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CMB Redshift Question (Visible Wavelengths)

  1. Apr 28, 2009 #1
    I've looked quite a bit on this forum and also elsewhere online and I cannot really find any information on my inquiry. If this is a double post please delete and send me in the right direction.

    Now I may be totally off on my thought process, but as I understand it the CMB first started off as gamma ray radiation and has since redshifted to the radio.

    At some point in this redshift journey it must have passed through the visible spectrum.

    Which leads me to my question: If we were alive during that time when it was shifting through the visible (assuming there was a time), would the "night's" sky have been bright? Since the CMB is literally everywhere, would we not even be able to see due to overexposure?

    Or would the CMB have not been intense enough to even visibly see during this time?

    Once again thanks for any insight!
  2. jcsd
  3. Apr 28, 2009 #2


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    Yes the night sky would have been bright. Uniform temperature of about 3000 kelvin.

    According to the standard cosmo model the CMB photons date from a time about 380,000 years from start of expansion and the CMB redshift is about 1100.

    Before that the hydrogen gas filling space was sufficiently ionized so as to be effectively opaque---photons had a limited mean free path and were always being scattered. The temperature was above 3000 K. The CMB photons date from the moment that the medium cooled enough to become effectively transparent.

    You are mistaken about the CMB starting as gammaray. It would have started as mostly infrared with a small percentage of visible, like the glow off a star that is 3000 K, more orange than the sun, not so hot has the sun.

    If you remember the CMB redshift z = 1100, and the presentday CMB temperature T = 2.728 K, then you can multiply to get the temperature back then when the photons began their journey.

    2.7 x 1100 = about 3000

    The same factor works for the ratio of wavelengths.

    As you can imagine, anyone back then bathed in that early light would be rapidly cooked.
    Last edited: Apr 28, 2009
  4. Apr 28, 2009 #3
    Hmm.. awesome. Thanks for the reply and correction about my gamma ray misconception.
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