Comment on ``Violation of the Greisen-Zatsepin-Kuzmin Cutoff: A Tempest   in a (Magnetic) Teapot? Why Cosmic Ray Energies above $10^{20}$ eV May Not   Require New Physics''
  In a recent letter [1] with the same title, Farrar and Piran offered an explanation for the near isotropy of the arrival directions [2] of ultrahigh energy cosmic rays (UHECRs) and the apparent absence [3] of the so called `GZK cutoff' in their spectrum around $10^{20}$ eV due to pion photoproduction on the cosmic background radiation (CMB) that was predicted independently by Greisen [4] and by Zatsepin and Kuz'min [5]. They suggested that the extragalactic magnetic fields near the Milky Way are strong enough to deflect and isotropise the arrival directions of the UHECRs from a few nearby sources for which their travel time to Earth is shorter than their attenuation time by pion photoproduction on the CMB. They also estimated that this allows active galactic nuclei (AGNs) or gamma ray bursts (GRBs) to be the source of the UHECRs. However, these suggestions are inconsistent with various observations 
Arnon Dar