Breaking the axion abundance-isocurvature relation: Scale invariance and high-scale inflation

A QCD axion accounting for dark matter through pre-inflationary misalignment is strongly constrained by CMB isocurvature bounds, which place the classic axion window in tension with high-scale inflation. The tension arises because the same decay constant controls both the relic abundance and the primordial axion fluctuations. We show how this link can be broken in a broad class of scale-invariant models, without introducing any dimensionful parameter into the classical action. Rather than imposing a fixed Peccei-Quinn scale, the axion's kinetic normalisation is set by a dynamical field ratio. The resulting field-space metric controls the abundance through its value today and the primordial fluctuations through its value at horizon crossing. A sufficiently large metric during inflation suppresses isocurvature, allowing high-scale inflation in the classic axion window with an untuned misalignment angle and sub-Planckian decay constants. The exact scale and Peccei-Quinn symmetries also enforce a non-turning inflationary attractor, conserving the super-Hubble curvature perturbation and preserving the single-field adiabatic spectrum. We realise the mechanism in the Higgs-Dilaton model by deforming the Peccei-Quinn kinetic term. The same deformation turns the inflaton sector into an $α$-attractor pinned near the Starobinsky point. This explicit realisation has a super-Planckian present decay constant, which places it in the anthropic window, now reopened at high inflationary scales, with an ultralight QCD axion constrained by black-hole superradiance.

Preparing paper insights from the available abstract and paper details.

Authors

Mariaveronica De Angelis; Javier Rubio

Published

Oct 16, 2026

Paper type

preprint

Source

Arxiv