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Comment by theobreuerweil | Hacker News Reader

I’m not 100% on this but the way I understood this is by analogy to the 2D surface of the Earth. The surface of the Earth is finite yet has no reachable edge. The universe is the same in 3D. Like if the Earth grew then there would be more land but still no more or less “edge” of the world, and the same could hold for the universe. That said I think this is one theory rather than accepted fact?

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> The universe is the same in 3D.

This would mean the universe has positive curvature. Experimental evidence points towards the universe being flat (zero curvature), though there is some margin for error that could go either way (positive or negative curvature).

> This would mean the universe has positive curvature.

There are several flat compact manifolds, they're just not simply connected.

In particular, almost none of them are isotropic. The assumption that the universe is isotropic is part of the cosmological principle and the foundation of modern cosmology. It's a very natural assumption to make, so the vast majority of cosmologists are quite comfortable with it, but at the end of the day it's just an assumption that could be wrong.

In the case of a universe of finite size, this analogy explains how there can be such a thing as a finite space without there being boundaries provided the space is (slightly) curved.

So this has less to do with an infinitely sized universe and more with the question of “What exists beyond the edge of the universe if it would be finite in size?”

Our universe is probably a black hole in another, larger universe we cannot reach. And then turtles all the way down.

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