String Theory seminar - Enrico Pajer Seminar

Abstract: According to our standard model of cosmology, inflation gives us the privilege to observationally test perturbative quantum field theory in curved spacetime with quantised dynamical gravity. The natural observables are correlation functions of metric perturbations. In this talk I will discuss how symmetries of the laws of physics constrain the shape of correlators. For non-linearly realised symmetries, examples include adiabatic modes in single field inflation in both flat and curved universes, and shift symmetric theories. Then I'll present two no-go theorems on the possible linearly realised symmetries, a cosmological analog of the Coleman-Mandula theorem. Assuming scale invariance, homogeneity and isotropy, the largest amount of symmetries of a generic scalar correlator is the de Sitter group, equivalently the conformal group in three Euclidean dimensions. Moreover, assuming a single-clock evolution, the correlators of curvature perturbation can be de Sitter invariant only if the theory is free.