Photon-photon scattering of gamma rays at cosmological distances

The total scattering rate, the optical depth of the universe, and the scattered photon spectrum are derived from gamma-ray photons scattering in an isotropic field of blackbody photons. By solving the kinetic equation describing repeated photon-photon scattering for both monoenergetic and power-law injection of photons, it is shown how the photon spectra resulting from pair-photon cascades are further reprocessed by photon-photon scattering. The power of photons injected at the photon energy epsilon(max), where the pair cascades terminate, is reprocessed into a characteristic spectral jump at epsilon(min). Photon-photon scattering thus reduces the spectral cutoff energy from epsilon(max) to epsilon(min). Photon-photon scattering is likely to be important in consideration of the constraints imposed by the primordial abundances on the properties of exotic unstable particles.