Violation of the Greisen-Zatsepin-Kuzmin Cutoff: A Tempest in a (Magnetic) Teapot? Why Cosmic Ray Energies above1020eVMay Not Require New Physics

The apparent lack of suitable astrophysical sources for the observed highest energy cosmic rays within $\approx 20\mathrm{Mpc}$ 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 $\mu \mathrm{G}$ 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.