6.1. Extragalactic Backgrounds

With the exception of the CBR, establishing the existence of electromagnetic backgrounds at other wavelengths has been a difficult chore. Here we summarize, in order of increasing wavelength, the current evidence for different backgrounds and what the possible interpretations are. In general, these backgrounds have been discovered via satellite observations. These observations are of rather low resolution and therefore can not effectively distinguish between backgrounds that represent a uniform distribution of hot or cold gas from those that are due to an aggregate of discrete sources. This is much like making a naked eye observation of the plane of the Milky Way and concluding that there is a diffuse, continuous band of light that runs across the sky. Higher resolution observations, of course, reveal the source of the radiation to be discrete stars. Finally much care must be taken to isolate extragalactic backgrounds from galactic backgrounds. This requires an all-sky survey and subsequent analysis of the data in galactic coordinates.