Would traveling outward from any point in space ultimately lead you back to where you started?
The answer to this hinges on the fate of the universe. Right now, the universe is expanding. The galaxies are flying away from each other. But — according to astronomical theories — the universe might have one of several destinies.
Its fate depends on the strength of its 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 — and that’s what astronomers don’t know yet. Not enough density, and the universe is “open” — it’ll expand forever. With sufficient density, the universe is “closed” — it’ll 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!