A model of the universe which is no longer generally favoured by current thought. In a closed universe, the universe will one day start retracting instead of expanding forever, ending in an event called the big crunch. The opposite is the open universe, with a third model described as balancing between the two. When the closed universe one day ends, time and matter will cease to exist. Many physicists deem this kind of non-existence impossible, and so theorize that a new big bang would follow, with time and matter recreated. Some cosmologists talk of a pulsating universe. Certain theories suggest that each new universe would be larger and would last longer than the previous one, until the last universe would be open.
In a closed universe, gravitation's retracting force will one day conquer the expanding motion energy. The situation can be compared to that of escape velocity. If one throws a rock up from the Earth's surface, the rock will return to the ground. If the rock's speed is sufficient, as when it is aided by a rocket, the rock will leave the Earth. Although the
universe has no actual boundary, it can be thought to exist about 12 billion light-years away. Thus, the universe's escape velocity would be precisely the speed of light.
