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arXiv · 2607.09969

Suppressing Extra-Dimensional Axion Isocurvature Dynamically

Gongjun Choi·Tony Gherghetta
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Abstract

Extra-dimensional QCD axion is well motivated by string compactifications and enjoys enhanced protection against quality-violating effects. If present during inflation, however, its quantum fluctuations generate isocurvature perturbations that strongly constrain the inflationary scale. We propose a dynamical suppression mechanism in warped five-dimensional models, where a radion-inflaton coupling sets the radion minimum at small inter-brane separation during inflation, temporarily enhancing the effective four-dimensional axion decay constant. After inflation, the radion minimum shifts to larger separation, restoring the standard QCD axion window. In a warped orbifold GUT with Goldberger-Wise stabilization, this mechanism can satisfy CMB isocurvature bounds while allowing substantially higher inflationary scales than in conventional pre-inflationary axion cosmology.

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Gongjun Choi, Tony Gherghetta. 2026-07-10. Suppressing Extra-Dimensional Axion Isocurvature Dynamically. https://arxiv.org/abs/2607.09969

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