Theoretical Framework


General relativity plus Robertson-Walker metric plus Friedmann equation give us a complete mathematical description of the evolution of space-time.

The description is deterministic - if we know the content and expansion rate at one time, then we know them at all times, both past and future.
So we usually talk about the values at the present time for convenience.

Theory assumes that the Universe is smooth - which it clearly isn't on small scales.
Nevertheless on big enough scales it is...