The expansion of the universe is fueled by a continuous transition to lower-energy vacuum states. Unlike the normal "false vacuum" model, though, there are a lot of these lower-energy states, which become closer and closer together until they reach a limiting value.
The graph of these states would probably look familiar - it's similar to the electron transitions for the hydrogen atom, only with the orbitals replaced with "time since the Big Bang". The net result matches the lower value of the vacuum energy... and there's the possibility that this also explains inflation as being equivalent to the transition between n=1 and n=2 (whereas we're currently at something on the order of n=10^35).
Granted, there's no guarantee that I'm right (and in fact I'm probably not, since I have no formal training in cosmology), but it looks like a model that fits the current knowledge.