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Cosmic Microwave Background Radiation, why from 380,000?

  1. May 30, 2012 #1
    I just have a quick question about CMB.

    Why is it from 380,000 years after big bang? Why not before?

    Will you please tell me if my explantion is right?

    Before 380,000 years, the universe was too dense to have any neutral atoms (free electrons and protons / plasma). And apparently, these free electrons cause the universe to be opaque (what does this mean? Does this mean energy from light is absorbed? Not allow to be transmitted/or re-emitted?) Because the light is absorbed almost instantly as it is emitted, it is not allowed to move out into space, and can never get to us (travel into the future sort of).

    Okay if my reasoning is right, does that mean free electrons absorb lights and hold onto it?
    Whereas, at 380,000 years, atoms are created, and now atoms do not hold the light but
    let the lights be transmitted through? (Is this because, it only absorbs lights at specific wavelength? and even those that are absorbed, gets re-emitted when electrons come back down to a lower energy orbitals?)

    Okay, and my last question is, if it started 380,000 years, when did it end?
  2. jcsd
  3. May 30, 2012 #2


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    Your description is right.

    Free charges absorb photons and emit different photons (at the same time). Therefore, photons could not survive before most of the atoms became neutral. The photons from the remaining free charges could just fly through the universe - and continue to do so up to now.

    Photons from the CMB will continue to fly around forever, or until something destroys the universe.
  4. Jun 2, 2012 #3
    Charged particles -- electrons and ions -- scatter radiation. When they combine into an atom then this is neutralized. (Don't ask me how, because I don't know.)

    Something like a hydrogen atom only absorbs and emits light at specific wavelengths. It would scatter that light but that hardly matters, I think.
  5. Jun 2, 2012 #4
    Good question....there is not a precise answer, I think, because the CMBR is formed from decay processes, sort of like radioactive decay...when does THAT end??

    Here is an interesting discussion on the issue:


    Last edited: Jun 2, 2012
  6. Jun 2, 2012 #5
  7. Jun 2, 2012 #6
    "Free charges absorb photons and emit different photons (at the same time. "

    no. no.
    As a fundamental particle an electron by itself cannot absorb energy...it has no degrees of freedom, no underlying structure, with which to do so. In an atomic structure, there ARE degrees of freedom so energy can be emitted and absorbed, but it is not individual electrons but rather their bound orbital electron energy levels within the atom that accomplish this. The additional degrees of freedom are provided by the structure of the system, not by the electrons

    Before CMBR was emitted, everything was so dense it never got out....like light trying to penetrat wood.
    More here:
    http://en.wikipedia.org/wiki/Recombination_(cosmology [Broken])
    Last edited by a moderator: May 6, 2017
  8. Jun 2, 2012 #7


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    To elaborate on what Naty1 is saying, free charges can readily absorb EM radiation resulting in an acceleration of the particle. (I think so at least. Someone correct me if I'm wrong) In the very early universe before the CMB was emitted light was essentially absorbed by a charged particle immediately upon being emitted, with the particle being accelerated, followed immediately by it crashing into another charged particle and being accelerated by that particle, emitting radiation in the process, only for it to be absorbed almost immediately again.
  9. Jun 2, 2012 #8


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    Please provide a better description for the process e+gamma -> e+gamma then. I do not think 4everphysics is interested in details of Feynman graphs.

    And the trees were the free charges, which can scatter light as described before.
  10. Jun 2, 2012 #9


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    You are right, of course. You answered 4ever's question already in post #2. I'm not sure why Naty said "no no" :biggrin: or why the thread continued as it did.
    Anyway thanks, Mfb!

    You could have supplied an authoritative source to expand on what you said. Joe Silk is about as credible as they get in Astrophysics and he and Eric Gawiser have this 2000 document:
    It is a pedagogical review of the CMBR. Here is the relevant page:

    It says that Compton scattering (which you described in lay terms) was one of three most important scattering processes at that era.

    In fact Compton scattering was, I believe, by far the primary process that Joe Silk mentions as thermalizing the photons until "last scattering" The other two were "double Compton" and "Bremstrallung" which is kind of what Drakith described. The photon is absorbed by giving the electron an acceleration kick, the reverse of the other thing he described where it comes to an abrupt halt and emits a photon.
    Simple Compton which you describe in post #8 as
    e + γ → e + γ'
    would have been the main reason the hot hydrogen was not transparent.

    It only takes a small percentage to be ionized for the cloud to be optically dense. Like the surface of a star at 3000 kelvin is sure not transparent! And yet it is mostly neutral hydrogen. You probably know more about this than I do. I hope you do anyway. Haven't met you before. Welcome.
    Last edited: Jun 2, 2012
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