  We study the self-energies of weakly interacting scalar fields in de Sitter
space with one field much lighter than the Hubble scale. We argue that
self-energies drastically simplify in this light limit. We illustrate this in
theories with two scalar fields, one heavy and one light, interacting with one
another through either cubic or quartic interactions. To regulate infrared
divergences, we compute these self-energies in Euclidean de Sitter space and
then carefully analytically continue to Lorentzian signature. In particular, we
do this for the most general renormalizable theory of two scalar fields with
even interactions to leading order in the coupling and the mass of the light
field. These self-energies are determined by de Sitter sunset diagrams, whose
analytic structure and UV divergences we derive. Even at very weak couplings,
the light field can substantially change how the heavy field propagates over
long distances. The light field's existence may then be inferred from how it
modifies the heavy field's oscillatory contribution to the primordial
bispectrum in the squeezed limit, i.e. its cosmological collider signal.
