The expansion of the universe is a manifestation of the principle of Conservation of Energy.

Thread Index of Key Points
Introduction (below)
The Actual Expansion Rate #7
Cosmic "Sea-level" #15
Dealing With Infinity #24
The Basic Picture #27
The Crux of the Matter #55, p2
The Big Picture #56
Einstein's Sublest Error #57
Contraction Energy Estimate #33 p3
Expansion Energy Est. (revised) #256 p 9
On-going Discussion page 3 +

Figure 1 & Figure 2[Print these two figures to follow along in the text. The original urls no longer work , and the printed versions are easier to understand anyway]

Everywhere in the universe, matter is "falling together," radiating energy "into space" as it does so. What happens to all this energy radiated "into space"? It causes "space" to expand! General Relativity allows for this. In GR, space can bend and stretch, and it does so in such a way that the total energy content of the universe is constant. Space expands in GR as naturally as water flows downhill.

Except for the trivial case of a cold, collapsed body, all gravitational systems either expand or contract. Contraction always tends to occur near "centers of mass," and expansion tends to occur away from the center. The sun's core is contracting, but its outer envelope expanding. Satellites near to earth spiral in; those beyond the geosynchronus orbit spiral away. The cores of globular clusters contract; their outer halos expand or get torn apart by tidal forces. The core of a supernova collapses, the outer parts blown away.

Since all gravitational systems are unstable, either contracting or expanding, and since all contraction tends to take place "locally," it logically follows that on the largest scales, the residual effect must be expansion. The universe is in almost perfect balance...but not quite. Locally, the net effect is contraction, accompanied by the release of energy. Cosmically, the net effect is expansion. The expansion of the universe, acceleration and all, is due to "balancing the books" on energy.