TeV gamma rays from blazars beyond z=1?

At TeV energies, the gamma-ray horizon of the Universe is limited to redshifts z << 1, and, therefore, any observation of TeV radiation from a source located beyond z = 1 would call for a revision of the standard paradigm. While robust observational evidence for TeV sources at redshifts z >= 1 is lacking at present, the growing number of TeV blazars with redshifts as large as z similar or equal to 0.5 suggests the possibility that the standard blazar models may have to be reconsidered. We show that TeV gamma rays can be observed even from a source at z >= 1, if the observed gamma rays are secondary photons produced in interactions of high-energy protons originating from the blazar jet and propagating over cosmological distances almost rectilinearly. This mechanism was initially proposed as a possible explanation for the TeV gamma rays observed from blazars with redshifts z similar or equal to 0.2, for which some other explanations were possible. For TeV gamma-ray radiation detected from a blazar with z >= 1, this model would provide the only viable interpretation consistent with conventional physics. It would also have far-reaching astronomical and cosmological ramifications. In particular, this interpretation would imply that extragalactic magnetic fields along the line of sight are very weak, in the range 10(-17) G < B < 10(-14) G, assuming random fields with a correlation length of 1 Mpc, and that acceleration of E >= 10(17) eV protons in the jets of active galactic nuclei can be very effective. DOI: 10.1103/PhysRevD.87.063002