Abstract:
In this thesis we analyze interacting massive scalar fields in de-Sitter space, specifically the issue of infrared divergences which cause the conventional in-out perturbation theory inapplicable. We then review the use of Schwinger-Keldysh formalism which is the most reliable tool to study non-equilibrium systems like QFT in de-Sitter spacetime. We review the previous work on correlation functions (specifically the propagators) the re-summation of propagators and use the technique to show that the propagators calculated have well defined flat-space limit. Next, we discuss the work done on the construction of an S-matrix on the global de sitter space and provide our own construction of an S-matrix in the expanding Poincaré patch. We show that our S-matrix shows good behavior under CPT operation, is unitary and has the expected flat space limit.
