Meaning of a "Flat," "Open," and "Closed" Universe Hello all, I'm reading up on cosmology and the potential shapes of the universe based on its density in relation to the critical density. When one says that a universe is "flat," what precisely does this mean? Does this mean that the space-time of the universe is described by Euclidean geometry? Similarly, what does it mean to say the universe is "open" or "closed"? Thanks in advance!