Cosmic microwave background

[Brendan]

What wavelengths were the photons of the microwave background at when they were being created? If the background ever passed through the part of the spectum that is visible to humans, it could be shown by using Selden's idea of moving through nested spheres. Maybe someone could come up with textures representing the early universe's appearence at different stages and use them as textures on the insides of the 3ds spheres. That could make a good educational activity about the Big Bang.

Brendan

[granthutchison]

What wavelengths were the photons of the microwave background at when they were being created?

Radiation decoupled from matter when the temperature of the Universe was around 3000K - so the initial cosmic background was at the same temperature as a red giant star, and would have looked orange (assuming you could peek in there for a moment).

Grant

[Brendan]

How quickly did the decoupling happen? Energy would be released when the electrons got attached to the nuclei, the of opposite what happens with ionization. Would the different ionization energies of hydrogen, helium and lithium have noticable effects during the decoupling?
So if the universe looked orange at decoupling, it must have gone through the colors of the rainbow. Did it start with gamma rays, go down to x rays, ultraviolet, visible light, then infrared and the present day microwaves?

[granthutchison]

Over a few hundred thousand years, the temperature of the Universe fell from a level at which all matter was ionized by the endless bombardment of high-energy photons, to a state in which the average photon energy was too low to ionize hydrogen or helium. So in the early Universe there were gammas and X-rays (in a hot soup of particles at millions of degrees K), but they didn't become the background radiation ... because they lived relatively short lives before interacting with matter.
Only when the temperature of the Universe fell as low as 3000K did a substantial quantity of matter become un-ionized, because the ambient photons in general lacked the energy to maintain ionization. Those photons, in a black-body spectrum with a peak at the long-wavelength end of the visible range, could then sail on across the Universe without being absorbed. And as the Universe expanded, the wavelength of these photons was subsequently stretched down through the spectrum, so that for a while the Universe glowed deep red, and then was filled with a fading infrared background, and then today's microwave background.

I don't know enough to say anything about your question concerning specific ionization energies.

Grant