  We consider the effect of various particles on the cosmic expansion rate
relative to that of the graviton. Effectively massless fermions, gauge bosons
and conformally coupled scalars make only minuscule contributions due to local
conformal invariance. Minimally coupled scalars can give much stronger
contributions, but they are still sub-dominant to those of gravitons on account
of global conformal invariance. Unless effectively massless scalar particles
with very particular couplings exist, the leading effect on the expansion rate
is furnished solely by the graviton. An upper bound on the mass of such scalar
particles is obtained.
