A model of the universe in which the curvature of space is roughly spherical, entailing that the universe has finite size. An object moving in a straight line in a closed universe would eventually return to its starting point. According to most current cosmological theories, the universe is closed if it is sufficiently dense, and therefore possesses enough gravitational force to stop or reverse the expansion started by the big bang (resulting in what is called the big crunch). Compare open universe. See Note at big bang.
If there is enough matter in the universe to stop the expansion associated with the big bang, we say that the universe is closed. In a closed universe, the current period of expansion will be followed by a period of contraction (sometimes called the Big Crunch) as the gravitational force pulls matter back in. Searching for matter to “close” the universe is a major task of modern cosmology. (See also dark matter, flat universe, and open universe.)