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.