Axions as Quintessence in String Theory

TL;DR

The authors construct a string-theoretic quintessence model where an axion, arising from the RR sector, acquires a linear potential through axion monodromy induced by NS5/anti-NS5 branes in highly warped throats. The model integrates KKLT-style moduli stabilization and SUSY breaking, showing that warping and brane backreaction can yield a slowly varying potential consistent with current cosmology, while predicting many ultra-light throat states and a possible rotation signal in CMB polarization. A key result is that the axion potential can remain linear and of the correct energy scale despite high SUSY-breaking scales, thanks to warp-factor suppressions and near-cancellation of problematic terms under a ${f Z}_2$ symmetry between throats. The framework offers observational handles via a potentially detectable evolution of the dark-energy equation of state and a limited, but interesting, CMB polarization rotation signal, while highlighting challenges such as the lack of tracker behavior and the anthropic treatment of the cosmological constant. Altogether, the work provides a UV-complete (string-theory) realization of quintessence with distinctive phenomenology tied to warped extra dimensions.

Abstract

We construct a model of quintessence in string theory based on the idea of axion monodromy as discussed by McAllister, Silverstein and Westphal arXiv:0808.0706. In the model, the quintessence field is an axion whose shift symmetry is broken by the presence of 5-branes which are placed in highly warped throats. This gives rise to a potential for the axion field which is slowly varying, even after incorporating the effects of moduli stabilization and supersymmetry breaking. We find that the resulting time dependence in the equation of state of Dark Energy is potentially detectable, depending on the initial conditions. The model has many very light extra particles which live in the highly warped throats, but these are hard to detect. A signal in the rotation of the CMB polarization can also possibly arise.

Figures (1)

• Figure 1: A schematic diagram of the internal space showing the two throats related by the ${\mathbb Z}_2$${\cal R}$ symmetry containing the NS5-brane and the $\overline{\rm NS 5}$-brane respectively.