  A modified-gravity theory is considered with a four-form field strength F, a
variable gravitational coupling parameter G(F), and a standard matter action.
This theory provides a concrete realization of the general vacuum variable q as
the four-form amplitude F and allows for a study of its dynamics. The theory
gives a flat Friedmann-Robertson-Walker universe with rapid oscillations of the
effective vacuum energy density (cosmological "constant"), whose amplitude
drops to zero asymptotically. Extrapolating to the present age of the Universe,
the order of magnitude of the average vacuum energy density agrees with the
observed near-critical vacuum energy density of the present universe. It may
even be that this type of oscillating vacuum energy density constitutes a
significant part of the so-called cold dark matter in the standard
Friedmann-Robertson-Walker framework.
