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I Why was the early universe opaque?

  1. Jul 17, 2016 #1
    I just learned from an Astronomy course that when the sub atomic particles were not bound together, they were moving around freely, but after the universe expanded and cooled, they began to combine to form Hydrogen atoms. Now, before this combination period, the photons couldn't pass through those particles, but after they did, they could. What I don't understand is why? I've read the introductory part here, https://en.wikipedia.org/wiki/Recombination_(cosmology), but I couldn't understand much. Any help?
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  3. Jul 17, 2016 #2


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    Did you follow the link in the wiki article to the description of Thomson scattering ? - that's the physical process responsible. You need free electrons in abundance.
  4. Jul 17, 2016 #3
    I did and that's my question. How could the particles scatter the EM waves earlier, but not after they combined to form Hydrogen atoms?
  5. Jul 17, 2016 #4


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    Because you need a free charged particle for a photon to scatter off and atoms are electrically neutral.
  6. Jul 17, 2016 #5
    Ok, that makes sense, but I can even picture a photon coming in contact with just the electron of the atom or just the proton. Then wouldn't it be possible for that photon to be scattered again? Because when the photon will strike either the electron or the proton, it wouldn't really strike the neutral atom, it'll strike only either of those two particles.
  7. Jul 17, 2016 #6

    Ken G

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    No one before 100 years ago could have answered your question either-- it is called quantum mechanics. It means that when electrons are bound in an atom, they are only allowed to have discrete (special) energies, so they cannot absorb most of the photons present because those photons don't have the right energy. Since most of the atoms are in the "ground state", it means the electrons can only interact with special ultraviolet energies, yet most of the CMB was in the visible or redder, so there is very little interaction between them. For a classical analog, you can imagine that each bound electron is attached, in effect, to a tight spring, so it is very difficult to make them oscillate slowly, you can only make them oscillate rapidly, near the "resonant frequency" of the spring.
  8. Jul 18, 2016 #7
    That's perfect. Thank you so much! :)
  9. Jul 18, 2016 #8
    A photon is like a steak and an electron is like a dog. While the electrons moved freely through the universe because it was too hot for atoms to form, they constantly interacted with the photons, making it opaque. When the temperature hit a specific point, the protons in the universe almost all at once put their electrons on leashes and photons could be free. That's the Cosmic Microwave Background.
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