Nonlinear axion dynamics and the formation of cosmological pseudosolitons

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 T~1 GeV can lead to very dense pseudosoliton configurations we call axitons. These configurations evolve to axion miniclusters with a present density ρₐ10^-8 g cm^-3. This is high enough for the collisional 2a→2a process to lead to Bose-Einstein relaxation in the gravitationally bound clumps of axions, forming Bose stars.