The GZK Cutoff
- Calculations show that high-energy cosmic rays lose energy to interactions with the
cosmic microwave background.
- At 10^19 eV, protons lose energy via photo-pion reactions on a scale of ~10 Mpc.
(The scale decreases at even higher energies.)
- However, there have been several observations of UHECRs up to 2x10^20 eV.
- UHECRs must come from relatively nearby, but no sufficiently energetic sources have
been conclusively identified.