In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) or extreme-energy cosmic ray (EECR) is a cosmic ray particle with an extreme kinetic energy, far beyond both its rest mass and energies typical of other cosmic ray particles.
These particles are significant for astrophysics and fundamental physics theory, because they have energies comparable to the Greisen–Zatsepin–Kuzmin limit, which occurs at about 5×1019 electron volts (8 J). This limit should be the maximum energy of cosmic ray particles that have traveled long distances (about 160 million light years), due to the theoretical energy losses of higher-energy ray particles and to scattering from photons in the cosmic microwave background.
Read more about Ultra-high-energy Cosmic Ray: Observational History, Active Galactic Cores As One Possible Source of The Particles, Other Possible Sources of The Particles, Pierre Auger Observatory, Ultra-high-energy Cosmic Ray Observatories
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—Ralph Waldo Emerson (1803–1882)