  The scale--independence of the primordial curvature perturbation suggests
that it comes from the vacuum fluctuation during inflation of a light scalar
field. This field may be the inflaton, or a different `curvaton' field. The
observation of primordial non--gaussianity would be a smoking gun for the
curvaton model, while the observation of gravitational waves originating during
slow--roll inflation would rule out the model.
