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B Why do we still see cosmic background radiation

  1. Nov 28, 2016 #1
    In a discussion with a friend I am unable to explain to him why at this moment we still can detect cosmic background radiation. According to his reasoning the radiation that originated from the big bang should have passed us long ago. Where in fact does this radiation that we now detect come from? And where does it go to?
  2. jcsd
  3. Nov 28, 2016 #2


    Staff: Mentor

    It comes from the surface of last scattering. This is the surface when the universe cooled down to the point where it became transparent.

  4. Nov 28, 2016 #3
    It happens at a moment called recombination, where the whole universe suddenly became clear. As universe is broad (or may be infinite), lights from a moment need different time to reach earth as their sources are at different places.
  5. Nov 28, 2016 #4
    This radiation was and is present in the entire observable universe, except for the smallish amount which has interacted with matter etc at a different temperature.
    The expansion of space is diluting and red shifting this radiation, thus reducing its measured temperature.
    The distance from which we receive CMBR photons is increasing and the temperature is still (very slowly) decreasing..
  6. Nov 28, 2016 #5


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    Science Advisor

    The Big Bang did not happen at a point in space. It happened everywhere in the Universe. As CMB radiation from one region of the universe passes by us, new CMB radiation from a new, more distant region arrives.
  7. Nov 29, 2016 #6


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    The CMB photons we observe originated at a distance of 42 million light years from us and required 13.7 billion years to reach us due to expansion. Our distance from the source of CMB photons is completely irrelevant. We will never observe photons that originated earlier than CMB photons, the only thing that matters is the age of the observer relative to the age of the universe. As the universe ages we will observe photons that are equally more ancient. The CMB photons will eventually redshift beyond detectability, but, never because they have outrun us. It is impossible to outrun the age of the universe.
    Last edited: Nov 29, 2016
  8. Nov 29, 2016 #7
    Thank you all for your answers. I think I understand it better now and I will have my friend read your responses. Maybe it will be clearer to him as well.
  9. Dec 4, 2016 #8
    Here "suddenly" means approximately 100 thousand years.
  10. Dec 5, 2016 #9
    Understood, thanks.
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