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Homework Help: Galaxy counts and the expansion of the universe

  1. Oct 3, 2008 #1
    1. The problem statement, all variables and given/known data
    You want to count all the galaxies out to a certain distance R(naught). If the universe were not expanding, then N (number of galaxies) would be proportional to R^3. Justify/derive.

    For an expanding universe, take into account the finite time that light takes to reach us and just describe (without derivation) the relation that would be observed between N and R.

    2. Relevant equations

    3. The attempt at a solution
    Is this as simple as saying that the density is mass/volume, so the more volume of space you have, the more galaxies you have? That seems to easy for this class...
  2. jcsd
  3. Oct 4, 2008 #2


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    Number is equal to density times volume is fine. So if the universe were not expanding R^3 is a good factor. But if it is, as you look back in time, not only is R^3 increasing the density is increasing, isn't it?
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