What does "expansion of the universe" mean?

[PeterDonis]

But it wasn't!

I have no idea what you mean. "The expansion of the universe" is standardly defined, in cosmology, to mean "the expansion of the congruence of worldlines of comoving observers".

[Moderator's note: Edited to delete comment on which forum this discussion was taking place in; discussion has now been moved to Cosmology.]

 

[Dale]

The disk is not spacetime; it's a family of worldlines in spacetime. The universe is spacetime.

Ah, I see the difference. I am thinking about the "dust" as defining the universe, and you are thinking about the spacetime. I agree that the spacetime does not single out any specific congruence, but the dust does.

 

[PeterDonis]

I agree that the spacetime does not single out any specific congruence, but the dust does.

Exactly.