"In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray particle with a kinetic energy greater than 1×1018 eV, far beyond both the rest mass and energies typical of other cosmic ray particles.

An extreme-energy cosmic ray (EECR) is an UHECR with energy exceeding 5×1019 eV (about 8 joule), the so-called Greisen–Zatsepin–Kuzmin limit (GZK limit). This limit should be the maximum energy of cosmic ray particles that have traveled long distances (about 160 million light years), since higher-energy ray particles would have lost energy over that distance due to scattering from photons in the cosmic microwave background. 

It follows that EECR could not be survivors from the early universe but are cosmologically "young", emitted somewhere in the Local Supercluster by some unknown physical process."