The axion is a hypothetical pseudo-Goldstone boson, arising from the breaking of the Peccei-Quinn U(1)-symmetry, which allows to solve the strong CP-problem in QCD. It is also one of the most popular candidates for dark matter. To detect dark matter axions, it is very important to understand, how they exist in our universe at present. The axions produced in the early universe by a misalignment mechanism were non-relativistic and coherent. Later, a collection of axions could have condensed into a bound Bose-Einstein condensate, called an axion star. It is thus possible that a significant fraction of the axion dark matter exists in the form of axion stars. This would make some efforts to identify the dark matter axions more challenging, but it would also open new possibilities. In this talk, I will review the recent progress in the study of axion stars.
Peter Thirolf (LMU) / Norbert Kaiser (TUM)