# The time expression of the matter-raditation equality era

• Magister

#### Magister

1. The problem statement

I have already find the red shift of this era, so I just need to find the expression of time in terms of red shift.
From the Friedmann equation I can derive 2 relations between time and the red shift, one to the matter dominated era and another to the radiation dominated era.
I suppose that I will have to use the second one because from the beginning until the equality of the densities it was a radiation dominated universe.
My problem is that I saw, in the Kolb and Turner book, that they use the matter dominated expression. What am I missing??

I would guess that they are then computing the time between now and the equality time. But that's basically the age of the universe. The more interesting number would be time from big bang to equality time. Do you have a chapter page reference from Kolb and Turner?

The $t_{eq}$ is in page 89 (chapter 3.5) and the expression they use are in page 65 (chapter 3.2) (for $\Omega_0 = 1$).
I understand why do you say that they must be computing the age from the equality time until now but the they get a $t_{eq}=2.4 X 10^3$ years so it must be from the Big Bang until the equility time...