Dark Energy in Perturbative String Cosmology

TL;DR

This paper explores the challenges of defining an S-matrix in string cosmology with dark energy, arguing that metastable de Sitter vacua are unlikely in weakly coupled string theory and examining alternative explanations for dark energy.

Contribution

It analyzes the limitations of perturbative string theory in describing dark energy and de Sitter space, and discusses the potential role of axions as dark matter and dark energy.

Findings

Eternal quintessence unlikely in weakly coupled string theory

S-matrix cannot be computed perturbatively in these models

Most string-based explanations for dark energy are implausible

Abstract

The apparent observation of dark energy poses problems for string theory. In de Sitter space, or in quintessence models, one cannot define a gauge-invariant S-matrix. We argue that eternal quintessence does not arise in weakly coupled string theory, but point out that it is difficult to define an $S$-matrix even in the presence of perturbative potentials for the moduli. The solutions of the Fischler-Susskind equations all have Big Bang or Big Crunch Singularities. We believe that an S-matrix (or S-vector) exists in this context but cannot be calculated by purely perturbative methods. We study the possibility of metastable de Sitter vacua in such weakly coupled scenarios, and conclude that the S-matrix of the extreme weak coupling region cannot probe de Sitter physics. We also consider proposed explanations of the dark energy from the perspective of string theory, and find that most are implausible. We note that it is possible that the axion constitutes both the dark matter and the dark energy.