Natural Quintessence in String Theory

TL;DR

This paper presents a string theory model of quintessence with a naturally light, radiatively stable scalar field that couples weakly to matter, explaining dark energy without fine-tuning and compatible with observations.

Contribution

The authors develop a string theory-based quintessence model with a stable light scalar field arising from a modulus, embedded in a specific compactification with large extra dimensions.

Findings

The quintessence field's mass is protected by bulk supersymmetry.

The model naturally explains the small dark energy scale without tuning.

Couplings to Standard Model particles are suppressed, avoiding fifth-force constraints.

Abstract

We introduce a natural model of quintessence in string theory where the light rolling scalar is radiatively stable and couples to Standard Model matter with weaker-than- Planckian strength. The model is embedded in an anisotropic type IIB compactification with two exponentially large extra dimensions and TeV-scale gravity. The bulk turns out to be nearly supersymmetric since the scale of the gravitino mass is of the order of the observed value of the cosmological constant. The quintessence field is a modulus parameterising the size of an internal four-cycle which naturally develops a potential of the order (gravitino mass)^4, leading to a small dark energy scale without tunings. The mass of the quintessence field is also radiatively stable since it is protected by supersymmetry in the bulk. Moreover, this light scalar couples to ordinary matter via its mixing with the volume mode. Due to the fact that the quintessence field is a flat direction at leading order, this mixing is very small, resulting in a suppressed coupling to Standard Model particles which avoids stringent fifth-force constraints. On the other hand, if dark matter is realised in terms of Kaluza-Klein states, unsuppressed couplings between dark energy and dark matter can emerge, leading to a scenario of coupled quintessence within string theory. We study the dynamics of quintessence in our set-up, showing that its main features make it compatible with observations.