The extragalactic background light and a definitive resolution of Olbers's paradox

Abstract

There appears to be widespread confusion as to why the intensity of the extragalactic background light (EBL) is low. This concerns in particular the relative importance of factors associated with the lifetime of the galaxies and the expansion of the universe. To remedy this, an improved derivation is given of the main relations for the EBL. Then a new method is used to show explicitly what effect the expansion has on the EBL in Friedmann-Robertson-Walker models: the intensity is calculated for an expanding model and for an equivalent static model, and the ratio is formed. This is done for a wide range of plausible parameters, and a typical value for the noted ratio is about 1/2. It is thereby confirmed that the intensity of intergalactic light is determined to order of magnitude by the lifetime of the galaxies and only affected by a factor of about 2 by the expansion of the universe. This result also provides a definitive resolution of Olbers's paradox and should help stop persistent misrepresentation of this latter subject in the astronomical literature.