Runaway Quintessence, Out of the Swampland

TL;DR

This paper proposes a string-theory–motivated quintessence scenario based on a runaway string modulus that is lifted by leading non-perturbative effects, creating a de Sitter maximum followed by a runaway. If the modulus starts near the hilltop, Hubble friction freezes it for much of the cosmic history, yielding a nearly constant vacuum energy that can match dark energy observations, with a thawing transition at late times to a time-varying quintessence. The model maintains radiative stability and avoids fifth forces by sequestering the modulus from the visible sector, and it naturally yields an axion/axino sector that can contribute to dark energy and dark radiation. The authors show consistency with the refined de Sitter Swampland Conjecture and discuss implications for metastable de Sitter vacua, potential inflationary extensions, and anthropic considerations within heterotic orbifold frameworks.

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.

Figures (2)

• Figure 1: Potential \ref{['E:pot']} for $\alpha=\sqrt{2}$ and $A = e^{-1105/8}$ in Planck units.
• Figure 2: Solution to the cosmological equations \ref{['E:Frie']} with potential \ref{['E:pot']} and parameters $\alpha=\sqrt{2}$ and $A = e^{-138.122}$, choosing various initial conditions $\phi_{init}$, $\dot{\phi}_{init}=0$ and $\dot{\theta}_{init}=0$. We take $H_0=5.95 \times 10^{-61} M_{pl}$, $\Omega_M=0.31$ and $\Omega_r = 10^{-4}$. We plot variables with respect to the number of e-folds, with $N=0$ today, and $N_{init}=-15$ is some time between BBN and matter-radiation equality.