The GZK limit (or Greisen–Zatsepin–Kuzmin limit) is a theoretical upper energy threshold for cosmic rays—high-energy particles traveling through space—beyond which they lose energy when interacting with the cosmic microwave background (CMB) radiation.
Predicted in 1966 by Kenneth Greisen, Georgiy Zatsepin, and Vadim Kuzmin, the GZK limit is around 5 × 10¹⁹ electronvolts (eV). Cosmic rays above this energy should undergo collisions with low-energy CMB photons, producing pions and rapidly losing energy. As a result, such ultra-high-energy cosmic rays (UHECRs) cannot travel very far, typically not more than about 100 million light-years.
Key implications of the GZK limit:
- It constrains the maximum distance from which UHECRs can reach Earth.
- It helps identify possible cosmic sources, such as nearby active galaxies or black holes.
- It provides a natural test of high-energy physics and our understanding of particle interactions in extreme conditions.
Observations from experiments like the Pierre Auger Observatory have largely confirmed the suppression of ultra-high-energy cosmic rays at this limit, making the GZK cutoff a crucial concept in astroparticle physics and cosmology.