The universe was opaque to neutrinos until a few seconds after the Big Bang, at which time the temperature of the universe was roughly 10 billion K. Their present average temperature is about 2 K. In contrast, the cosmic microwave background decoupled at about 4500 K, with a photosphere about 9000 mpcin radius, and have a present peak temperature of about 3 K. From these facts, roughly how large was the "neutrinosphere" when neutrino decoupling occurred?

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     3000 pc
      6000 kpc
      9000 mpc
      1500 kpc
      4100 mpc
asked Sep 20, 2012 in Astronomy by anonymous
    

1 Answer

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3000 pc
answered Sep 22, 2012 by SkyStar ~Expert~ (2,855 points)

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