If the universe is dense enough, then space is “closed.” In that case, the universe resembles the surface of a sphere, and a light beam shining outward eventually returns to its starting point. If the universe is not dense enough, then the universe is “open,” negatively curved like the surface of a saddle. There’s also a third option. The universe could be not “open” or “closed,” but “flat” like a sheet of paper.

If the universe is dense enough, then space is “closed.” In that case, the universe resembles the surface of a sphere, and a light beam shining outward eventually returns to its starting point. If the universe is not dense enough, then the universe is “open,” negatively curved like the surface of a saddle. There’s also a third option. The universe could be not “open” or “closed,” but “flat” like a sheet of paper.

Right now, the universe is expanding. The galaxies are flying away from each other. But – according to astronomical theories – the universe may have one of several different destinies. That’s because gravity – the gravity of the universe as a whole – wants to stop the galaxies from flying apart and even pull them back toward each other.

The strength of this pull of gravity depends on how dense the universe is – how much mass the universe contains – and that’s what astronomers don’t know yet. They don’t know the density of the universe. _Without_ enough density, the universe is “open,” and it will expand forever. _With_ enough density – enough mass for the universe – the universe is “closed,” and it will eventually stop expanding and collapse back on itself.

If you shine a flashlight beam out into an open universe, the light keeps on moving outward … forever. But a closed universe is like a sphere. It doesn’t have a boundary, but it’s finite in extent. On a sphere – say, a planet – you could walk in a straight line and return to the place you started. Likewise, a light beam shining out into a closed universe will eventually return … from the opposite direction!