The Cosmic Gamma-Ray Spectrum from Secondary-Particle Production in the Metagalaxy

The purpose of this paper is to discuss the form and intensity of the spectrum of cosmic y-rays re- sulting from the production and decay of 7r°-mesons produced in metagalactic cosmic-ray p-p collisions. It is assumed that intergalactic space contains ionized hydrogen gas at a density of 1O~ cm3, as is consistent with recent X-ray observations at 0 25 keY. The Friedmann solution to the Einstein field equations of general relativity is used as a description of our expanding Universe, and a discussion is presented of the effects of redshift and spatial curvature on the generation and distortion of the local 7-ray spectrum from the decay of ir°-mesons Numerical calculations are presented for the Einstein-de Sitter solution, which is found to be an adequate model for these calculations Two models are presented to represent the possible flux of metagalactic cosmic rays In calculating metagalactic 7-ray spectra, the effect of 7-ray absorption at large redshifts is taken into account. A discussion of the results is given The results indicate that future 7-ray experiments in the region of 1-100 MeV may yield valuable information relating to cosmology, cosmogony, and the metagalactic cosmic-ray flux. In particular, the metagalactic 7-ray spectra predicted tend to peak near 70 (1 + Z~ax)1 MeV, where ~ the maximum redshift at which cosmic rays are produced, may correspond to the age of the Universe at the epoch of galaxy formation