  We derive a model of dark energy which evolves with time via the scale
factor. The equation of state $\omega=(1-2\alpha)/(1+2\alpha)$ is studied as a
function of a parameter $\alpha$ introduced in this model. In addition to the
recent accelerated expansion, the model predicts another decelerated phase. The
age of the universe is found to be almost consistent with observation. In the
limiting case, the cosmological constant model, we find that vacuum energy
gravitates with a gravitational strength, different than Newton's constant.
This enables degravitation of the vacuum energy which in turn produces the tiny
observed curvature, rather than a 120 orders of magnitude larger value.
