  The Cosmological Principle states that the universe is both homogeneous and
isotropic. This, alone, is not enough to specify the global geometry of the
spacetime. If we were able to measure both the Hubble constant and the energy
density we could determine whether the universe is open or closed.
Unfortunately, while some agreement exists on the value of the Hubble constant,
the question of the energy density seems quite intractable. This Letter
describes a possible way of avoiding this difficulty and shows that if one
could measure the rate at which light-rays emerging from a surface expand, one
might well be able to deduce whether the universe is closed.
