Sometimes the simplest questions have the most complicated answers. One such question is If the universe is expanding, what is it expanding into?
Simply said, the answer to this question is that it doesn’t make sense. The universe is everything in itself, so it isn’t expanding into anything. It is just expanding as all of the galaxies inside the universe are moving away from each other. Every region of space is getting stretched, but there’s no center they’re increasing from and no specific outer edge to expand into anything else. But this does not mean that the universe is infinite. Now, the answer becomes complex. To comprehend how something can be finite yet have no edge, consider the fabric of the universe to be similar to the surface of a balloon. As the balloon inflates, its surface stretches out making every point on that surface move away from every other point. Yet a tiny entity placed on the surface of that balloon could walk forever and come across the edge of its balloon universe. Without an edge, the balloon universe has a finite volume.
The Shape of the Universe
The balloon is just an analogy. Scientists aren’t really sure whether the universe is finite or infinite, or even the shape the universe is. There are three possible shapes: spherical, flat, or hyperbolic. Evidence from the earliest light in the universe suggests that the universe is in reality, flat.
So, if we assume the universe is flat and not balloon-shaped as our earlier example, we can think of it as a flat piece of paper. Two opposing edges can be folded to touch each other and create a cylinder. If a tiny 2-dimensional rocket ship travels from one of those edges to the other, it would arrive right back where it started. The same thing can be recreated in the perpendicular direction by connecting the two ends of the tube to each other to create a donut-shaped torus. In this case, the 2-dimensional rocket ship could travel in any direction without ever encountering an edge in spite of the fact that the paper torus has a finite volume.
Another point that needs clarification is that our paper is flat while the torus is curved. Our analogy still stands true because scientists have specifically defined “flat” as “Euclidean,” which means that parallel lines always run parallel and the sum of the angles of a triangle is always exactly 180 degrees. This is not possible on a sphere or a hyperbola, but only on a cylinder, a torus, and any other shape that can be made out of a flat piece of paper.
This leads to the exciting possibility that if we live in a flat universe, we can potentially travel in one direction long enough to reach right back to the starting point. Other shapes that twist back on themselves offer even thrilling possibilities like arriving back at a mirror image of where you started.
But irrespective of its shape, the universe is not expanding into anything and there is nothing outside of the universe as it has no edge.