Les Houches lectures on inflationary observables and string theory
Eva Silverstein
TL;DR
The lectures argue that inflationary observables must be understood within a UV-complete framework, with string theory offering concrete mechanisms such as axion monodromy, warped brane inflation, and DBI inflation that yield distinctive signatures in $r$, $n_s$, and non-Gaussianity. They connect low-energy EFT descriptions of perturbations to high-energy physics, illustrating how Planck-suppressed operators and multi-field dynamics can alter the power spectrum and bispectrum. The notes also discuss the structure of string compactifications, moduli stabilization, and flux configurations that realize inflation or its exit, and explore thought-provoking holographic approaches to defining observables in de Sitter spacetimes. Overall, the work emphasizes the synergy between bottom-up EFT analyses and top-down string constructions in guiding data analysis and interpreting future observations.
Abstract
These lectures cover the theoretical structure and phenomenology of some basic mechanisms for inflation. A full treatment of the problem requires `ultraviolet completion' because of the sensitivity of inflation to quantum gravity effects, while the observables are elegantly parameterized using low energy field theory. String theory provides novel mechanisms for inflation, some subject to significant observational tests, with highly UV-sensitive tensor mode measurements being a prime example. Although the ultraviolet completion is not directly accessible experimentally, some of these mechanisms have helped stimulate a more systematic analysis of the space of low energy theories and signatures relevant for data analysis, including searches for non-Gaussianity and additional structure in the power spectrum. We include a pedagogical overview of string compactifications, with a focus on candidate inflatons and their symmetry structure. In the last lecture we attack the problem of thought-experimental observables in inflation, developing a generalization of gauge-gravity duality that relies on the structure of the scalar potential in string theory.