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B The Nature of the Cosmic Microwave Background

  1. Apr 19, 2016 #1
    When a "layman" hears of the cosmic microwave background, and this layman also has a passing knowledge of big bang theory, it can be difficult for said layman to wrap his head around what the CMB really is.

    That layman might ask, wouldn't this primordial radiation have "passed" us by now, and we wouldn't be able to see it? In other words, the universe originated in a "big bang" and has been expanding since then at some speed less than the speed of light. Therefore this primordial radiation would exceed the boundaries of the matter which comprises the known universe, and you wouldn't be able to see it any more.

    Obviously this is an oversimplification of big bang theory and CMB, and I was wondering if those of you knowledgable about the details of cosmology can elaborate for me. I.e., debunk the idea that the CMB would have passed us by now. Or, explain why the CMB has not passed us.

    -Scott V.
  2. jcsd
  3. Apr 19, 2016 #2
    It hasn't all passed us because it's not coming from a particular point in space, but from all of space.
    Light which was emitted from more distant parts takes longer to get here.
  4. Apr 19, 2016 #3
    OK, thanks for the answer! I guess one way of looking at it is, in the young universe, space as we think of it now didn't really exist, or quite exist as much as it does now, so all around us are the remnants of that early universe?
  5. Apr 20, 2016 #4


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    Following the Big Bang, the universe was filled with hot plasma, all of it glowing and all of it absorbing light. At some point it cooled enough that it started to form atoms and light had a chance to propagate. It's the glow emitted then that we are seeing. It came from every point in the universe, so one year after atoms started to form you would have been seeing the glow from one light year away; ten years after you would have been seeing the glow from atoms ten light years away. Now we are seeing light emitted a very long way away, 13.9bn years ago. It doesn't look really hot because the universe is expanding and that has red-shifted the glow to around 3K.

    Think of Earth as a rock sticking out of a pond. Throw a stone into the pond and it makes ripples which spread out, wash over the rock and carry on. This is analogous to a supernova - a short (on cosmological timescales) flash at a single point in space. If you aren't looking when the flash washes past Earth, you've missed it.

    Now build a grid of squares an inch by an inch with a stone at each junction. Hold it over the pond and add a mechanism that makes it drop all of its stones at once. There will be ripples everywhere when they all drop. Stones near the "Earth" will make ripples that will wash quickly past and be gone. But there are ripples from the next stone right behind, and more ripples behind those ones. There will always be ripples because ripples started from everywhere. There will always be CMB because it came from everywhere.

    Does that help?
  6. Apr 20, 2016 #5
    Yes, thanks. Also, maybe another way to look at it is that the universe has always been unbounded, contrary to an uninformed belief that the universe started at a point, or a little sphere. So at one time, when the universe was very young, the universe consisted of this unbounded plasma. Then it cooled enough to create this background of microwave radiation. The older the universe gets the further away this background gets. Is that a valid way of looking at it?
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