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Mass-dominated vs. radiation-dominated.

  1. Nov 9, 2009 #1
    I have a simple question:
    What does it mean that the universe is mass-dominated or radiation-dominated?
    I cant find the answer anywhere on the internet. But i kind of think that it means that the energy-density of the universe is either dominated by the energy from mass or radiation. Is this correct? Thanks alot.
  2. jcsd
  3. Nov 9, 2009 #2
    Yes, that's correct.
  4. Nov 9, 2009 #3
  5. Nov 9, 2009 #4
    If you call [tex]a[/tex] the expansion factor, the energy density of the radiation goes as [tex]\rho_{rad}\propto a^{-4}[/tex], while for the matter you have [tex]\rho_{mat}\propto a^{-3}[/tex]. If you plot logarithmically these quantities, you will see that at a certain [tex]a_{eq}[/tex] the two densities will be equal: there it is the equivalence epoch. Before equivalence the radiation energy is greater that the matter energy, so you talk about "radiation-dominated era". For [tex]a>a_{eq}[/tex], you have the matter-dominated era.
  6. Nov 9, 2009 #5
    And this means because of Hubbles law, v=Hr, you can say that a [tex]\propto[/tex] r? Since the expansion factor is proportional to the speed that objects move from each other in space?
  7. Nov 9, 2009 #6
    Not exactly, since in cosmology, H(t) is defined as (da/dt)/a (sort of like your v/r).

    Hubble's law is really just a local observation of the universe expanding.
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