What was the extreme spectral shift observed for the light that now constitutes the Cosmic Microwave Background (CMB)?

Answer

It was redshifted from visible light all the way into the microwave band.

The CMB originated as visible light when the universe was very young (380,000 years old), but subsequent expansion by a factor of over a thousand has redshifted that radiation into the microwave portion of the spectrum.

What was the extreme spectral shift observed for the light that now constitutes the Cosmic Microwave Background (CMB)?

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