I wanted to say I love the posts on this Cosmology forum - very good reads. My question is whether there is enough to large-scale curvature alone to cause the expansion of the universe as we measure it? Why can't galaxies accelerating away from us be moving in curved space? I agree with the idea of infinite matter-energy density at some time in the past and also with inflation. I believe I understand homogeneity and isotropy. The question in part is whether large-scale curvature satisfactory to move galaxies in ever increasing velocities with further distances would be consistent with observations like COBE data and also would it be symmetrical. Clay Cravens 713 858 2724
Expansion isn't due to 'curvature', per se. Curvature is defined at a point (or region) of spacetime, while expansion is the behavior of curvature over time---the 'velocity' of the curvature, if you will. Accelerated expansion is a level further away from curvature itself, its like the 'acceleration' of the curvature (again, a loose metaphor) and requires a driving force beyond the curvature itself. But galaxies certainly are moving in curved space.
Actually, it's entirely the other way around. The expansion of space is quite precisely and accurately described as space-time curvature. It is not, however, spatial curvature. The spatial curvature of our observable universe is very close to zero. But it does have space-time curvature, and that manifests itself as expansion.