  The quantum gravitational back-reaction on inflation is based on the
self-gravitation of infrared gravitons which are ripped out of the vacuum
during inflation. The only quantum part of this process is the creation of
gravitons; after they have emerged from the vacuum their behaviour is
essentially classical. To test the thesis that a sufficiently dense ensemble of
classical gravitons can hold the universe together in pure gravity with a
positive cosmological constant, we compute the initial value and first time
derivative of an invariant measure of the expansion rate for arbitrary
classical initial value data. Our result is that the self-gravitation from the
kinetic energy of an initial ensemble of gravitons can indeed slow expansion
enough to hold the universe together.
