Greisen–Zatsepin–Kuzmin limit

The Greisen–Zatsepin–Kuzmin limit (GZK limit or GZK cutoff) is a theoretical upper limit on the energy of cosmic ray protons traveling from other galaxies through the intergalactic medium. The limit is 5×1019 eV, or about 8 joules (the energy of a proton traveling extremely near the speed of light).

Source: Wikipedia — Greisen–Zatsepin–Kuzmin limit (CC BY-SA 4.0)