I have not found any place on the Internet discussing evidence if the universe is bounded or unbounded. I’ve been giving this idea some thought and currently debating this in the Christianity Debate Forum in Is the universe bounded or unbounded?
Before I explain why I believe the universe is bounded, first let me explain what is meant by a bounded or unbounded universe.
A bounded universe means that there is an “edge” to the universe. Beyond this “edge”, our universe does not exist.
An unbounded universe means that there is no edge to the universe. Space curves in on itself so that if you go in one direction far enough, you will eventually come back to where you started. It can be illustrated like a surface of a sphere. On the surface of a sphere, if you start at any point and then go in a straight line, you will eventually come back to the starting point. Now, instead a 2-D surface on a sphere, the universe is a 3-D topology that curves in on itself.
From my observances, most astronomers/cosmologists believe that the universe is unbounded. But, I have not seen any arguments supporting why they believe it is unbounded.
Let me give three arguments why I believe the universe is bounded.
1. We do not see light all around us.
If the universe was unbounded and started from the Big Bang, even if there was only one star in the universe, we would see light from this single star all around us all the time.
Let me illustrate it this way. Suppose the entire 3-D universe is represented by the surface of a sphere. I am standing at one point on the sphere. On the other side of the sphere there exists a light source. The light source would radiate light in all 2-D directions. And I would see light from the light source coming at me from all 2-D directions. And I would still experience this even if the sphere was expanding (as long as it’s expanding less than the speed of light).
2. All our calculations for things in our universe are based on Euclidian mathematics.
If the universe is unbounded, then it would follow that the universe has a non-Euclidian geometry. More precisely, the universe would have an elliptic geometry. But, as far as I know, all rocket scientists use Euclidian geometry to determine how to fly a spacecraft in space. Also, an elliptical geometry universe would mean that all triangles that exist in the universe is greater than 180 degrees. Perhaps in our small part of the universe, we can approximate all triangles to have 180 degrees, but in fact, all triangles would have more than 180 degrees. So, when our geometry teachers say that the angles of all triangles sum to 180 degrees, they are in fact wrong if the universe is unbounded.
3. We have no evidence that space is being created or stretched.
If the universe is unbounded, then in order for the universe to be expanding, then space itself would either need to be “stretched” or “created”. Suppose the universe is represented by the surface of an expanding balloon, as the balloon gets inflated, the balloon itself will get stretched. In the same way, the universe would be stretched at every point in space in all directions. But, does that mean that the earth and everything in it is expanding also? Suppose you paint a dot on the balloon as it gets blown up, the dot itself would also be growing. But, if the earth is not expanding, and other places are, what exactly is preventing the earth from expanding?
So, based on these three arguments, the universe is bounded.