Greisen-Zatsepin-Kuzmin Limit

CORE DEFINITION

The GZK limit, named after the initials of its proposers Greisen, Zatsepin, and Kuzmin, is a theoretical upper limit describing the maximum energy that cosmic rays originating from distant sources should have. This limit was calculated in 1966 by Kenneth Greisen, Vadim Kuzmin, and Georgiy Zatsepin, based on the expected interaction between cosmic microwave background radiation and cosmic rays. The prediction states that if a cosmic ray carries energy exceeding the threshold of 5×10¹⁹ electron volts, it will interact with photons of the cosmic microwave background, producing pions. Such interactions will continue until the particle's energy drops below the pion production threshold. Because the mean free path for this interaction is very short, for example, extragalactic cosmic rays originating from distances greater than 50 megaparsecs from Earth with energies above this threshold cannot be observed on Earth; and within this distance, there are no known sources capable of producing such high-energy cosmic rays. Some observations from the Akeno Giant Air Shower Array experiment have shown that cosmic rays from distant sources carry energies above this limit (known as ultra-high-energy cosmic rays).