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Homework Help: Temperature vs. time

  1. Nov 23, 2012 #1
    Assume that we live in a Robertson-Walker universe with matter, radiation and curvature. The present mass density is ρm = 3Ω0H20/(8πG), where H0 = 100 h km s-1 Mpc-1 and Ω0(1+ρr,0/ρm;0) ≤ 1 (i.e., k ≤ 0). The present radiation temperature is T0 = 2.725 K.
    Assume that only photons, with present temperature T0 = 2.725K contribute to the radiation; ignore neutrinos in this problem.
    How old was the universe when the radiation temperature was 1 MeV? 1GeV?
    1014 GeV? (Hint: you need g*, the effective number of relativistic spin states contributing to the energy density. At 1 GeV g* = 61.75 and at 1014 GeV, g* = 106.75 without supersymmetry or double this with SUSY.)
    Thanks so much!!!
  2. jcsd
  3. Dec 1, 2012 #2


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    Staff Emeritus
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    This looks like a homework question, as such I am moving to the relevant homework forum. What attempts have you made the answer the question?
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