Colour of the Luminous Background of the Extra-Galactic Nebulæ

Abstract

ACCORDING to Milne, the multitude of the distant galaxies form a continuous luminous background. It is interesting to investigate what its colour should be. By Hubble's law, the spectra of the galaxies are shifted towards the red. It is easy to calculate the integral colour of the background if we adopt Milne's view that the galaxies are really receding. Then the light of a galaxy is shifted towards the red according to Doppler's law. The intensity from a receding nebula is less than it would be if the nebula were motionless at the same distance. Indeed, a quantum of light if shifted to the red has less energy than the original quantum; in addition, when a source of light is receding, its quanta fall upon the observer less frequently than they would if the source were motionless. Under the combined action of both causes the intensity is diminished doubly.