P: 1,261 Hi Sahota, great question! There was definitely a time when the CMB was the COB (the Cosmic 'Optical' Background ;). The frequency of radiation changes as $\nu \propto \frac{1}{1+z}$ for the cosmological redshift 'z'. The CMB is currently at a wavelength of about a millimeter, so the redshift for an optically-peaked CMB would be something like z~2500. This is in the very very early universe, long long before the first stars, galaxies, etc etc; and even before the first atoms had formed (everything was in a plasma). Because of this, the ambient medium of plasma was very 'optically thick' or 'opaque' (it absorbs the light, keeping it from free-streaming out, like it does now). I think this would mean everything would be in a warm, gaseous glow. and at the time every point in the sky would look like surface of the sun----exc