Lawrence M. Krauss wrote a tremendous (in my opinion) book called A Universe From Nothing, which pretty neatly explains how stuff exists, and how “nothing” gives rise to everything.
Part of the book is dedicated to explaining the expansion of space, and what that means for us and far-distant future astronomers. For us, it means that we live in precisely the right moment to observe that this expansion is actually happening, but for the future astronomers that situation is more bleak.
Krauss predicts that eventually, the universe will recede so far and fast that only a single galaxy cluster will remain, causally isolated from everything else. Today, we know the universe is expanding by observing the receding galaxies far away from us, while this hypothetical future alien astronomer can make no such observations. It is therefore virtually impossible for this future astronomer to create an accurate model of the universe which includes expansion. This astronomer will never even know he is wrong, as there would be no prediction or experiment he could make, the results of which which could confirm expansion.
My questions is, so what? This future astronomer wouldn’t necessarily be wrong. In fact, if there’s absolutely no evidence (from his perspective) that the universe is expanding, no phenomenon that could be better explained by an expanding universe than a static one, no accurate prediction that an expanding universe would make better than another explanation could,…wouldn’t it be perfectly reasonable (and even accurate) to say that space is NOT expanding, from the perspective of this hypothetical astronomer? Wouldn’t he actually be correct to infer a static (or maybe even collapsing?) universe? In what sense would his conclusion be wrong?