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Cosmological expansion

  1. Oct 19, 2014 #1
    1. The problem statement, all variables and given/known data
    If light travelled a distance L = [itex] H_{eq}^{-1} [/itex] at M-R equality, how large does this distance expand to at present? (in Mpc)

    2. Relevant equations
    [itex] z_{eq} = 3500 [/itex]
    [itex] \Omega_m = 0.32 [/itex] at present
    [itex] \rho_c = 3.64 \times 10^{-47} GeV^4 [/itex] present critical density

    3. The attempt at a solution
    Not entirely certain where to begin for this one. I think it's asking for the horizon length at present, so perhaps need to use the equation
    [tex] L =a(t) \int \frac{da}{a^2 H} [/tex]
     
  2. jcsd
  3. Oct 19, 2014 #2

    Orodruin

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    Since the problem quotes ##L = H_{\rm eq}^{-1}##, I suspect that what they want you to do is to compute (roughly) the present size of a region that was in causal contact at the time of matter-radiation equilibrium.
     
  4. Oct 19, 2014 #3
    So would this be done by calculating [itex] H_{eq} [/itex] at equality, and then expanding with scale factor, [itex]L(z=0) = L_{eq} (1 + z_{eq})[/itex] ?
    If I do that, it gives a value a little under 150 Mpc.
     
  5. Oct 19, 2014 #4

    Orodruin

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    This is the approach I would take - assuming that my interpretation of the problem is correct.
     
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