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arXiv · 2512.20290

A non-compact QCD axion

Georgios K. Karananas·Mikhail Shaposhnikov·Sebastian Zell
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Abstract

We investigate the cosmology of an axion that is fundamentally non-compact. During inflation, fluctuations of the effectively massless field populate many QCD vacua, thereby evading conventional isocurvature constraints while generating domain walls -- without accompanying cosmic strings. A small non-QCD contribution to the axion potential is required to trigger the timely collapse of domain walls; as a consequence, a residual amount of CP violation in the strong sector must exist, potentially within reach of planned experiments. Non-compact axions can account for the entirety of the dark matter abundance, and the collapse of domain walls sources a stochastic gravitational-wave background at nanohertz frequencies. Such axion dynamics can be embedded in top-down constructions -- such as Weyl-invariant Einstein-Cartan gravity -- where the tilting of the axion potential arises automatically.

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Georgios K. Karananas, Mikhail Shaposhnikov, Sebastian Zell. 2025-12-23. A non-compact QCD axion. https://arxiv.org/abs/2512.20290

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