It’s at least logically possible to observe the whole of space as a whole, but not to observe the whole of time.
In practice, we lay claim to a hypothetical knowledge of the temporal whole of the universe by assuming that the laws will be invariant. But it’s strange to call something a scientific hypothesis which could only be confirmed at the end of history. Such an end will either come or not: if it comes it will come to late to be of any value to us; if it doesn’t we are still left with no confirmation.
It might be objected that we assume the temporal invariance of law all the time: in the face of an oncoming bus we don’t wonder if the relation between mass and force will hold after the time of contact; and this is true even when we study the universe. If this sort of assumption doesn’t count as presumptive and rational, what would?
But this argument conflates regularities with law, since law adds the idea that some sort of abstraction/ impersonal reality is at work behind the regularity. Assumption of regularity – which any person does spontaneously – does not get us to law, only the assumption that this regularity is impersonal (en passant, this speaks to another oddity in the Analytic account of the abstract as non-causal. It would be closer to the truth to say that we usually assume the causal is ultimately abstract.)