In this two-week journal club, we will discuss how symmetry and consistency principles constrain these primordial correlators.
In the first talk, I will focus on cosmological soft theorems. In flat-space quantum field theory, soft theorems describe the universal behavior of scattering amplitudes when the energy of an external particle is taken to zero. In the early universe, the analogous question is not about the S-matrix, but about late-time correlation functions of primordial fluctuations. A long-wavelength curvature perturbation acts only as a local change of scale; symmetry fixes how it can affect the shorter-wavelength modes. This leads to consistency relations that relate the higher-point function in the soft limit to a lower-point function. As a key example, I will discuss the single-field inflationary consistency relation, in which the scale dependence of the power spectrum fixes the squeezed limit of the scalar three-point function.
While soft theorems constrain special kinematic limits of primordial correlators, the second talk will continue from this viewpoint toward reconstructing higher-point correlators using symmetry as well as locality, unitarity, and singularity structure. Together, the two talks will present a modern perspective on how early-universe physics can be inferred directly from the structure of late-time correlation functions.
