# Age of Universe (Liddles Modern Cosmology)

Hi,
I am currently reading Liddles Introduction to Modern Cosmology (2nd Ed) and having trouble with problem 8.4, about the age of the universe with a cosmological constant.
The question asks to derive the formula for the age by first writing the Fridemann equation in such a model as
$\frac{da}{dt}$ = $H^{2}_{0}$ $[Ω_{0}a^{-1}$ + $\left(1-Ω_{0})a^{-2}\right]$

But i'm not sure how this step is found, so can't proceed any further. I would appreciate any help to get me started.

## Answers and Replies

cepheid
Staff Emeritus
Science Advisor
Gold Member
Welcome to PF hawker3! Hi,
I am currently reading Liddles Introduction to Modern Cosmology (2nd Ed) and having trouble with problem 8.4, about the age of the universe with a cosmological constant.
The question asks to derive the formula for the age by first writing the Fridemann equation in such a model as
$\frac{da}{dt}$ = $H^{2}_{0}$ $[Ω_{0}a^{-1}$ + $\left(1-Ω_{0})a^{-2}\right]$

But i'm not sure how this step is found, so can't proceed any further. I would appreciate any help to get me started.

I'd be happy to help you, but in order to do so, you need to tell me what form of the Friedmann equation you are starting with. Then I can give you suggestions on what manipulations to perform in order to get it into the form given above.

BillSaltLake
Gold Member
I'm having trouble finding a set of conditions in which the formula is correct. Look at the matter-only case for example: a $\propto$t2/3. Then the left side is $\propto$ t-1/3 whereas the 2 right side terms are $\propto$ t-2/3 and t-4/3.

cepheid
Staff Emeritus
Science Advisor
Gold Member
I'm having trouble finding a set of conditions in which the formula is correct. Look at the matter-only case for example: a $\propto$t2/3. Then the left side is $\propto$ t-1/3 whereas the 2 right side terms are $\propto$ t-2/3 and t-4/3.

I think that the relation a ~ t2/3 is specific to the "critical" or "Einstein-de Sitter" universe which is not merely matter-dominated, but also happens to have a critical density of matter so that Ω0 = Ωm = 1. Hence (1-Ω0) = 0.

Even so, the equation is also a bit off. At the risk of giving too much away, it should be (da/dt)2 on the left-hand side. You can see that this makes things work out for the critical case.

The a-2 dependence on the (1-Ω0) term also seems wrong to me. If you assume that there is no spatial curvature, and that the matter shortfall (1-Ω0) is made up of "something" that has a constant energy density (e.g. dark energy), then this term should have an a2 dependence. If you assume that there is nothing else aside from matter, then (1 - Ω0) is the curvature term, and it should have an a0 dependence (i.e. no dependence on a at all).