Absorption of high-energy cosmic photons through double pair production in photon-photon collisions

At photon energies above 7.2 x 1020 eV, absorption by γ + γ' → 2 e+ + 2 e– dominates that from the lower-order process γ + γ' → e+ + e– for high-energy photons traversing a 2.8 K blackbody photon gas. Existence of the 2 e+, 2 e– process and the cosmic blackbody radiation thus guarantees that the universe is opaque for all photon energies above 1014 eV. The question of single-pair production in collisions with cosmic radio-wave photons is discussed, and the uncertainty in our knowledge of the important low-frequency end of the radio spectrum is emphasized. Other processes (γ + γ' → muons, hadrons, etc.) are discussed briefly, and it is concluded that they are of minor importance.