Mass-dominated vs. radiation-dominated.

[borvis]

I have a simple question:
What does it mean that the universe is mass-dominated or radiation-dominated?
I can't 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 a lot.

 

[mikeph]

Yes, that's correct.

 

[borvis]

Thanks!

 

[MykDee]

If you call $$a$$ the expansion factor, the energy density of the radiation goes as $$\rho_{rad}\propto a^{-4}$$, while for the matter you have $$\rho_{mat}\propto a^{-3}$$. If you plot logarithmically these quantities, you will see that at a certain $$a_{eq}$$ 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 $$a>a_{eq}$$, you have the matter-dominated era.

 

[borvis]

And this means because of Hubbles law, v=Hr, you can say that a $$\propto$$ r? Since the expansion factor is proportional to the speed that objects move from each other in space?

 

[mikeph]

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.