The following simple argument shows. Any radiation produced near the sun, in our galaxy, or even in nearby galaxies would undoubtedly be unevenly distributed. Therefore, we assume that the sources of the radiation are evenly distributed throughout space. Suppose we divide the universe into a large number of concentric and equally spaced spherical shells, all centered on and enclosing the earth. In this case, the amount of radiation coming from sources within any pair of adjacent shells is the same, because the area of a sphere increases with distance in just the same way as the intensity of the radiation decreases. The number of emitting sources increases with the area, and the flux from each source decreases as the square of it's distance. A uniform background radiation must come from mostly distant parts of the universe, where the majority of the sources are found. Very little of the radiation could originate in our local region of space and any isotropic background radiation must be produced at cosmological distances.
