Neutrinos produced by ultrahigh-energy photons at high red shift
  Some of the proposed explanations for the origin of ultrahigh-energy cosmic rays invoke new sources of energetic photons (e.g., topological defects, relic particles, etc.). At high red shift, when the cosmic microwave background has a higher temperature but the radio background is low, the ultrahigh-energy photons can generate neutrinos through pair-production of muons and pions. Neutrinos produced at high red shift by slowly evolving sources can be detected. Rapidly evolving sources of photons can be ruled out based on the existing upper limit on the neutrino flux. 
Alexander Kusenko and Marieke Postma