The Universe Could Be Flat

The shape of the universe is influenced by the struggle between the pull of gravity (based on the density of the matter in the universe) and the rate of expansion. If the density of the universe exceeds a certain critical value, then the universe is closed, like the surface of a sphere. This implies that the universe is not infinite but has no end. In this case, the universe will eventually stop expanding and start collapsing in on itself, in an event known as the Big Crunch. If the density of the universe is less than the critical density value, then the shape of the universe is open, like the surface of a saddle. In this case, the universe has no bounds and will continue to expand forever.