Determine the phase-transition wall Lorentz-factor parameter from simulations

Determine the phase-transition parameter \(\alpha\), defined through the bubble-wall Lorentz factor at bubble coalescence, from simulations of axion relic-pocket formation.

Background

The equilibrium radius, mass, and phenomenology of axion relic pockets depend on the parameter α\alpha, which characterizes the Lorentz factor of the phase-transition walls at bubble coalescence. The paper adopts the upper-bound value α≃1.6\alpha\simeq 1.6 for its phenomenological estimates, but does not derive this parameter dynamically. Numerical simulations of the phase transition are therefore required to determine its physically realized value.

References

We expect this number to be constrained by the inequality 0<\alpha\lesssim 1.6. In Sections \ref{sec:icecube}--\ref{sec:upward}, we assume that \alpha saturates the bound \alpha\simeq1.6 for simplicity, and leave the determination of this parameter from simulations to future work.

— The phenomenology of Axion Relic Pockets  (2609.24714 - Khater et al., 21 Sep 2026) in Section 2, subsection “The main scenario: weak axion-photon coupling”