I have been googling Cosmic Background Radiation, but am still confused as to the location of its source. Is it just very old light finally arriving from very distant sources? Or is earth also surrounded by nearby CBR sources that in the fullness of time will arrive at very distant galaxies?
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According to the Big Bang model, the entire universe originated from a single point. That is, at age t=0, all the energy content of the universe was packed into a single point. Then as we progress in time, the volume of the universe expanded (and is continuing to expand).
In this model, the early universe was still so dense that most light would bounce off other objects. So in effect light was everywhere and the universe was opaque dense plasma. After a certain point, the universe expanded enough and cooled enough (same thing really) that it became transparent – that bouncing light now stopped bouncing and is free to travel straight through space.
So the CMB we perceive today is exactly the light that happened to be traveling this way from exactly the right distance from us when that transition from opaque to transparent happened. Notice we are not special in this – everywhere you go in the universe you will also observe the same thing (but the shell of light will just be a different one); similarly as time progresses we will just observe CMB from a farther and farther shell.
So just imagine a deflated balloon with some colored fog inside. This fog is the CMB and the balloon is the universe (starting out with low volume). Imagine the balloon is filled with regular air now (this doesn’t represent anything – just the expansion of space). And imagine we are at any given point inside the balloon – in all directions we will see a bit of that colored fog.
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