Challenges for String Cosmology

TL;DR

The paper critically evaluates whether string theory can describe the observed universe and whether cosmological data can constrain string theory. It highlights that progress is hindered by an incomplete understanding of string theory, reliance on multiple overlapping approximations, and the absence of universally controlled de Sitter vacua. It identifies moduli stabilization, initial conditions, bounces, singularities, string-inspired approaches, and the inverse problem of compactifications as central obstacles, and it argues for a pragmatic path forward. The authors advocate focusing on explicit, stabilized compactifications and careful treatment of naturalness and heavy-field effects, aiming for predictions in UV-sensitive cosmological observables rather than broad, overly optimistic claims.

Abstract

We critically assess the twin prospects of describing the observed universe in string theory, and using cosmological experiments to probe string theory. For the purposes of this short review, we focus on the limitations imposed by our incomplete understanding of string theory. After presenting an array of significant obstacles, we indicate a few areas that may admit theoretical progress in the near future.