  We define a scalar measure of the local expansion rate based on how
astronomers determine the Hubble constant. Our observable is the inverse
conformal d'Alembertian acting on a unit ``standard candle.'' Because this
quantity is an integral over the past lightcone of the observation point it
provides a manifestly causal and covariant technique for averaging over small
fluctuations. For an exactly homogeneous and isotropic spacetime our scalar
gives minus one half times the inverse square of the Hubble parameter. Our
proposal is that it be assigned this meaning generally and that it be employed
to decide the issue of whether or not there is a significant quantum
gravitational back-reaction on inflation. Several techniques are discussed for
promoting the scalar to a full invariant by giving a geometrical description
for the point of observation. We work out an explicit formalism for evaluating
the invariant in perturbation theory. The results for two simple models are
presented in subsequent papers.
