[Archy (George)]

Originally Posted by sjastro

Evidently my response went over your head, so let me try to make as simple as possible for you.

The mathematical equations behind GR cannot be solved directly. Solutions have been provided by a process similar to reverse engineering, a solution is constructed based on certain physical assumptions. The solution is then plugged into the field equations. If they satisfy the field equations you have a model you can build on.
Cosmological models are built on this foundation. The important point is that GR provides the gravitational theory for these models and that the physical assumptions of the model do not violate the field equations.

A typical physical assumption for a cosmological model is whether the Universe expands, contracts, oscillates, rotates or remains static.
There are various cosmological models reflecting these scenarios all of which conform to the field equations. GR doesn't determine these assumptions.

As Carl has pointed out time dilation is a property of an expanding Universe cosmology, not a relativity issue.

What an arrogant reply!

You say:
"time dilation is a property of an expanding Universe cosmology, not a relativity issue."

Yet Wikipedia says:
"Gravitational time dilation is the effect of time passing at different rates in regions of different gravitational potential; the lower the gravitational potential, the more slowly time passes. Albert Einstein originally predicted this effect in his theory of relativity and it has since been confirmed by tests of general relativity."