  Inflation leads us to expect a spectrum of gravitational waves (tensor
perturbations) extending to wavelengths much bigger than the present observable
horizon. Although these gravity waves are not directly observable, the energy
density that they contribute grows in importance during the radiation- and
dust-dominated ages of the universe. We show that the back reaction of tensor
perturbations during matter domination is limited from above, since
gravitational waves of wavelength $\lambda$ have a share of the total energy
density $\Delta \rho(\lambda)/\rho$ during matter domination that is at most
equal to the share of the total energy density that they had when the mode
$\lambda$ exited the Hubble radius $H^{-1}$ during inflation. This work is to
be contrasted to that of Sahni, who analyzed the energy density of gravity
waves only insofar as their wavelengths are smaller than $H^{-1}$. Such a
cut-off in the spectral energy of gravity waves leads to the breakdown of
energy conservation, and we show that this anomaly is eliminated simply by
taking into account the energy density and pressure of long wavelength
gravitational waves as well as short wavelength ones.
