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Location Of CMB Particles

  1. Nov 4, 2008 #1


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    Are the particles that comprise the CMB in the shape of a "shell" so that all of these particles are very far from our Solar System, or are they in the form of a solid "ball" so that they are evenly distributed throughout our Observable Universe in which case some would be located right here near the sun.?

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  3. Nov 4, 2008 #2


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    you need to distinguish between the matter particles which emitted the photons, and the photons of light (now stretched to microwave photons) which actually comprise the CMB.
    CMB means cosmic microwave background, so it consists of photons----the word background suggests something evenly distributed throughout all space. And indeed the CMB photons are evenly distibuted throughout all space as far as we know.

    And yes, there are plenty of them located right here near the sun! And near the earth, because we are always detecting them with microwave antennas.


    the matter which long ago emitted the CMB is another issue

    ALL matter which existed at that time (380,000 years into expansion) participated in emitting light which eventually became CMB for somebody somewhere. The matter in your and my bodies took part in that emission of light.

    That event was the first great emission of light not destined to be immediately re-absorbed and scattered by previous foggy conditions. Every bit of matter took part. Mostly hydrogen atoms. It was a great whoopee because the fog had cleared. And then that matter proceeded to form stars and planets and go about its matterly business.

    But the light emitted by the matter in your and my bodies is now far away from here (46 billion lightyears and rising) so it is not the CMB light that we here see.
    The CMB light that we can currently detect with antennas was emitted from other matter similar to ours which happens to be (in a shell) 46 billion lightyears away from us in all directions because that distance is exactly the distance that light must have traveled in the 13.7 billion years since it was emitted.

    Light from anything closer has already been here and gone.
    Light from anything farther has not gotten here yet.
    So the matter we currently see, when we make maps of the CMB sky, must be located in a spherical shell with that radius. That shell is a mathematically defined sphere called the surface of last scattering. It is not a physical object---just the location where the matter is that we are now viewing with our microwave eyes.
  4. Nov 5, 2008 #3
    81+ – Just a small follow up on the comments:
    That does mean the light has traveled 46 billion lightyears in only 13.7 billion years; obviously now traveling FTL away from us.
    Just to be clear that is only “apparently faster than light” as Hubble expansion accounts for how space distances are increasing while the speed of light remains “c” for the local area it is traveling in.
    So as the light gets farther away from is origin (Us or its source matter that eventually become us), Hubble expansion actually causes that distance to grow even larger – leading to the apparent FTL effect. Likewise for the CMB coming to us and the distance behind it.
    Last edited: Nov 6, 2008
  5. Nov 5, 2008 #4


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    Absolutely. That's a good additional point to make about this. It is something you can see graphically happening in, say, Ned Wright's computer-animation balloon model. I have links to that in the balloon analogy sticky.

    Or you can just google "wright balloon model" and you will get two different animations, watch both!

    Something like a compound interest savings account happens with the light that our matter emits in the early universe. The distance it initally gets away from us is amplified by subsequent expansion of distances, and that is amplified by even later expansion.

    So it gets to be, as you see 81+, quite substantially farther than it would have been in a static universe. (But in any case a static universe is practically speaking a physical impossibility, assuming Gen Rel is right, so static's not a case to worry about.)
    Last edited: Nov 6, 2008
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