  We study the renormalized energy-momentum tensor of gravitons in a de Sitter
space-time. After canonically quantizing only the physical degrees of freedom,
we adopt the standard adiabatic subtraction used for massless minimally coupled
scalar fields as a regularization procedure and find that the energy density of
gravitons in the E(3) invariant vacuum is proportional to H^4, where H is the
Hubble parameter, but with a positive sign. According to this result the scalar
expansion rate, which is gauge invariant in de Sitter space-time, is increased
by the fluctuations. This implies that gravitons may then add to conformally
coupled matter in driving the Starobinsky model of inflation.
