Curvature of the Universe
Since gravity is spacetime curvature, the
density sets the geometry. Only in a “flat” Universe do parallel lines stay next to each
other. In a “closed” Universe, in principle a beam of light eventually comes
back upon itself.
In principle, we can measure the geometry of
spacetime through geometrical tests. For example, you can count the number of
galaxies in a given patch of sky as you go further out. These tell
us our ultimate fate.