Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Science Advisor
    Gold Member

    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

    Buzz Bloom

    User Avatar
    Gold Member

  6. Dec 14, 2015 #5
  7. Dec 14, 2015 #6

    Buzz Bloom

    User Avatar
    Gold Member

    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,

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook