Where/what is emitting cosmic background radiation, and when did it come into ex

Question

Where/what is emitting cosmic background radiation, and when did it come into existance, was immediately after the big bang?

I know that the universe isn't 3D in the traditional sense, and I don't pretend to understand it. However if the big bang originated from a single point, and the cosmic radiation was emitted from that point at the very start, then unless he explosion/expansion/inflation (whatever happened) wasn't faster the speed of light, then the CMB would have reached the edge of the universe and gotten out of it.

Explanation / Answer

When the Universe was very young, the matter in it was very hot. That matter emitted thermal radiation, in the same way that other hot things do. For quite a while, the radiation was in equilibrium with the matter: photons of radiation were constantly being both emitted and absorbed by the matter (mostly the electrons that were zipping around).

At a certain point, the matter "decoupled" from the radiation, meaning that interactions between photons and other particles became very rare. The main reason for this is that most of the matter formed neutral atoms, rather than consisting of free charged particles (electrons and nuclei). Neutral atoms don't interact with radiation nearly as strongly as charged particles.

From that time on, most of the photons that were in existence simply flew through space, not interacting with anything. Those are the CMB photons that we see today.

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