[Ahban]

Could there be a very simple explanation for the speeding up of the rate of expansion? Every second since stars started fusing they have been turning a small portion of their mass into energy. The less mass, the less gravity. That means that the total amount of gravitational force in the universe has been constantly decreasing since the time the first stars began shining. So the breaking force of gravity has been getting constantly weaker since that time.

In addition, I understand that black holes have a gravity well that is not that wide, but exceptionally steep once entered. Perhaps this configuration means that most of its own gravity is swallowed up at long distances.

Ordinary stars may pull on every other object in the universe, but black holes only pull (with their full strength) on things inside their event horizons. Things outside are pulled on with a quantum tithe of gravitry, i.e.- Only those gravity "waves" able to escape the event horizon due to the fluctating nature of the quantum world near the edge of the event horizon.

Whut U ThAnk?

[VadeRetro]

The total energy in the universe is the same. Stars turn nucleon mass into photons, yes. Here's the funny part. A photon has energy and thus mass-equivalence. (E = mc² and all that.) Energy is mass, mass energy. Light not only curves in space, it causes space to curve.

The curvature of the universe is unchanged by the energy transformations.

BTW, I've been bit by that one myself on a different question. Physicist had to slap me awake.

[RightWhale]

black holes have a gravity well that is not that wide, but exceptionally steep once entered

Sort of. Black holes act gravitationally-speaking just like an ordinary body of the same mass if you are outside the critical radius. If there were a black hole in place of the sun, and if that black hole had the same mass as the sun, earth and the other planets would continue orbiting just as they do now. In the dark.