[photon]

Originally Posted by Cowperson

Again, there was a big bang. The resulting universe is expanding outward.

I don't think "outward" is the right way to think about it, as that implies an outward to expand into which afawk doesn't exist, and it also implies an inward which also isn't accurate.

For expanding, just think that everything is getting further from everything else.

Originally Posted by Cowperson

Photon is right to say that if I took a long walk around the outside of our universe I'd end up in the same place, the North Pole, because I'm walking on a bubble.

The universe isn't a bubble though, when I used the analogy of a sphere of the earth and there's no north of the north pole, imagine the entire universe being contained on the surface of that sphere. There is no up or down, nothing exists beyond the surface of the sphere. You can go in any direction on the sphere and end up back where you started, but you can't go up or down.

Similarly if our universe is finite and bounded, you could go in any direction in 3 dimensions and eventually end up back where you started (well not necessarily because of expansion, but assuming you could stop time).

The bubble visualization isn't accurate, you couldn't walk around the outside of the universe because there (as far as we know and can theorize anyway) IS no outside of the universe.

Originally Posted by Cowperson

The universe is expanding into an emptiness. Or empty as far as we know.

Nope, it's simply expanding. If you have a balloon, and you blow it up, the surface is expanding. From the point of view of the surface of the balloon, you aren't expanding into anything. Points of the surface of the balloon are getting further apart.

Originally Posted by Cowperson

Are there other big bangs and other expanding universes, in this dimension, also expanding outward into nothing? Perhaps closing the gap with our universe as an example.

That we don't know. There's different ideas about it, some ideas have new universes being spawned from black holes in different dimensions, some have many universes being parallel but separate from each other in higher dimensions, at collisions of branes, and some ideas have there being only one infinte realm of spacetime, and our visible universe expanding is expanding into existing space, and there's a small chance that a quantum fluctuation could produce a whole new "visible universe" inside our own.

Originally Posted by Cowperson

What lies beyond our universe, in this dimension?

Likely the question again is meaningless, there probably isn't a beyond our universe, either it's finite and bounded or infinite and unbounded, so in either case there is no beyond.

In the case that there's only one realm of spacetime, there would be a "beyond our universe", but because of inflation we're causally isolated from it by the speed of light so we'd never end up knowing.