ErnieM wrote:When astronomers say "The expansion of Space is accelerating", how many dimensions of Space are they referring to?
They are generally referring to the three spatial dimensions.
What do you mean with "because it isn't a three-dimensional structure"?
The Universe is a four-dimensional structure. Its geometric center contains t=0, a point that we can't observe. If the Universe if finite, its geometric center of mass will also be a four-dimensional point, which will not lie on the three-dimensional manifold we can observe.
I see this expansion making the already large Universe larger and the gravitational hold between galaxy structures weakening to inconsequential.
Gravity is a weak force to begin with. Certainly, its influence between points that are very far apart (but still causally connected) is likely to be very tiny. But not zero. Any bounded region of the Universe will have a center of mass.