Axion miniclusters

Abstract

The formation of cold dark matter in the form of coherent axion fields is expected to produce large-amplitude (δϱ≅) isocurvature fluctuations in axion density. These are later the first fluctuations to collapse, forming tiny bound “miniclusters” of axions which cluster in a gravitational hierarchy until the standard inflationary fluctuations become non-linear. The existence of these miniclusters can have observable consequences for gravitational lensing, may have a role in seeding the collapse of the first stars, and could affect the signal observed in axion-detection experiments.