How Will It All End ?

Most things have a beginning, exist for a period of time, and then come to a end. It is also the nature of the universe to begin, exist and end. The universe was born in an unparalleled explosion called the "Big Bang." The universe as a whole has been getting cooler and cooler, but how much longer can this go on? Is the universe going to come to an end? Or will it continue to expand and cool for ever? What is the fate of the universe?

Imagine that a ball is thrown into the air. As it travels upward, it slows down. Eventually it will stop rising and fall back to Earth. The gravitational attraction between the Earth and the ball causes this to happen. If the ball is thrown with greater force, it will travel higher before it falls. Can the ball be thrown with sufficient force to overcome the Earth's gravity and not fall back down but continue to fly away forever? Yes, if humans could throw a ball with enough force, the ball would not return to Earth. Of course, if earth were less massive and had less gravity, the ball would require less force, or speed, to overcome the gravity. So whether or not the ball returns to earth or flies away forever depends on two factors: the speed with which the ball is moving and the strength of the gravitational attraction between Earth and the ball.

Matter within the universe is also flying away. All the galaxies and galaxy clusters are moving away form one another, much as the thrown ball flies away form Earth. The question now becomes, are the Galaxies flying apart with sufficient force so that they will continue to fly apart, or will they slow down, stop, and eventually come back together ? As in the case of the thrown ball, the answer depends on the speed with which the galaxies are moving away for one another and the strength of gravitational attraction between the galaxies.

Astronomers have accurately calculated the speed of the galaxies. ( Hubble noticed form his study of galaxies that the father away a galaxy is, the faster it is moving away for us. A galaxy one light-year away is receding at a speed of nearly 10 miles/sec. [ 16 km/sec ]. A galaxy 10 light years away is receding 10 times as fast, at a speed of nearly 100 miles/sec.) Although the universe is expanding, its rate of expansion is slowing.

If there is not enough matter in the universe, it will expand forever. Cosmologists refer to such a universe as "Open." Although the rate of expansion is slowing with time, it will never stop, not even at infinite time.

If there is enough matter in the universe, it will stop expanding and start contracting. This contraction would logically continue until the universe is once again a dense, hot, small body as it was prior to the Big Bang. Cosmologists have referred to this squeezing together as the Big Crunch. A universe that operated this way is referred to as " Closed," since it reached a limit in size, with a measurable outer boundary at its greatest expansion. Astronomers have had theories that if there were enough matter in the cosmos for this to happen, the history of the universe may very well be an endless succession of the Big Bangs expansions, and the Big Crunches contractions, with the most recent Big Bang happening 15 milling years ago. This, of course. Implies no beginning and no end.