EFT for de Sitter Space
The physics of de Sitter space is essential to our understanding of our cosmological past, present, and future. It forms the foundation for the statistical predictions of inflation in terms of quantum vacuum fluctuations that are being tested with cosmic surveys. In addition, the current expansion of the universe is dominated by an apparently constant vacuum energy and we again find our universe described by a de Sitter epoch. Despite the success of our predictions for cosmological observables, conceptual questions of the nature of de Sitter abound and are exacerbated by technical challenges in quantum field theory and perturbative quantum gravity in curved backgrounds. In recent years, significant process has been made using effective field theory techniques to tame these breakdowns of perturbation theory. We will discuss how to understand the long-wavelength fluctuations produced by accelerating cosmological backgrounds and how to resolve both the UV and IR obstacles that arise. Divergences at long wavelengths are resummed by renormalization group (RG) flow in the EFT. For light scalar fields, the RG flow manifests itself as the stochastic inflation formalism. In single-field inflation, long-wavelength metric fluctuations are conserved outside the horizon to all-loop order, which can be understood easily in EFT terms from power counting and symmetries.