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I At which time did the CMB become dark?

  1. Feb 13, 2017 #1
    At the time the CMB was emitted it was a glowing yellowish-white radiation at 3000 K. From there as space expanded it became redder and redder eventually falling into what for humans is non-visible infrared.
    At which age of the universe did the CMB background turn from red to infrared?
    Thanks
     
  2. jcsd
  3. Feb 13, 2017 #2
    To estimate this, you need to know:
    -the dependance of the temperature ##T## of the CMB with time/scale factor ##a## of the universe, which is roughly ##T \propto \frac{1}{a}##
    -the evolution of the scale factor with time, from the Friedman equation which gives for a matter-dominated universe ##\frac{a}{a_0}=(\frac{t}{t_0})^{2/3}##
    Then you can get the ##t## you look for by choosing the temperature you want in the following equation: ##t = t_0(\frac{T_0}{T})^{3/2}## where ##t_0## would be present time (13.8 billion years) and ##T_0## would be the present temperature of the CMB, 2.73 Kelvin degrees.
     
  4. Feb 13, 2017 #3

    mfb

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    There is no hard threshold, it just got darker over time. Based on those arbitrary categories, thermal radiation becomes visible somewhere around 525°C, or ~800 K. Today the temperature is 2.7 K, so we are talking about z=300. Based on this cosmology calculator, this was around 3 million years after the Big Bang.

    "Cherry red" at 1000 K corresponds to z=370, 2.2 million years after the Big Bang.
     
  5. Feb 13, 2017 #4
    Thanks a lot.
     
  6. Feb 14, 2017 #5

    Chronos

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    Photons do not play well in plasma, largely due to a phenomenon known as Compton scattering. This same effect makes it difficult to see anything below the atmosphere of a star. For futher discussion, see; https://ned.ipac.caltech.edu/level5/Sept05/Gawiser2/Gawiser1.html, ORIGIN OF THE COSMIC BACKGROUND RADIATION.
     
  7. Feb 14, 2017 #6

    mfb

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    Everything discussed in this thread happened after the plasma recombined enough to make scattering negligible. That is the whole point of the CMB. The hydrogen got re-ionized much later (>100 million years), but at that time it was so spread out that it didn't make the universe opaque any more.
     
  8. Feb 14, 2017 #7

    PeterDonis

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    Moderator's note: some off topic posts have been removed.
     
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