De Sitter vs Quintessence in String Theory

TL;DR

This work evaluates the status of de Sitter vacua versus quintessence in string theory, focusing on IIB flux compactifications, KKLT, LVS, and uplift mechanisms, and contrasts them with quintessence scenarios driven by moduli or axions. It argues that dS constructions have achieved substantial concreteness and calculational control, though full non-perturbative understanding remains elusive, while quintessence faces tighter theoretical hurdles and typically requires additional tuning or hidden-sector dynamics. The analysis emphasizes the swampland constraints, especially the Higgs–quintessence coupling problem, and explores ultra-light axions as viable quintessence candidates within LVS and axiverse frameworks. Observational data favoring $w\approx -1$ supports dS-like behavior, but potential tensions in Hubble parameter measurements keep dynamical dark energy models as an active area of investigation.

Abstract

De Sitter solutions have been recently conjectured to be incompatible with quantum gravity. In this paper we critically assess the progress and challenges of different mechanisms to obtain de Sitter vacua in string compactifications and compare them to quintessence models. We argue that, despite recent criticisms, de Sitter models reached a level of concreteness and calculational control which has been improving over time. On the other hand, building string models of quintessence appears to be more challenging and requires additional fine-tuning. We discuss the tension between the swampland conjecture and the Higgs potential and find examples which can evade fifth-force bounds even if they seem very hard to realise in string theory. We also comment on the tension with low-redshift data and explore ultra-light axions from string theory as dark energy candidates.

Figures (4)

• Figure 1: Examples of potentials that allow for hilltop quintessence. The red domains schematically represent the regions of the potentials where slow-roll can take place.
• Figure 2: In the case $\Lambda \gg \Lambda_\ell$ slow-roll can happen also in the region close to the inflection point of the potential, and given (\ref{['eq:IPQ']}) this does not require a super-Planckian axion decay constant.
• Figure 3: We illustrate how the equation of state oscillates while the axion oscillates around its minimum (time is in units of the axion mass). Contrary to the dark matter case in which the average $w$ vanishes, here the presence of $\Lambda$ causes the average to be non-zero. This behaviour can be compared with data from low-redshift observations, in order to explore the existence of axions with mass around $H_0$.
• Figure 4: We plot the function $\Psi$ in the $\chi$ direction at $h = v$.