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

Abstract
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.