String Cosmology: A Review

TL;DR

This paper surveys string cosmology, focusing on how a UV-complete theory informs early-universe dynamics, inflation, moduli stabilization, and dark energy. It analyzes both initial-singularity physics and a wide array of string-inspired inflation models, including open- and closed-string scenarios, warped geometries, and axion/multi-field constructions, highlighting observational predictions and current constraints. Major contributions include organizing the inflationary problem into moduli-stabilization–driven eta and mass issues, outlining viable stringy mechanisms like KKLT and DBI, and examining landscape-inspired proposals and their phenomenology. The work emphasizes that while many models remain fine-tuned or model-dependent, forthcoming cosmological data will increasingly discriminate among them and potentially reveal signatures characteristic of string theory, such as non-Gaussianity, cosmic strings, or suppressed tensor modes.

Abstract

We give an overview of the status of string cosmology. We explain the motivation for the subject, outline the main problems, and assess some of the proposed solutions. Our focus is on those aspects of cosmology that benefit from the structure of an ultraviolet-complete theory.