Warning may contain stupid questions.

1. Does Universe has to meet some prerequisite before it can be considered a Universe?
2. Can we have a microscopic Universe?
3. Can there be a Universe withing a Universe?

Thx

There are theories that describe more than one universe, but this is in Astronomy and Cosmology, and the best current models tell us there is only one, and it is expanding but not into anything, since the universe is all there is. The same models describe a time when our universe was (what we would now call) microscopic, extremely dense, and extremely hot.