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After recapitulating the discussion of last week on how to define sensible correlation functions for massive fields on de Sitter space, I will move on to the more interesting case of massless fields.
I will begin by discussing how there are no dS invariant vacua for free massless fields, and then discuss the breakdown of perturbation theory for interacting theories in the Bunch- avies vacuum (Burgess, Holman, Leblond, Shandera, 1005.3551 [hep-th]). I will briefly discuss the resummation of leading logs using a Boltzmann type equation (Starobinsky and Yokoyama astro-ph/9407016, Riotto and Sloth 0801.1845 [hep-ph]), and finally discuss some recent work (Rajaraman, arXiv:1008.1271 [hep-th]) on how to recover the resumed answer using a correct path integral treatment of massless interacting fields.
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