  Understanding mechanisms capable of altering the vacuum energy is currently
of interest in field theories and cosmology. We consider an interacting scalar
field and show that the vacuum energy naturally takes any value between its
maximum and zero because interaction affects the number of operating field
modes, the assertion that involves no assumptions or postulates. The mechanism
is similar to the recently discussed temperature evolution of collective modes
in liquids. The cosmological implication concerns the evolution of scalar field
$\phi$ during the inflation of the Universe. $\phi$ starts with all field modes
operating and maximal vacuum energy in the early inflation-dominated epoch. As
a result of inflation, $\phi$ undergoes a dynamical crossover and arrives in
the state with one long-wavelength longitudinal mode and small positive vacuum
energy predicted to be asymptotically decreasing to zero in the late epoch.
Accordingly, we predict that the currently observed cosmological constant will
decrease in the future, and comment on the possibility of a cyclic Universe.
