TASI lectures on cosmological observables and string theory

TL;DR

This paper offers an updated pedagogical overview of cosmological observables, string theory mechanisms for inflation, and the implications of holographic duality, emphasizing the importance of ultraviolet completion and observational constraints.

Contribution

It introduces novel inflation mechanisms within string theory, discusses their observational tests, and highlights the role of holographic duality in cosmology.

Findings

String theory backgrounds have positive potential energy with a constrained landscape.

Some inflation mechanisms are testable through cosmological observations.

Long-term data analysis can reveal physics beyond the inflation energy scale.

Abstract

These lectures provide an updated pedagogical treatment of the theoretical structure and phenomenology of some basic mechanisms for inflation, along with an overview of the structure of cosmological uplifts of holographic duality. A full treatment of the problem requires `ultraviolet completion' because of the sensitivity of inflation to quantum gravity effects, including back reaction and non-adiabatic production of heavy degrees of freedom. Cosmological observations imply accelerated expansion of the late universe, and provide increasingly precise constraints and discovery potential on the amplitude and shape of primordial tensor and scalar perturbations, and some of their correlation functions. Most backgrounds of string theory have positive potential energy, with a rich but still highly constrained landscape of solutions. The theory contains novel mechanisms for inflation, some subject to significant observational tests. Although the detailed ultraviolet completion is not accessible experimentally, some of these mechanisms directly stimulate a more systematic analysis of the space of low energy theories and signatures relevant for analysis of data, which is sensitive to physics orders of magnitude above the energy scale of inflation as a result of long time evolution (dangerous irrelevance) and the substantial amount of data. Portions of these lectures appeared previously in Les Houches 2013, "Post-Planck Cosmology" .