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Question about Recombination Redshift values

  1. Dec 13, 2015 #1
    Wikipedia gives the redshift of recombination at z = 1500 (roughly 4,000 K). The Plank report list a 'redshift for which the optical length equals unity' and gives a value of 1090. What is the difference between the two?
     
  2. jcsd
  3. Dec 13, 2015 #2

    Chronos

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    z=1500 is approximated based on achieving a 50% ionization fraction under the Saha equation. This calculation, however, assumes the universe was in a state of thermal equilibrium, which it was not. Under the Peebles equation, which does not assume equilibrium, the 50% ionization fraction is achieved around z=1210. The value of z=1090 merely represents the ratio between the reionization temperature of hydrogen and the current measured temperature of the CMB.
     
  4. Dec 14, 2015 #3
    Sorry, I didn't follow all of that. In layman's terms, which value represents the red-shift where the electrons travelled freely through the universe. Also, if it isn't asking too much, could you also give a layman's description of optical depth, τ. I see it's a parameter to ΛCDM and if you Google it, you get tons of information about various attempts to measure and constrain it, but I can't find a conceptual guide to tell me what this parameter is physically.
     
  5. Dec 14, 2015 #4
  6. Dec 14, 2015 #5
  7. Dec 14, 2015 #6
    Hi Dr. Strange:

    You question requires expertise way over my head. The two people who have been particularly helpful in answering my questions about recombination are
    @Chronos and @Chalnoth. Perhaps they can answer your question,

    Regards,
    Buzz
     
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