Vortices and waves in light dark matter

Abstract

In a galactic halo like the Milky Way, bosonic dark matter particles lighter than about 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 to, for instance, the QCD axion as well as to lighter axion-like particles such as fuzzy dark matter. We show that the interference of waves inside a halo inevitably leads to vortices, locations where chance destructive interference takes the density to zero. The phase of the wavefunction has nontrivial winding around these points. This can be interpreted as a non-zero velocity circulation, so that vortices are sites where the fluid velocity has a non-vanishing curl. Using analytic arguments and numerical simulations, we study the properties of vortices and show they have a number of universal features: (1) In three spatial dimensions, the generic defects take the form of vortex rings. (2) On average there is about one vortex ring per de Broglie volume and (3) generically only single winding (±1) vortices are found in a realistic halo. (4) The density near a vortex scales as r 2 while the velocity goes as 1/r, where r is the distance to vortex.(5) A vortex segment moves at a velocity inversely proportional to its curvature scale so that smaller vortex rings move faster, allowing momentary motion exceeding escape velocity. We discuss observational/experimental signatures from vortices and, more broadly, wave interference. In the ultra-light regime, gravitational lensing by interference substructures leads 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. We advocate the measurement of correlation functions by axion detection experiments as a way to probe and capitalize on the expected interference substructures.

Beyond Schrödinger-Poisson: nonrelativistic effective field theory for scalar dark matter

“…Since the mass of such axion-like fields is typically taken to be m a 10 −5 eV, the effects of wave dynamics can occur on macroscopic or even astrophysical scales, giving rise to the possibility of distinguishing such dark matter from other dark matter candidates. Novel dynamics such as the formation of solitons [13][14][15][16], interference patterns [14], transient vortices [17], and suppression of power below the de Broglie length-scale of the particles [8,18,19] is quite generic in axion-like dark matter. For recent reviews, see refs.…”

Ultra-light dark matter

“…During the final preparation of this review a paper studying the possibility of the formation of vortices in gravitationally bounded BECs appeared (Hui et al 2020). In this paper, they study the possibility of formation of a vortices in the FDM model and of other topological defects with different topologies.…”

“…There is a small amount of studies in the literature that investigate the presence of vortices (Rindler-Daller and Shapiro 2012; Zinner 2011; Kain and Ling 2010) (and recently Hui et al 2020). Given that to investigate vortices one needs to study the Schrödinger-Poisson equations, the main model where those were studied is the simplest superfluid model given by the SIFDM model.…”

“…However, to fully study the presence and consequences of a rotating halo in those models one needs to perform wave simulations. One study that takes that in consideration is Hui et al (2020).…”

“…However, some authors claim this equivalence is not so clear. Some authors also claim that since the quantum pressure tensor has non-diagonal components, and it is not isotropic, so it cannot be called pressure(Hui et al 2020).…”