Axion Miniclusters: Formation, Structure and Observational Signatures

Abstract

The axion is one of the leading candidates for cosmological dark matter. It arises as the pseudo-Goldstone boson of a spontaneously broken global U(1) symmetry. If this symmetry is broken after inflation, cosmic string topological defects form. The decay of these defects leads to the formation of large overdensities called axion minicluster seeds. These seeds then collapse under gravity and merge to form a very dense type of substructure called axion miniclusters. Due to their high density, it has been suggested that it may be possible to detect axion miniclusters using gravitational microlensing. This thesis attempts to shed light on this possibility by studying the formation of axion miniclusters to predict their structure today. This is done by applying a modified version of the Peak Patch excursion set formalism to compute the mass and size distribution of axion miniclusters from a realistic initial density field obtained from numerical simulations of axion string decay. These results are then used to identify the densest minicluster seeds. The fate of these dense seeds is determined by tracking the subsequent gravitational collapse in cosmological N-body simulations. Finally, modelling the final radial density profiles as either NFW or single power-law, the lensing capability of these objects is calculated.