## Vortices and waves in light dark matter.(English)Zbl 1486.83075

The article is of evident interest for a very actual area of dark matter research and connection of particle physics with astrophysics and cosmology. The authors argue, that in galactic haloes like the Milky Way, bosonic dark matter particles lighter than 100 eV have a de Broglie wavelength larger than the average inter-particle separation and are therefore well described as a set of classical waves. This applies for example to the QCD axion as well as to lighter axion-like particles as fuzzy dark matter. The authors show that the inerference of waves inside a halo leads to vortices. They became sites where the fluid velocity has a non- vanishing curl. Using analytic methods and numerical simulations the authors proove that 1. In 3 spatial dimensions the generic defects take the form of vortex rings. 2. On average there is about one vortex ring per de Broglie volum. 3. Generically only single winding $$( +/- 1)$$ vortices are found in a realistic halo. 4. The density near a vortex scales goes as $$r^2$$ while the velocity is going as $$1/r$$, where r is the distance to vortex. The authors discusse observational/experimental signatures from vortices and wave interference. In the ultralight regime gravitational lensing by interference substructure lead to flux anomalies of 5–10 % in strongly lensed systems. For QCD axions vortices lead to a diminished signal in some detection experiments but not in others. The authors suggest the measurement of correlation functions by axion detection experiments as a way to probe on the expected interference substructures. The article is well structured, argued and referenced.

### MSC:

 83C56 Dark matter and dark energy 83F05 Relativistic cosmology 35C07 Traveling wave solutions 81V25 Other elementary particle theory in quantum theory 83C55 Macroscopic interaction of the gravitational field with matter (hydrodynamics, etc.) 76B47 Vortex flows for incompressible inviscid fluids