2019-11-08 at 11:36 PM UTC
It's flat as far as we can tell but we can really only put a lower bound on how big it could be if it wasn't flat. It's very possible for the global geometry of the universe to be curved while locally being flat.

In fact we see it in our everyday lives, and run a test yourself: try to walk and return to where you started by taking only three 90° turns.

Ya can't do it.

Except you can.

Imagine starting at the North pole. Then you walk down to the equator. Take a 90 degree turn left. Walk any distance, take another 90° turn left and walk back to the north pole.

So it's very possible for the global geometry to be curved, but with an emergent flat local geometry.

There's no transitional point between the two. It just appears a certain way at a particular scale.

I actually think this is the most elegant possible universe geometry, to be closed but simply so huge that it is indistinguishable from flat on any local scale that matters.

The speed of light and time since the Big Bang create a cosmological event horizon regardless of your scale, where causality simply stops. The universe only needs to be big enough to appear flat on such a scale

2019-11-08 at 11:38 PM UTC
I mean a closed geometry would be elegant. An open curved geometry would be even worse from a philosophical standpoint. Filthy math.