Cosmic Filament Spin from Dark Matter Vortices
  The recent observational evidence for cosmic filament spin on megaparsec
scales (Wang et al, Nature Astronomy 5, 839-845 (2021)) demands an explanation
in the physics of dark matter. Conventional collisionless cold particle dark
matter is conjectured to generate cosmic filament spin through tidal torquing,
but this explanation requires extrapolating from the quasi-linear regime to the
non-linear regime. Meanwhile no alternative explanation exists in the context
of ultra-light (e.g., axion) dark matter, and indeed these models would naively
predict zero spin for cosmic filaments. In this Letter we study cosmic filament
spin in theories of ultra-light dark matter, such as ultra-light axions, and
bosonic and fermionic condensates, such as superfluids and superconductors.
These models are distinguished from conventional particle dark matter models by
the possibility of dark matter vortices. We take a model agnostic approach, and
demonstrate that a collection of dark vortices can explain the data reported in
Wang et al. Modeling a collection of vortices with a simple two-parameter
analytic model, corresponding to an averaging of the velocity field, we find an
excellent fit to the data. We perform a Markov Chain Monte Carlo analysis and
find constraints on the number of vortices, the dark matter mass, and the
radius of the inner core region where the vortices are distributed, in order
for ultra-light dark matter to explain spinning cosmic filaments.
