including positive protons, negative electrons, or entire atomic nuclei, that constantly shower down upon Earth as they interact with our atmosphere.

As cosmic rays enter Earth’s upper atmosphere, they collide with oxygen and nitrogen gas nuclei, creating numerous secondary particles. These secondary particles travel a short distance before repeating the process, resulting in a cascade of billions of particles that scatter across the surface. Detecting and studying these secondary showers require the use of large detector arrays like the Telescope Array. These surface detectors provide valuable information about each cosmic ray, including its trajectory and energy.

Due to their charged nature, the flight path of cosmic rays is often erratic, tugged and pushed by electromagnetic fields and the cosmic microwave background. Tracking the trajectory of most cosmic rays, especially those with lower energy, is extremely challenging. Even high-energy cosmic rays can be distorted by the microwave background. However, particles with energies similar to the Oh-My-God and Amaterasu particles can penetrate intergalactic space with relatively minimal deflection. Only the most powerful cosmic events have the capacity to generate such highly energetic particles.

Traditional energetic events like supernovae are far from being energetic enough to produce particles of this magnitude. The generation of ultra-high-energy cosmic rays requires immense amounts of energy and high magnetic fields to confine and accelerate the particles, according to John Matthews, Telescope Array co-spokesperson and co-author of the study.

Ultra-high-energy cosmic rays must exceed 5 x 1019 eV, which is equivalent to the kinetic energy of a major league pitcher’s fastball. This level of energy is tens of millions of times higher than what human-made particle accelerators can achieve. Scientists have calculated a theoretical limit, known as the Greisen-Zatsepin-Kuzmin cutoff, which is the maximum energy a proton can possess when traveling long distances before the interactions with microwave background radiation deplete its energy.

Known sources that could potentially produce such high-energy cosmic rays, such as active galactic nuclei or black holes with accretion disks emitting particle jets, are typically located over 160 million light-years away from Earth. However, the newly discovered particle with an energy of 2.4 x 1020 eV and the Oh-My-God particle with an energy of 3.2 x 1020 eV far surpass this theoretical limit.

In addition to energy analysis, researchers also study the composition of cosmic rays in their quest to uncover their origins. Heavier particles, like iron nuclei, are more susceptible to bending in a magnetic field due to their greater charge compared to lighter particles made up of protons. Based on this information, scientists believe that the newly discovered particle is likely a proton. Particle physics dictates that a cosmic ray with energy surpassing the Greisen-Zatsepin-Kuzmin cutoff cannot be significantly affected by the microwave background and instead points towards empty space.

Various explanations have been proposed to solve this mystery, including the possibility of stronger magnetic fields than previously believed or alternative theories that challenge existing observations. However, scientists remain perplexed, and the expansion of the Telescope Array experiment is currently underway to capture more events and shed light on these enigmatic ultra-high-energy cosmic rays. Once completed, the expansion will incorporate an additional 500 scintillator detectors, increasing the sampling area to 2,900 km2 (1,100 mi2), comparable to the size of Rhode Island. This expanded footprint will hopefully yield further discoveries and provide crucial insights into the origin and behavior of these cosmic rays.



  1. Source: Coherent Market Insights, Public sources, Desk research
  2. We have leveraged AI tools to mine information and compile it