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I'm not talking about gradual red-shifting, that happens slowly anyway, but with a universe of finite size, we should actually see Cosmic Background Radiation turn off in a sense, first at a point, then slowly disappearing in an expanding circle, like it's being turned off? - or am I thinking about this wrong?

The only reason we see CMB now from all directions is cause the universe was greater than 13.5 billion light years radius from where we are now at the time of the background radiation, some 13.5 billion years ago - kinda. In other words, the universe had to be at least 13.5 billion light years radius at just 380,000 years of age.

Of-course, if the universe is much larger than that, this may not happen for an enormously long time, but in a finite universe this should still happen?

or am I thinking about this all wrong. - Thanks.

Jim
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userLTK
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    The Big Bang didn't happen at a point. It happened everywhere, so the CMB is present everywere. – John Rennie Feb 25 '15 at 17:24
  • CMB isn't from the big bang. – userLTK Feb 25 '15 at 17:29
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    The CMB is a consequence of the evolution of the universe from the Big Bang. If the Big Bang happened everywhere (which it did) then recombination happened everywhere as well. tl;dr - you are thinking about this all wrong. – John Rennie Feb 25 '15 at 17:42
  • Even if the universe is finite and even if there is some physical edge or ending to it just beyond our observable limits (which there definitely isn't), we would still never be able to observe or reach this edge. Expansion works in such a way that at the moment we expect to never be able to see anything that is currently outside the limits of the observable universe. And we expect that the CMB, which happened at every point, will continue to be visible. It will just shift to lower temperatures as time goes on – Jim Feb 25 '15 at 17:55
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    "thinking about it all wrong?" I'm not sure how that's supposed to be helpful.

    OK, as I understand it, the early universe, before CMB was Opaque, not exactly like the inside of the sun, but kind of like that, photons were basically bouncing back and forth between electrons and atomic Nuclei.

    Once it cooled enough for Electrons to bind to atomic Nuclei and form what we think of as normal atoms - light basically flashed, and, I know it wasn't instantaneous, but the CMB flash was temporary and much brighter than any thermal radiation that followed? or no?

     – userLTK Feb 25 '15 at 18:12
  • flashed? What do you mean flashed? It was thermal radiation. The CMB is the best known example of thermal radiation. I'm not sure how you mean it was temporary. – Jim Feb 25 '15 at 18:14
  • OK, that works. I thought it was something more than thermal radiation. - Thanks. – userLTK Feb 25 '15 at 18:17

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