I'm not sure if this is meant for a math or cosmology section but GR is what had me on this train of thought so I'll post it here. What is flat space? Does that simply mean that the universe as a whole has the same geometry as we have here on Earth (minus the gravity well?). Or is it something more fundamental that I'm missing. Basically, if the universe was curved in such a way that there were three right angles in a triangle, would we have defined that as flat? Is flatness itself a relative term to the observer? If so, why is the observation that the universe appears perfectly flat not expected, wouldn't we only to be able to detect gradients?