So, with some assumptions, it seems like we can put some lower bounds on the size of the unobservable universe.

Suppose the the universe is unbounded, closed, finite, and flat (is that right?). Can we think of it like a gigantic 4D sphere? If so, does that give us a way to compute its minimum size?

As I understand it we know the size of the “observable universe,” and across nearly this entire part of the universe space looks flat. This means its curvature is indistinguishable from zero. But we only know this to some number of significant digits (how many and in what units?). So, thinking of it as a 4D sphere, and assuming that there is a tiny postive curvature, the maximum possible within measurement error, we should be able to compute the total surface (a volume in this case) of the whole universe (observable and unobservable). Perhaps we could report it as multiples of the observable universe.

Is this a sensible way to estimate the minimum size of the unobservable universe (given those starting assumptions)? I wonder if the @physicists know…