Annu. Rev. Astron. Astrophys. 2002. 40: 319-348 Copyright © 2002 by Annual Reviews. All rights reserved

7. GZK LIMIT

Greisen (1966) and Zatsepin & Kuzmin (1966) pointed out that EeV cosmic rays lose energy due to photo-pion production through interaction with the CMB. These losses limit the propagation distance for 10²⁰ eV particles to about 60 Mpc. Yet no clear correlation has been found between the arrival direction of high energy cosmic rays and the most likely sites of origin for EeV particles, namely, AGN at distances less than 60 Mpc (Elbert & Sommers 1995, Biermann 1999). One solution to the GZK paradox is to assume the energetic particles are isotropized in the IGM by tangled magnetic fields, effectively randomizing their observed arrival direction. Such isotropization requires fields in the local super-cluster 0.3 µG (Farrar 2000, Isola, Lemoine, & Sigl 2001).