[btilly]

That only works if radiation density and expansion are connected. General Relativity says that they aren't.

[mnky9800n]

How does general relativity say that?

[btilly]

The term for the stress-energy tensor has the wrong impact on expansion for a simple relationship of any kind.

You can rescue it with pseudo-tensors, but then you force the existence of a preferred reference frame. Which undermines the principles of GR.

Some still argue for that. Others don't.