The double-soft limit in cosmological correlation functions and graviton
  exchange effects
  The graviton exchange effect on cosmological correlation functions is
examined by employing the double-soft limit technique. A new relation among
correlation functions that contain the effects due to graviton exchange
diagrams in addition to those due to scalar-exchange and
scalar-contact-interaction, is derived by using the background field method and
independently by the method of Ward identities associated with dilatation
symmetry. We compare these three terms, putting small values for the slow-roll
parameters and $(1-n_{s}) = 0.042$, where $n_{s}$ is the scalar spectral index.
It is argued that the graviton exchange effects are more dominant than the
other two and could be observed in the trispectrum in the double-soft limit.
Our observation strengthens the previous work by Seery, Sloth and Vernizzi, in
which it has been argued that the graviton exchange dominates in the
counter-collinear limit for single field slow-roll inflation.
