The Serendipitous Axiodilaton: A Self-Consistent Recombination-Era Solution to the Hubble Tension

TL;DR

This work investigates a minimal axio-dilaton cosmology that links dark energy, dark matter, and time-varying particle masses through a two-field scalar sector. By implementing the model in CLASS with HyRec and confronting it with Planck, SPT-3G, DESI BAO, Pantheon+, and SH0ES data, the authors show that the framework can reduce the Hubble tension to below 3σ, while predicting correlated variations in the electron mass around recombination. The analysis reveals that cosmology prefers nonzero dilaton–matter couplings of order $g\sim10^{-2}$–$10^{-1}$, which today would clash with solar-system tests unless screening or environmental effects are invoked. Compared with phenomenological $\Lambda$CDM+$m_e$ and CPL extensions, the axio-dilaton model offers a more predictive, dynamically consistent account of early- and late-time observations, with SH0ES priors pushing the preferred region toward higher $H_0$ values that remain compatible with the CMB and BAO within the model’s constrained structure.

Abstract

Axio-dilaton cosmology provides a minimal benchmark model for both Dark Matter (DM) and Dark Energy (DE) that is well motivated by fundamental physics. The axion and dilaton arise as pseudo-Goldstone modes of symmetries that predict particle masses depend on the dilaton, and therefore to evolve cosmologically, leading to correlated modifications of recombination physics, the sound horizon, and late-time expansion and growth histories. We confront this model with Planck 2018 temperature, polarisation, and lensing data, SPT-3G high-$\ell$ measurements, DESI DR2 BAO, and Pantheon$+$ supernovae, assuming that the axion makes up all of the dark matter and that the dilaton plays the role of a dark energy field. We find that it fits the data somewhat better than $Λ$CDM cosmology, with the $χ^2$ lowered by $\simeq 7$ for three additional parameters, and significantly raises the inferred Hubble constant to $H_0 \simeq 69.2\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$, reducing the Hubble tension to $\lesssim 3σ$ and thereby allowing a joint fit of CMB and SH0ES data. The model fits this enlarged data set as well as the $w_0w_a$ model with an electron mass modified by hand at recombination, though it does so with calculable dynamics. Axio-dilaton self-interactions robustly fake a phantom equation of state in DESI measurements. There is a sting: cosmology prefers dilaton-matter couplings $|\mathbf{g}|\sim 10^{-2}$-$10^{-1}$, which are large enough to have been detected in solar-system tests of General Relativity. These results show how axio-dilatons can provide a viable cosmology preferred by current data at surprisingly large couplings, within a framework that links dark energy, dark matter, and time-dependent particle masses in a coherent way. They suggest both new observable signals and new theoretical directions, aimed at resolving the apparent inconsistency with non-cosmological observations.

Figures (11)

• Figure 1: CMB angular power spectra for different initial displacements of the dilaton field in the Yoga model variants. The quantity $\chi_i - \chi_{\rm min}$ parametrises the initial distance from the local minimum of the Albrecht--Skordis dilaton potential, given by \ref{['eq:albrecht-skordis']}. We fix the dilaton--matter coupling to $\mathbf{g} = -0.3$, the axion--dilaton kinetic coupling to $\zeta = 0.1$, and set the six standard $\Lambda$CDM cosmological parameters to their Planck 2018 best-fit values Planck:2018vyg.
• Figure 2: Triangle plot comparing the minimal Yoga model with fixed initial field values and varying initial conditions (VI). Both analyses use the same CMB-A DESI PP dataset combination. The two setups exhibit the same correlation directions between $\mathbf{g}$ and $\zeta$, but differ in the correlation with $H_0$, with large $|\mathbf{g}|$ (i.e., values far from zero, either positive or negative) correlating with large $H_0$ only in the VI case.
• Figure 3: Triangle plot comparing the standard cosmological parameters of $\Lambda$CDM and $\Lambda$CDM+$m_e$ with the minimal Yoga model. All cases use the same CMB-A DESI PP dataset combination.
• Figure 4: Best-fit background evolution in different model variants using the CMB-A DESI and PP datasets (calibrated with the SH0ES prior where stated). Top left: Fractional energy densities $\Omega_i(a)$ for the Yoga-VI model in solid and with the SH0ES prior in dashed. Top right: Dilaton field evolution $\chi(a)$. Bottom left: Effective equation of state $w_{\chi}^{\mathrm{eff}}(z)$ showing deviations from $-1$ and damped late-time oscillations in the Yoga cases. Bottom right: Relative electron and other Standard Model particle mass variation $m/m_{0}$; the PPSH0ES fit exhibits the largest excursion before relaxing at late times.
• Figure 5: Comparison of the main common parameter constraints for the Exponential and Yoga axio-dilaton models.