Photon Density and the Gamma-Ray Flux at a Point in an Expanding Universe

Abstract

The investigation deals with certain cosmological questions involved in the interpretation of the gamma-ray flux measurements made by the satellite Explorer XI. An expanding universe filled with similar sources of optical radiation is considered and the photon density in the neighborhood of an observer is obtained. Numerical values are given for the density of 1-eV photons when the sources are regarded as blackbodies at a temperature of 5000°K. The necessity of specifying the bandwidth, the model of the universe, and the scale of distance before a density can be stated, is emphasized. The models of the universe considered are those of general relativity and that of the steady-state theory. It is concluded that a photon density can be specified, in the present state of astronomical knowledge, only as lying within wide limits.