Spiralling into Dark Energy: Axio-Dilaton Attractors in Light of Current Data

Abstract

In string compactifications, dilaton-like moduli always come paired with axions, and the two fields interact through a curved field-space metric. We study the cosmology of such an axio-dilaton system, in which the dilaton rolls down an exponential potential corrected by a polynomial prefactor and couples to a massless axion through the kinetic function $W=e^{-ξϕ_1/M_p}$. A dynamical systems analysis shows that the late-time attractor follows a non-geodesic trajectory in field space with equation of state $ω_ϕ=(λ-2ξ)/(λ+2ξ)$, so accelerated expansion is possible for steep potentials provided $ξ\ggλ$. No physical fixed point is phantom. A Bayesian analysis with DESI DR2 BAO, binned Pantheon supernovae and Planck 2018 distance priors gives $λ\gtrsim7$--$9$ and $\log_{10}ξ\gtrsim2.5$ at 95% confidence, together with a degeneracy between the two parameters. The model is slightly favoured over $Λ$CDM by current data, while the different polynomial corrections cannot yet be distinguished. This contribution is based on arXiv:2609.22539.