A Closed Universe

The last type of universe that we could possibly live in is called a closed universe. The best way to describe a closed universe is one that does not comes back in on itself, or in other words, it closes in on itself. The best example of a closed 2-D space is the surface of a sphere. Unlike, a 2-D flat space or a 2-D open space, a closed 2-D space does not have an edge. When it expands, it expands equally in all directions, uniformly. The spaces are said to be positively curved, because they do not expand in opposite directions.

Looking at the sphere, you can see that as it expands, all points on the surface expand uniformly from the center, and are equidistant from the center at all times. A 3-D closed universe (positively-curved space) would be similar, but in higher dimensions. Another unique thing about a closed universe, is that no matter where you started on a path, eventually, you'd end up at your starting point even without turning, though it may take an infinitely long time. In both open and flat universes, you could never reach your starting point without turning.

Closed universes are also different in lines that start out parallel, end up converging at large distances. Take lines of longitude on the Earth. They start out parallel at the equator, but as they approach either pole, the converge. Because of this, the sum of the interior angles of a triangle are always greater than 180°.

How would we know if the universe is closed? In this case, the actual density of the universe will have to be larger than the critical density. This means that eventually gravity in the universe will slow down and stop expansion at some maximum size. Gravity will then take over and start attracting all the matter and energy together again. When the matter and energy reach the singularity, the Universe will end in a Big Crunch. It is believed that if this is the way the Universe ends, it may be the beginning of a new universe with another Big Bang. This type of universe is called an oscillating universe.