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Homework Help: Astronomy coursework question

  1. Mar 15, 2010 #1
    1. The problem statement, all variables and given/known data

    Assume that the vast majority of the photons in the present Universe are cosmic microwave radiation photons that are a relic of the big bang. For simplicity, also assume that all the photons have the energy corresponding to the wavelength of the peak of a 2.73K black-body radiation curve. At Approximately what redshift will the energy density in radiation be equal to the energy density in matter?

    (hint: work out the energy density in photons at the present time. Then work it out for baryons, assuming a proton for a typical baryon. Remember how the two quantities scale with redshift to work out when the energy density is the same.)

    2. Relevant equations

    [tex] \rho_M \propto a^{-3}[/tex]

    [tex] \rho_\gamma \propto a^{-4}[/tex]

    T \propto a^{-1}

    1 + z = \frac{v}{v_0} = \frac{\lambda_0}{\lambda} = \frac{a(t_0)}{a(t)}

    3. The attempt at a solution

    Not sure where to start.. how do I work out the energy density for photons and protons at the present time? Do I use E = mc^2?
  2. jcsd
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