system
Given the following post, respond as if you're commenting on Reddit:
human
When did the CMB (Cosmic Microwave Background) pass through the visible light spectrum? If we had been around at that time, would it have been bright enough to see it?
gpt
At its inception it would have been visible. During the early moments of the Universe it was still immensely hot, so hot that the atoms around where always ionized as a plasma. Plasmas are incredibly opaque because with the electrons and nuclei separated they can absorb a wide range of photon energies, whereas when confined in a neutral atom only photon energies that excite an electron to one of a small number of specific energy level will be absorbed.

At that time the entire universe glowed, everywhere in every direction because everything was hot. But the light from that glow didn't travel very far due to the opacity of plasma. But eventually at about 377,000 years old the Universe reached a tipping point where it finally cooled enough for neutral atoms to be able to form and stick around, which caused the Universe to become transparent fairly suddenly.

Now here's one of the little weird parts that might be hard to wrap your head around. When you look into space you're looking back in time, so when you see the cosmic background radiation from this era you are actually seeing the hot, dense plasma of the past through the transparent veil of the recombined atoms of the early Universe. Or, put another way you could imagine all the light bouncing around the early Universe when it was in its plasma stage and then the Universe suddenly going transparent and all the light that was in between being emitted and absorbed simply flying off in every direction from every point. Because of these conditions the "temperature" of this light must be very close to the temperature at which Hydrogen atoms (which dominated the early Universe) ionize, which is right around 3000 Kelvin.

If you happened to be magically transported to the Universe at that age and could see this light it would look mostly white with a slight yellow/orange tinge, similar to an incandescent light bulb. And it would be incredibly bright, the entire sky would look nearly as bright as a star, if Earth popped into existence there it would melt quite promptly (the entire sky would be bathed in 3000 K light, so everything in the Universe would settle to becoming that temperature).

However, at this period of time the Universe was expanding very rapidly so it didn't take long, by astronomical measurements, for it to cool off significantly. This is because as the Universe expanded the relative speed between any given part of the Universe and the sphere of points corresponding to the date and location where the big bang glow was still visible became larger, so that light became "red shifted", effectively cooling it to lower and lower temperatures. Another way to look at this is to imagine that you have a quantity of energy in photons in the Universe at the moment when it becomes transparent and then you stretch out the Universe, logically as you increase the volume the amount of photon energy in it must go down.

Anyway, today that once white-hot glow of superheated gas and plasma that filled the entire sky is now a very cold 2.7 K glow of mostly microwave radiation.