Axion-Dilaton Destabilization and the Hubble Tension

TL;DR

Problem: The Hubble constant tension suggests new early-universe physics beyond $\Lambda$CDM. Approach: A string-inspired axion-dilaton system with a dilaton potential $V(\phi)=V_0 e^{-\lambda \phi/m_{pl}}$ and an axion potential $V(\chi)= m_\chi^2 f^2 \cos(\chi/f)$; early Hubble friction holds the fields, and when $H< m_\chi$ the axion oscillates and destabilizes the dilaton, triggering a fast-roll phase with $w=1$ and $\rho_\phi \propto a^{-6}$, realizing Early Dark Energy that dissipates near recombination. Key results: a simple coupling $V(\phi,\chi)$ yields Planck-scale decay constant for $m_\chi \sim 10^{-27}$ eV and a viable timing for EDE; a full perturbation analysis is necessary, though prior work supports effective-fluid descriptions. Significance: this provides a UV-mensible, moduli-driven route to EDE and helps address the Hubble tension, linking cosmological observations to string-theoretic moduli dynamics and swampland considerations.

Abstract

The discrepancy in measurements of the Hubble constant indicates new physics in dark energy, dark matter, or both. Drawing inspiration from string theory, where axions interact with the other moduli fields, including the dilaton, here we demonstrate that the dynamics of an interacting dilaton and axion naturally realizes the proposal of Early Dark Energy. In this setup, stabilization of the the dilaton is in part due to the axion, and in the early universe the dilaton contributes to dark energy. The combined axion-dilaton system is destabilized when the Hubble constant falls below the mass of the axion, triggering a phase of fast-roll evolution of the dilaton wherein its equation of state is $w=1$, and the early dark energy redshifts away as $a^{-6}$.

Figures (5)

• Figure 1: Evolution of the equation of state for a quintessence field with potential $V=V_0 e^{- \lambda \phi/m_{pl}}$, with $\lambda=400$ and initial conditions $x(0)=0$, $y(0)=1/\sqrt{10}$, in a background with equation of state $w_b=0$.
• Figure 2: Evolution of the energy density for a quintessence field with potential $V=V_0 e^{- \lambda \phi/m_{pl}}$, with $\lambda=400$ and initial conditions $x(0)=0$, $y(0)=1/\sqrt{10}$, as a fraction of the total energy density of the universe, in a background with equation of state $w_b=0$.
• Figure 3: Evolution of the fields in the coupled axion-dilaton system, with $\chi$ in units of $\pi f$ and $\phi$ in units of $m_{pl}$. The axion (black, solid) begins to oscillate at $t \sim 10^6$ triggering a destabilization and subsequent fast-roll of the dilaton (red, dashed).
• Figure 4: Redshift evolution of the dilaton equation of state in the coupled axion-dilaton system \ref{['V']}, in a matter dominated universe. Time flows right to left, with the present time given by $z=0$. Dashed lines are $w=-1$ and $w=+1$, corresponding to slow and fast roll respectively.
• Figure 5: Redshift evolution of the dilaton equation of state for the modified axion-dilaton coupling \ref{['betaphic']}, in a matter dominated universe. Time flows right to left, with the present time given by $z=0$. Dashed lines are $w=-1$ and $w=+1$, corresponding to slow and fast roll respectively.