In an open universe , the scale factor increases forever. The expansion begins at the big bang, but never ends. A flat universe is right on the border between open and closed. It expands forever, but just barely. Simply depends on the amount of matter in the universe.
Gravity slows the expansion.
If there's enough matter in the universe, the pull of everything for everything else will slow, stop, and reverse the expansion. If there is too little matter, the expansion continues forever. If we were to throw the ball with the same velocity on a much more massive planet with a much stronger gravitational pull, the thrown ball would come back down to the ground. Distinguishing between models. The most important factor in determining whether the universe is open, closed, or flat is the average density of matter.