Most* astronomers believe that the universe+ began with an event dubbed the Big Bang.
Since then, the universe has been expanding, but the expansion is slowing down. When we think about the "expnding universe", we can measure its state of expansion in terms of the mean distance between any two galaxies.
In the past, galaxies were closer together.
In the near future, they will be further apart.
But what will happen very far into the future? Will the attractive force of gravity overcome the outward expansion?
If the gravitational mass of the matter in the universe is not enough to halt the expansion, then the universe will expand forever, and we say the universe is open.
If the gravitational mass of the matter in the universe is great enough to halt the expansion, then the galaxies will reach some maximum separation and then will begin to collapse back on top of each other. In this case, and we say the universe is closed, and we predict that sometime in the distant future, there should also be a Big Crunch.
If there might be one Big Bang and one Big Crunch, perhaps there are many, and the universe is said to oscillate.
We don't know which case holds; the determination of the true mean mass density of the universe is one of the fundamental pursuits of astronomers.
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