I know some folks may get tired of questions about the finite/infinite scope of the universe. Sorry for that. But as you know, many concepts are hard to wrap one's head around. Let me make my question as clear as possible from the outset: -I am NOT asking whether the universe is infinite or finite. I accept that we don't know. -I only wonder what it would mean to say that the "universe" is "infinite." -I know that to answer this question depends on the proper use of specific definitions, and I may not be fully aware of what the standard definitions are. Anyway, in the FAQ tells us: "Standard cosmological models come in three flavors, open, flat, and closed,[Carroll] whose spatial curvatures are negative, zero, and positive. The open and flat types have infinite spatial volume." And NASA tells us: "If the density of the universe is less than the critical density, then the geometry of space is open (infinite), and negatively curved like the surface of a saddle. If the density of the universe exactly equals the critical density, then the geometry of the universe is flat like a sheet of paper, and infinite in extent." Now let me just assume an "open" universe, at less than critical density. The above suggests that the universe has "infinite spatial volume," and is "infinite in extent." Which of the following would it be fair to assume, in this case?: A) There exists matter/energy at a distance greater than 500 Quintillion (5*10^20) light years from earth. I.e., much much beyond the limits of the "observable universe." B) Matter/energy may be assumed to extend beyond the limits of the "observable universe," though there's no accounting for its extent. We can assume that the density beyond what is observable is the same or similar to the density that is observable. C) There is no reason to assume the existence of matter/energy even one micrometer beyond the limits of the observable universe. However, the observable universe is able to expand without limit, and without ever having to contract. As a tangential question, when a physicist uses the term "infinite", does that generally mean "a damnably large number so far beyond what is measurable that it approximates infinity," or is it expected to fit literally and absolutely the mathematical abstraction of "infinity" (which seems largely paradoxical in its formulation)?