Ask Ethan: Why Is The Universe Flat?
▻https://www.forbes.com/sites/startswithabang/2021/03/05/ask-ethan-why-is-the-universe-flat
The fact that our Universe exists with the properties we observe tells us that, very early on, the Universe had to be at least very close to flat. A Universe with too much matter-and-energy for its expansion rate will have positive curvature, while one with too little will have negative curvature. Only the perfectly balanced case will be flat.
But it is possible that the Universe could be curved on extremely large scales: perhaps even larger than the part of the Universe we can observe. You might think about drawing a triangle between our own location and two distant galaxies, adding up the interior angles, but the only way we could do that would involve traveling to those distant galaxies, which we cannot yet do. We’re presently limited, technologically, to our own tiny corner of the Universe. Just like you can’t really get a good measurement of the curvature of the Earth by confining yourself to your own backyard, we can’t make a big enough triangle when we’re restricted to our own Solar System.
Thankfully, there are two major observational tests we can perform that do reveal the curvature of the Universe, and both of them point to the same conclusion.
