Runaway Quintessence, Out of the Swampland

TL;DR

This paper proposes a string theory-based model where a runaway scalar potential leads to a form of quintessence, aligning with swampland constraints and avoiding issues with traditional de Sitter vacua, with potential observable cosmological effects.

Contribution

It introduces a novel runaway quintessence scenario in string theory that satisfies swampland constraints and offers a viable alternative to metastable de Sitter vacua.

Findings

The model aligns with current dark energy observations.

The scalar field remains frozen near the hilltop, acting as quintessence.

Concrete string models satisfy the refined dS Swampland Conjecture.

Abstract

We propose a simple, well-motivated and robust alternative to a metastable de Sitter vacuum in string theory, consistent with current observations of dark energy and naturally satisfying conjectured swampland constraints. Inflation ends in a supersymmetric Minkowski minimum, with a flat direction that is protected by non-renormalisation theorems. At some scale non-perturbative effects kick in, inducing a runaway scalar potential. The tail of this runaway potential cannot sustain a late-time dominating, slow-roll quintessence. However, the potential always contains a dS maximum. If the modulus starts close to the hilltop, it remains frozen there by Hubble friction for much of the cosmological history, at first sourcing a tiny classical vacuum energy and then constituting a rolling quintessence, with observable consequences. So long as the modulus is localised away from the Standard Model in the extra dimensions, there are no observable fifth forces nor is there time-variation of fundamental constants, and the modulus mass is protected from radiative corrections. We revisit concrete string models based on heterotic orbifold compactifications, and show that their de Sitter extrema satisfy the recently refined dS Swampland Conjecture.