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 approximately 1 GeV can lead to very dense pseudosoliton configurations we call axitons. These configurations evolve to axion miniclusters with a present density rho(a) greater than or similar to 10(-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.