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Photons vs neutrinos energy ratio

  1. Oct 1, 2011 #1
    What is the ratio of energy contained in photons vs energy contained in neutrions in the present universe? Also, how is that ratio changing with time? Thanks.
     
  2. jcsd
  3. Oct 2, 2011 #2

    nicksauce

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    The ratio is, assuming massless neutrinos,

    [tex]\frac{\rho_{\gamma}}{\rho_{\nu}} = \frac{21}{8}\left(\frac{4}{11}\right)^{4/3}[/tex]

    or, approximately, 0.68. The number won't change with time, since both are evolving as free radiation, [itex]\rho\sim a^{-4}[/itex].
     
  4. Oct 3, 2011 #3
    Thank you nicksauce.
     
  5. Oct 3, 2011 #4

    Drakkith

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    nick, how much does that change assuming neutrino's have mass?
     
  6. Oct 4, 2011 #5

    nicksauce

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    The usual expression is in terms of [itex]\Omega[/itex] is, I think,

    [tex]\Omega=\frac{m_{\nu}}{94h^2eV}[/tex]

    I'm not sure what people think the masses are these days.
     
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