  We discuss the mechanism by which the field vacuum energy varies as a result
of strong self-interaction. We propose a non-perturbative approach to treat
strong interactions and discuss the problem in terms of quasi-particles
describing the motion of field modes. The resulting vacuum energy is variable
and depends on the state of the system. If the interacting scalar field is
related to the cosmological field, an acceleration equation with a stable
equilibrium point follows in a simple model, predicting the oscillatory
behavior of the scale factor and other cosmological effects.
