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The (3+1) -dimensional evolution of an inhomogeneous axion field configuration around the QCD epoch is studied numerically, including important nonlinear effects due to the attractive self-interaction. It is found that axion perturbations on scales corresponding to causally disconnected regions at T1 GeV can lead to very dense pseudosoliton configurations we call axitons. These configurations evolve to axion miniclusters with a present density ₀10^-8 g cm^-3. This is high enough for the collisional 2a2a process to lead to Bose-Einstein relaxation in the gravitationally bound clumps of axions, forming Bose stars.
Kolb et al. (Sun,) studied this question.