Loop corrections in the separate universe picture

Understand

In inflationary models that produce a spike of power on short scales, back-reaction of small-scale substructure onto large-scale modes is enhanced.

• We argue that the separate universe framework provides a highly convenient tool to compute loop corrections that quantify this back-reaction.
• Each loop of interest is characterized by large hierarchies in wavenumber and horizon exit time.
• The separate universe framework highlights important factorizations involving these hierarchies.