  While the expansion rate of a homogeneous isotropic Universe is simply
proportional to the square-root of the energy density, the expansion rate of an
inhomogeneous Universe also depends on the nature of the density
inhomogeneities. In this paper we calculate to second order in perturbation
variables the expansion rate of an inhomogeneous Universe and demonstrate
corrections to the evolution of the expansion rate. While we find that the mean
correction is small, the variance of the correction on the scale of the Hubble
radius is sensitive to the physical significance of the unknown spectrum of
density perturbations beyond the Hubble radius.
