Abstract
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.
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