Investigating Unitarity and Symmetry Constraints on Cosmological Observables

Abstract

This thesis explores the application of the principles of unitarity and symmetries in possible theories governing early universe in deriving corresponding identities that must be satisfied by the cosmological observables that we measure. In the first part of the thesis, an attempt at re-deriving cosmological cutting rules is made using a recently-proposed definition of BunchDavies S-matrix. In the second part, we make an attempt in offering a resolution to some puzzle in the literature about soft theorems involving pure-graviton correlators.