Is the Universe a sphere? If not, which shape it is? How do we know that?

Artists illustration of the expansion of the Universe

There are two things you could mean by “the Universe”, which sometimes get conflated. One is the “observable Universe”, which is the part that it is theoretically possible for us to measure at this point in the history of the cosmos. The other is the entire Universe, which goes far beyond that.

The Observable Universe

This is at least very nearly a sphere, as a result of what it means for something to be “observable”.

Nothing can travel faster than the speed of light, and the Universe is “only” about 13.7 billion years old, and so some things are just too far away for us to be able to see them, even with the best telescopes imaginable. This means there’s some “maximum distance” that we can theoretically observe (which is gradually changing with time), and so our “observable Universe” is everything within a sphere of that radius.

What is that radius? It would make a lot of sense (it might even seem obvious!) to say that it was 13.7 billion light-years, but this is not the case. Because space itself is expanding, and all distances used to be a lot smaller, the current distance between us and the edge of our observable Universe is roughly 46 billion light-years!

The Entire Universe

We can’t be totally sure about anything that’s outside of the part of the Universe that we can observe, because… well… we can’t observe it. However, there are two principles in Cosmology which have served us well so far, and, if they continue to hold, they allow us to say things about the structure of the Universe as a whole. These principles are:

• Homogeneity. No point in space is physically “special”; if you average over large enough regions, every region looks more or less like every other.
• Isotropy. No direction in space is physically “special”; if you average over large enough regions, looking in any direction is more or less like looking in any other.

Given these two assumptions, as well as General Relativity, you can show that the Universe must have one of only three possible shapes, as described by the FLRW metric. These options are:

• “Flat”. A “flat” Universe would be shaped in the way we normally think about space, using Euclidean geometry. When you think about 3-D spaces, this is almost certainly what you’re thinking about. The 2-D analogue would be a geometrical plane. If the Universe is “flat”, then it extends infinitely in all directions.
• “Open”. An “open” Universe would have a structure that would be very difficult to visualize. The 2-D analogue would be a saddle. If the Universe is “open”, then it extends infinitely in all directions.
• “Closed”. A “closed” Universe is the only option that would be finite. The 2-D analogue would be the surface of a sphere. If the Universe is “closed”, then straight lines eventually “loop back around” and go through the same point again, much like moving “in a straight line” along the surface of the Earth will eventually return you to your starting point.

As far as we can tell, our Universe is “flat”. However, it’s impossible for us to know for certain, because the curvature might just be so small that we can’t measure it yet (much like how people 10,000 years ago couldn’t measure the curvature of the Earth, and so thought that it was flat). This means that all three of the above options are still possible, and so the size and shape of the Universe remains an open question in Cosmology.

However, just as early humans couldn’t measure the curvature of the Earth because the Earth is big, if our Universe is curved, we can’t notice the effects because it’s big. In this case, that means much bigger than the part of it that we can measure. This is why I claimed that the entire Universe went “far beyond” the observable part.

Source: Quora.com