Why Does the Universe Expand?

Edward Arthur Milne's proposal was that the universe started out expanding. That is Milne cosmology, and oddly enough it's equivalent to Standard cosmology when you take out the gravitational effects of mass-energy. What's odd about that is that in Milne cosmology you have a big bang at one point with the edge moving at arbitrarily close to c and with uniform distribution of stuff and velocities, while in Standard cosmology we speak of the big bang happening everywhere all at once. This oddity is resolved when you realize that Milne is hyperbolic, so you if you "unfurl" a Milne universe it then looks the same as Standard excluding gravity.