  We consider the possibility that, in the semiclassical Einstein equation for
cosmological spacetimes, gravity is sourced by the amount of stress-energy that
is above that of the instantaneous ground state. For this possibility to be
consistent, the Bianchi identities must continue to hold. This is nontrivial
because it means that the ground state expectation value of the stress-energy
tensor must be covariantly conserved in spite of the fact that the ground state
is generally a different state at different times. We prove that this
consistency condition does hold. As a consequence, we find that the vacuum
stress-energy which is above the instantaneous ground state does not
renormalize the cosmological constant, as long as the instantaneous ground
states and the instantaneous adiabatic vacua exist.
