There are three different, proposed futures to the universe; Open, Closed and Flat.
All of these different futures rely on the benchmark density called the critical density which defines how much density would be required in order to have a flat universe (non-expanding universe.)
(You don't have to state the density, but it is 4.5E-27 or 4.5E-26)
This graph shows the different futures for the universe.
(Chris Hamper, Physics Higher Level Book.)An Open Universe represents an ever-expanding universe, with less mass than the critical density so that the overall mass of the universe is less than the critical density and the universe's gravitational pull isn't strong enough to pull it back on itself, therefore it keeps expanding.
A Closed Universe represents a universe containing more mass than the critical density and thus it has a greater gravitational pull than the others which can pull the universe back to the starting position. This is called the 'big crunch.' It has been theorized that after this 'big crunch' occurs another 'big bang' will occur as all the mass has come together in one small area forming another singularity.
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