by Sahota
 P: 1 Hello there :) Been thinking about something for a while and i didn't really know where to seek out the answer, so i hope there is someone out there, whom might be able to answer it for me :) It's simply just something I've been wandering about, but here goes :) If i am to be correct, then the Microwave Bagground Radiation once used to be gamma radiation insted, right? If so would that meen that once upon a time, the Bagground Radiation would have been in the visible spectre? If so, what would it have looked like? and would we have been able to see it when we looked up upon the stars in the sky then? Hope it's okay i post here, didn't really know where else to put the post :) Best regards a curious Danish guy :)
 P: 2,070 Yes, at one time the CMB was visible radiation. Before a redshift of about 1000 (about 300,000 years after the big bang), the universe was made primarily of charged particles and radiation in thermal equilibrium with them. At this time the universe was opaque. As the universe cooled below a few thousand degrees K, the charged particles combined to form neutral atoms, and at this time the universe became transparent and the radiation propagated without scattering until we see it today. If we could have seen it at that point (this point is called 'decoupling'), it would have been a uniform visible glow. It would have looked more or less like what you would see (before you were incinerated!) if you were placed inside the outer atmosphere of the sun. However, at that point there were no stars, since the universe was still too uniform to have clumped up into stars. The first stars did not form until much later - how much later is a topic of current research, but probably at least hundreds of millions of years later.
 P: 1,262 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