After having read the whole thread, this is an answer I can think more about. The boundary between existence and non-existence.

But when physicists talk of an expanding universe, they are talking about something much more prosaic. That is the red shift you observe of light (EM radiation) from further galaxies because they are moving away from us.
Assuming we are not special, which is a big assumption, we can generalize that every galaxy is moving away from every other galaxy in space. And since The night sky is not full of galaxies in every direction, the universe must be finite. Or at least the number of galaxies (EM sources) in the universe must be finite. Ergo the universe is a closed curve with N dimensions. The problem of course is that every closed curve has an inside and outside and that is not possible for THE universe because everything exists inside.