Cosmological consequences of a variable cosmological constant model

We derive a model of dark energy which evolves with time via the scale factor. The equation-of-state is studied as a function of a parameter alpha introduced in this model as omega = (1 - 2 alpha)/(1 + 2 alpha). In addition to the recent accelerated expansion, the model predicts another decelerated phase. These two phases are studied via the parameter alpha. The age of the universe is found to be almost consistent with the observation. In the limiting case, the cosmological constant model, we find that vacuum energy gravitates with a tiny gravitational constant which evolves with the scale factor, rather than with 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.