Develop the cosmology of strongly coupled axion relic pockets

Develop the cosmological formation and evolution theory of axion relic pockets in scenarios where the axions are sufficiently strongly coupled to photons to co-thermalise with the Standard Model plasma.

Background

The main analysis assumes that axions do not co-thermalise with the Standard Model plasma. For larger axion-photon couplings, radiation pressure can delay or prevent dilaton-bubble expansion, while subsequent thermal decoupling can produce shock fronts and pockets whose properties depend on the decoupling time. The paper outlines these effects but leaves the corresponding cosmological formation scenario unresolved.

References

For sufficiently strong couplings, the axions co-thermalise with the Standard Model in the early universe, which significantly modifies the scenario of . In this paper, we focus on the original scenario of , and leave cosmological extensions for future work.

— The phenomenology of Axion Relic Pockets  (2609.24714 - Khater et al., 21 Sep 2026) in Section 1 and Section 2, subsection “Axions strongly coupled to photons”

However, the number of axions in such small pockets is {\cal O}(1), and classically, one would expect such regions to collapse. Two effects can alter this conclusion: first, consistently characterising such small pockets calls for a full quantum treatment; second, the high curvature of the pockets walls leads to non-thermal axion production inside the pockets, which may contribute to stabilising the pockets. We postpone a detailed investigation of these possibilities for future work.

— The phenomenology of Axion Relic Pockets  (2609.24714 - Khater et al., 21 Sep 2026) in Section 2, subsection “Axions strongly coupled to photons”

This leads to complicated phase transition dynamics involving axion shock fronts developing outside the bubbles that may fully or partially thermalise. In that case, stable pockets can form, but their macroscopic properties, like their mass or radius, depend on g_{a\gamma\gamma} through the decoupling time, t_d, in addition to the phase transition properties. We expect the resulting pockets to be significantly smaller than the Hubble radius at t_*. This broadens the range of phenomenologically interesting ARP models, but we leave a detailed exploration of co-thermalised models for future work.

— The phenomenology of Axion Relic Pockets  (2609.24714 - Khater et al., 21 Sep 2026) in Section 2, subsection “Axions strongly coupled to photons”

We leave the investigation of the general ARP quantum field theory, including also interactions to other fields, to future work.

— The phenomenology of Axion Relic Pockets  (2609.24714 - Khater et al., 21 Sep 2026) in Section 4, subsection “Quantum Theory”