Violation of the greisen-zatsepin-kuzmin cutoff: A tempest in a (magnetic) teapot? why cosmic ray energies above 10²⁰ eV may not require new physics

The apparent lack of suitable astrophysical sources for the observed highest energy cosmic rays within ≈20 Mpc is the “Greisen-Zatsepin-Kuzmin (GZK) paradox.” We constrain representative models of the extragalactic magnetic field structure by Faraday rotation measurements; limits are at the μG level rather than the nG level usually assumed. In such fields, even the highest energy cosmic rays experience large deflections. This allows nearby active galactic nuclei (possibly quiet today) or gamma ray bursts to be the source of ultrahigh energy cosmic rays without contradicting the GZK distance limit.