Opacity of the Universe to High-Energy Photons

Based on observational data, the spectra of cosmic radio, microwave, infrared, optical, and x-ray photons are estimated. The absorption probability per unit path length by the process of pair production in photon-photon collisions is then computed as a function of energy for high-energy photons traversing this photon gas, using the results of the previous paper. These calculations show that there should be a dip in the intensity of the high-energy cosmic photon spectrum by about a factor of 10 between ${10}^{12}$ and ${10}^{13}$ eV due to absorption by optical (\ensuremath{\sim} a few eV) photons. Above ${10}^{14}$ eV, the high-energy cosmic photon spectrum should essentially cut off because of the strong absorption by the cosmic 3\ifmmode^\circ\else\textdegree\fi{}K blackbody photons, and at higher energies by the cosmic radio photons.