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How Do We See The Background Radiation?

  1. Apr 24, 2015 #1

    bitznbitez

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    We see objects either because they emit light [particle or energy] toward us or because light bounces off them in our direction. This is true for all energy not just visible spectrum. We see stars because they emit light towards us and nebula because it is, generally, illuminated by some stellar light source. These emit light in all directions and we see the light heading our way.

    The CMB was released very early and, as I have heard it explained, has red shifted so far it is in the microwave spectrum as it moves "away" in all directions.

    If its moving "away" how do we actually see it ?
    If by way of answer its explained that in some sense part is heading toward us allowing us to see it why is it still shifted to the microwave end of things.

    I'm sure this is answered somewhere I simply have not run across it yet.
     
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  3. Apr 24, 2015 #2

    Orodruin

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    The CMB is not moving away from us. It is a radiation that is present everywhere in the Universe and it is composed of light that is moving in all possible directions. The CMB photons that we see are those which were emitted on a sphere around us at such a time that it has taken roughly 13 billion years for the light to reach us.
     
  4. Apr 24, 2015 #3

    wabbit

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    As I understand it, your description is fine up to the point "it moves away": the radiation emitted fills space uniformly and isotropically and is not moving away.

    What is getting farther is the surface of emission we see at a given time - the spatial location we are sampling, which is a sphere in the universe as it was 300k years old or so, is getting larger.
    Not that this is fast of course or detectable in itself - nor is it "motion" really: we look farther, that's all.

    Edit - added "as it was"
     
    Last edited: Apr 24, 2015
  5. Apr 24, 2015 #4

    Orodruin

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    Just to clarify here: The CMB was emitted everywhere in the Universe when it was ca 300k years old. It has since taken the CMB light that we receive now roughly 13 billion years to reach us and due to the travel time, the CMB light that will reach us tomorrow will have been emitted further away from us than the CMB light we receive today.
     
  6. Apr 24, 2015 #5

    wabbit

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    Ah yes thanks for the clarification - rereading my post I see one could take it to mean as 300k years ago - oops :)
     
  7. Apr 24, 2015 #6

    bitznbitez

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    So as the plasma cooled and the universe became transparent the photos were emitted, essentially simultaneously, in all directions from essentially all points in the universe. The coolness / redshift is not because of the photons all moving away in the sense of motion rather the coolness redshift is a function of space stretching.

    Is that what is being said essentially ?
     
  8. Apr 24, 2015 #7
    I think you got it.
     
  9. Apr 24, 2015 #8

    Chronos

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    In a nutshell, yes. Our best theories suggest the universe has expanded continuously since the moment it came into existence. The CMB photons we currently observe were emitted when the surface of last scattering was at a proper distance of about 42 million light years. Due to expansion, those photons needed ~13.7 billion years to catch up with us.
     
  10. Apr 24, 2015 #9

    ChrisVer

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    Yup. It's a different "Doppler effect" that doesn't come from relativity, but from cosmology...

    You could as well say that the photons were "freed" from the plasma "trap". It's not like at a single moment you got a sudden photon emission, rather than all the photons that were trapped (their mean free paths were very small) stopped interacting with the rest of the plasma soup (because of the creation of neutral atoms and the cooling -could not ionize the atoms). What happened was that their mean-free-path became very large, enough for the Universe to be transparent. That's true because photons before the time of last scattering were not absorbed by the atoms but rather underwent Compton (or later Thomson) scatterings. CMB photons that originated a little before the time of the last scattering, are a reason for the CMB-spectrum anisotropies (eg. Baryon Acoustic Oscillations).
     
    Last edited: Apr 24, 2015
  11. Apr 25, 2015 #10

    Chronos

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    CMB photons were not instantaneously unleashed in a blaze of glory at t=380,000 years. Some photons leaked out earlier, others later. We measure the average.
     
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