document: The microwave background is not the only universal photon field that has to be taken in consideration. Especially interesting is the isotropic infrared and optical background (IRB). The number density of IRB is smaller than that of MBR by more that two orders of magnitude. On the other hand, protons of lower energy can interact on the IRB, and the smaller number density has to be weighted with the higher flux of interacting protons. The present Universe is optically thin to 1019 eV and lower energy protons, but even at low redshifts the proton interaction rate quickly increases. This is different from the interactions on MBR, where the interacting protons quickly lose their energy even at z=0. The cosmological evolution of UHECR injection is thus of major importance for the contribution of such interactions to the flux of cosmogenic neutrinos.
doc_bio_tags: O B I O O O O B I I O O O O B I I O O O O B I I I I B O O O O B O O O O O B O O O O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O B I O B I O O O O O O O B I I O O O O O O B O O O O O O O O O B I I O O B I O O O O O O O O B I I I I O O O O O O O O O O O O O B I I I