Will the universe continue expanding? Will it just stop or even begin to contract?
The answer depends on the amount of mass that the universe contains, according to work done in the 1920's by Russian's mathematician Alexander Friedman. If the universe mass exceeds a certain crucial value, then gravity should eventually stop everything from flying away from everything else-just as, if you throw a ball straight up, you can depends on gravity to stop the ball and return it to you.
With enough mass the universe will eventually succumb to the attractive force of gravity and collapse once again into a single point - a fate often called the Big Crunch. But if there isn't enough mass, the universe will just keep on expanding. Scientists are still trying to find evidence for which of these fates our universe faces. Some say our universe alternates between Big Bang and Big Crunches in a perpetual display of the power of gravity.
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