The goal of this master thesis is to simulate the formation of axion miniclusters in the post-inflationary symmetry breaking scenario for QCD axion dark matter starting from realistic initial conditions. If the phase transition responsible for axion dark matter production occurs during the radiation-dominated epoch of the Universe, then this can produce large amplitude isocurvature perturbations that collapse into dense objects known as axion miniclusters. For this, initial condition of the size below one parsec was evolved, starting deep inside the radiation domination epoch, up until z = 1000. Initial particle setup, as well as, the Friedman equations for the radiation-domination were implemented in the cosmological code GADGET -2.
Miniclusters start to form early in the radiation-dominated epoch of the Universe, and around the matter-radiation equality, they start to merge. At the end of simulation, at z = 1000, about 55% of the mass, inside the simulation box, has collapsed into miniclusters. Power spectra and halo mass functions have been calculated as well. The halo profile was fitted with self-similar scale-free power-law profile, instead of the commonly accepted Navarro-Frenk-White (NFW), which is valid for the matter radiation epoch. With time, the profiles become steeper. At z = 1000 the distribution for the parameter in the power-law fit peaks at 2.7.