Signature neutrinos from ultrahigh-energy photons
  At high red shift, the temperature of cosmic microwave background is sufficiently high for the ultrahigh-energy photons to pair-produce muons and pions through interactions with the background photons. At the same time, the radio background and magnetic fields are too weak to drain energy out of the electromagnetic cascade before the muons and pions are produced. Decays of the energetic muons and pions yield neutrinos with some distinctive spectral properties that can be detected and can indicate the presence of ultrahigh-energy photons at high red shift. The neutrino signature can help identify the origin of cosmic rays beyond the Greisen-Zatsepin-Kuzmin cutoff. 
Alexander Kusenko