Closed Universe is indeed a strong and necessary assumption. For any math to be performed about the Universe, it *must* be considered Closed. The Math won’t work otherwise.
It goes at least back to Newton. It’s rather required. Even when regarding “multiverses”, one still has to consider the totality of the set which would be, the Universe… .at least they should, otherwise their math has lots of gaps in it that are leaking out into mysterious places which would be a bit of a cop-out imo.
*IS* the physical Universe REALLY closed? How the heck _can_ we know? smile emoticon
But for us to perform math with it, it’s assumed.