The contributions of heavy, spatially homogeneous, boson and fermion fields to the effective action for a Robertson-Walker space-time are calculated. For scale factors larger than the Compton wavelengths of the particles associated with these matter fields the equations of motion for this scale factor are the same as those for a matter-dominated universe. Some speculations about the forces driving the expansion of the very early Universe are presented. © 1992 The American Physical Society.