Well, my point is that potential energy isn't really all that important. What is important is energy period. As far as I am aware potential energy is mostly used to figure out how much energy we can get out of a system. For example in a hydroelectric power station the potential energy of the water at the top of the inlet with respect to the generator is very important. But if we are talking about space flight we are more interested in momentum.

As far as "some external force", dark energy isn't external, it is a part of the universe. What it is is, so far as I know, completely unknown. Any ideas as to what it is are basically just that, ideas. There is a lot of evidence that indicates it is there, but there is no good theoretical way to incorporate it into our view of the universe.

Bill Gill

C is not the speed of light in a vacuum.
C is the universal speed limit.