An Etymological Dictionary of Astronomy and Astrophysics

A theoretical limit of approximately 6 × 10¹⁹  → electron-volts for the energy of → cosmic rays above which they would lose energy in their interaction with the → cosmic microwave radiation background photons. Cosmic ray protons with these energies produce → pions on blackbody photons via the Δ resonance according to: γ_CMB + p → p + π⁰, or γ_CMB +
p → n + π⁺, thereby losing a large fraction of their energy. These interactions would reduce the energy of the cosmic rays to below the GZK limit. Due to this phenomenon, → Ultra-high-energy cosmic rays are absorbed within about 50 Mpc.

See also: Named after Kenneth Greisen (1966), Physical Review Letters 16, 748 and Georgiy Zatsepin & Vadim Kuzmin (1966),
Journal of Experimental and Theoretical Physics Letters 4, 78; → limit.