The Future of the Universe
At the moment the Universe is expanding – that is, the observable Universe is expanding: we believe in fact that the Universe is infinite. At the moment the furthest objects that we can see is about 12 billion light tears away, and as a result of the Big Bang these distant objects are moving away from each other with speed proportional to distance. If there were no forces between the galaxies then they would continue to recede from each other at the same constant speed, so the size of the observable Universe would be proportional to time.
In fact the speeds of the galaxies and the rate of expansion of the Universe cannot be constant. The force of gravity acts between all masses. The force of gravity is attractive and acts to slow the expansion of the Universe, which means that the rate of expansion must have been greater in the past. Whether or not the observable Universe continues to expand forever or reaches a maximum size and starts to contract depends on the current rate of expansion and the density of the Universe,
An open Universe is one that expands forever. Gravity slows down the rate of expansion but does not stop it. The densityof the Universe is not enough to stop the expansion.
An closed Universe is one that expands up to a maximum point and then starts to contract. Gravity slows down the rate of expansion and eventually stops it. The densityof the Universe is enough to stop the expansion.
A flat Universe has a critical densityand is the borderline case between an open and closed Universe. Gravity slows down the rate of expansion but does not stop it. The density of the Universe is not enough to stop the expansion. You may think of the Universe taking an infinite time to reach a maximum size.