[MasterChiToes]

In polar or spherical coordinates, r is often constrained to the positive values. Space and Time can be 'mixable' in General Relativity, since the transformations and behavior of distance and duration are essentially the same... and especially since the radial coordinate axis is "unreal" and choosing a measure of space or time (which have a 1:1 correlation) is unimportant. The past doesn't exist anymore, and the old space doesn't exist either. Whether one of the two is more conceptually real is unimportant to the math.

Yes, you can't travel through the middle, only the surface of the sphere is the universe, there is nothing else, unless you want the philosophical definitions that inside of the balloon is the past and outside the balloon is the future. However, you can also choose the philosophical definitions that inside the balloon are old universes and outside are coming universes.