MATTER CONTRIBUTIONS TO THE EXPANSION RATE OF THE UNIVERSE

We consider the effect of various particles on the cosmic expansion ra te relative to that of the graviton. Effectively massless fermions, ga uge bosons and conformally coupled scalars make only minuscule contrib utions due to local conformal invariance. Minimally coupled scalars ca n give much stronger contributions, but they are still sub-dominant to those of gravitons on account of global conformal invariance. Unless effectively massless scaler particles with very particular couplings e xist, the leading effect on the expansion rate is furnished solely by the graviton. An upper bound on the mass of such scalar particles is o btained. (C) 1998 Elsevier Science B.V. All rights reserved.