Vertical Theory provides a new model for infinity. At its simplest, Vertical Theory hypothesizes that in a multi-universe model, infinity stretches both up and down vertically from our universe. For this to work, the absolutely smallest indivisible unit or closed particle in the quantum world is a separate finite universe unto itself. There are a googillion of these units (could be some sort of sub-component of strings) and they serve as the first level building blocks for our universe. Next it states that our universe is one of a googillion other building block units for a universe much larger than ours, the next level up universe (not a mega universe because there is always a next level up). And so it goes, our universe a smallest component to a next larger universe, that universe a component to a next larger universe and so on infinitely. Alternately, the same model moves infinitely down to next smaller and next smaller universal building blocks.
That idea was the basis for a novel, but I can't remember what it was...
There are two possibilities according to physicists: the universe is either 'open,' or 'closed.' The possibilities are determined by how much mass the universe contains.
If the universe is open, that means it has no boundary, and is infinite in every direction. The gravity of all the mass in the universe is not strong enough to overcome its expansion, and it will expand for eternity. You can never escape the universe by traveling in a straight line, because the edges are expanding away faster than light. [The expansion of space itself has no speed limit; the relative speed of matter through space does]
If the universe is closed, that means it has a finite volume, but no boundary. Gravity has been able to overcome the universe's expansion, and billions of years in the future there will be a 'big crunch' (or as I heard one scientist say, the 'Gnab Gib'
) Space is curved back on itself, so that if you went in a straight line far enough, you would end up where you started. This is like living on the surface of a balloon; there is a finite surface area but no edges or boundaries. The balloon can expand or contract, and maybe bump into other balloons. If the universe is closed, we know that the distance you would have to travel to end up back where you are is greater than the diameter of the observable universe. Otherwise we would see a 'hall of mirrors' effect as we looked deep into space; the same galaxies would repeat once or many times in a given direction. We would also be able to observe our own Milky Way galaxy from a distance (as it was in the distant past, no less!).
Current theory says that the universe is OPEN, and that its expansion is accelerating due to the 'cosmological constant;' which is a force that pushes space apart, increasing with distance (an effect Einstein predicted, but then considered a ridiculous mistake).
There are lots of different theories about other universes. The 'many worlds' interpretation of quantum mechanics says that every possible quantum state, interaction and outcome has its own universe. In one universe a radioactive nucleus decays, causing a mutation that leads to the development of five digits instead six in some distant ancestor. In another (infinitely many, in fact), it does not decay, and people have six fingers. [But in the 'Copenhagen' interpretation (the one most physicists subscribe to), there are infinitely many _possible_ universes, but each quantum state collapses into one reality, in a single universe, upon interaction or observation.]
There is a theory that says universes reproduce semi-biologically. When a star collapses into a black hole, it pinches off a region of space-time which then expands into a new universe, having slightly different values for its physical laws. The universes that are a 'reproductive success' are the ones that are stable and can produce the most black holes, and thus the most baby universes. We live in one of those relatively stable universes.
Another says that higher-dimensioned universes float around hyperspace like soap bubbles, and where they bump and intersect, universes like ours are spawned. ['Brane' theory, short for 'membrane'] It says that gravity is so relatively weak because it 'leaks' out of the universe into higher space-time.
Some say there are infinitely many universe, but all at RIGHT ANGLES to each other.
Et cetera, and so on, and so forth....
The idea of other universes altogether is different than the idea that our universe has more dimensions than we can see. They say that our universe actually has 10 dimensions, but six of the dimensions are 'curled-up;' meaning that we can move in 10-D space, but the distances we can move in the other six are too small to notice (or measure, currently). [Think of sub-microscopic closed universes. The volume of those other six dimensions is so tiny that you immediately end up where you started]
To exit this universe would mean moving through higher dimensioned space-time, into whatever, if anything, is 'out there' (just like the escape from Flatland). Would you find another universe, a hyperverse, something else...? [I guess if you want an answer, that's what religion is for. For now anyway....]
Must a universe always be nested in a higher dimensioned one, and that one in a higher dimensioned one, and so on, upward infinitely? (That's the most boggling thing I've ever thought of...)
(and is the difference between 997 and 998 dimensions the same as the difference between 2 an 3, or 3 and 4?)