What is the Universe expanding into?
By definition, the universe encompasses all of space and time as we know it, so it is beyond the realm of the Big Bang model to postulate what the universe is expanding into. In either the open or closed universe, the only “edge” to space-time occurs at the Big Bang (and perhaps its counterpart the Big Crunch), so it is not logically necessary (or sensible) to consider this question.
This question is based on the popular misconception that the universe is some curved object embedded into a higher dimensional space, and that the universe is expanding into this space. There is nothing whatsoever that we have measured or can measure that will show us anything about this larger space. Everything that we measure is within the universe, and so we see neither edge nor boundary nor centre of expansion. Thus the universe is not expanding into anything that we can see or measure.
As far as we know, the universe isn’t expanding “into” anything. When we say the universe is expanding, we have a very precise operational concept in mind: the amount of space in between distant galaxies is growing. (Individual galaxies are not growing, as they are bound together by gravity.) But the universe is all there is (again, as far as we know), so there’s nothing outside into which it could be expanding. This is hard to visualize, since we are used to thinking of objects as being located somewhere in space; but the universe includes all of space.
This question is based on the ever popular misconception that the Universe is some curved object embedded in a higher dimensional space, and that the Universe is expanding into this space. This misconception is probably fostered by the balloon analogy which shows a 2-D spherical model of the Universe expanding in a 3-D space. While it is possible to think of the Universe this way, it is not necessary, and there is nothing whatsoever that we have measured or can measure that will show us anything about the larger space. Everything that we measure is within the Universe, and we see no edge or boundary or center of expansion. Thus the Universe is not expanding into anything that we can see, and this is not a profitable thing to think about. Just as Dali’s Corpus Hypercubicus is just a 2-D picture of a 3-D object that represents the surface of a 4-D cube, remember that the balloon analogy is just a 2-D picture of a 3-D situation that is supposed to help you think about a curved 3-D space,
