Effective Field Theory Approach to Cosmological Initial Conditions: Self-Consistency Bounds and Non-Gaussianities

TL;DR

This work formulates an EFT framework for cosmological initial conditions on inflationary backgrounds by introducing a boundary action at the initial time $t^*$, enabling a systematic parametrization of non-thermal states through local boundary operators with a UV cutoff $M$. It derives how such initial-state modifications alter the power spectrum and analyzes stringent back-reaction bounds on the boundary couplings, showing that observable changes are constrained by slow-roll dynamics. The study also demonstrates that non-Gaussianities can be enhanced relative to standard single-field predictions when generated by initial-state boundary terms, potentially yielding large, detectable signals if the initial time is not too far from the end of inflation. Overall, the results establish self-consistency conditions that govern the size of initial-state deviations and quantify the trade-off between observable signatures and back-reaction in EFT cosmology.

Abstract

Effective Field Theory (EFT) is an efficient method for parametrizing unknown high energy physics effects on low energy data. When applied to time-dependent backgrounds, EFT must be supplemented with initial conditions. In these proceedings, I briefly describe such approach, especially in the case of inflationary, almost-de Sitter backgrounds. I present certain self-consistency constraints that bound the size of possible deviations of the initial state from the standard thermal vacuum. I also estimate the maximum size of non-Gaussianities due to a non-thermal initial state which is compatible with all bounds. These non-Gaussianities can be much larger than those due to nonlinearities in the action describing single-scalar slow roll inflation