  Following fresh attempts to resolve the problem of the energy density of the
vacuum, we reconsider the case where the cosmological constant is derived from
a higher-dimensional version of general relativity, and interpret the
gauge-dependence of $\Lambda $ as a dynamical effect. This leads to a relation
between the change in $\Lambda $ and the line element (action) which is
independent of gauge choices and fundamental constants: $d\Lambda ds^{2}=-3$.
This implies that the (classical) vacuum is unstable, with implications for
particle production.
