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Question about pre-CMB & Lepton Number

  1. Sep 18, 2013 #1
    I have a question about the interaction between photons and leptons prior to recombination

    I understand that once the threshold temperature for electrons was reached early in the universe' history, the vast majority of the leptons and antileptons annihiliated into photons. Except for possibly the neutrinos and antineutrinos which interact very weakly. I also understand that the number of remaining leptons (the lepton number) after this annihilation event was very small relative to the number of photons. Something like 1*10(minus 9). So during the 380,000 years prior to recombination, when the universe was opaque to photons, it is said that the photons interacted very strongly with the electrons, continuously bouncing off of them, which is why the universe was opaque. However, if at this time there were something like 1,000 million photons for every electron and 1,000 million photons for every nucleon, how is this possible? It would seem that the universe would be so overwhelmed by photons relative to electrons that not all of the photons would interact with electrons and would indeed stream free. But according to BB cosmology they did not stream free until recombination. What am I missing in my understanding?
  2. jcsd
  3. Sep 18, 2013 #2


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    Staff: Mentor

    Remember that there was literally nowhere to "stream free" to. The entirety of the universe was this way. Even if a photon takes days to interact with a particle, it still had 380,000 years to do so.
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