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[part 2] > superfluid DM proposed Justin Khoury's version is a Bose-Einstein condensate (BEC) of axion-like particles. The BEC forms a halo and is fluid, like a gas, in most of the halo. In the halo's central regions the BEC undergoes a phase change and becomes superfluid, and in that region interactions with baryons are produced which bubble up to the thin nonsuperfluid phase, where they interact with baryons again as a "fifth force" where one would want to understand that as viscosity. Viscosity vanishes in a superfluid. Here the idea is that the superfluid phase does not interact much with baryons except to generate vortex-like ring-perturbations which rise (and I think magnify) from the core, where the sticky-viscosity of the non-superfluid phase deposits energy into baryons, with a bias driven by galaxy's bulk angular momentum (the kicks being more in the direction of rotation and radially outward, generating the flat velocity curve). It's very much a particle dark matter theory, and a somewhat complicated one. It shows that a more than one particle dark matter theory can reproduce empirical results from MOND. And yes, as with any field theory, one would want to take a Hamiltonian approach and consider the dynamic canonical variables (x, p) [position, momentum], so you will need to specify all (x, p) at some time t, and because that's intractable, one coarse-grains. Axions have yet to be directly detected in astronomical or laboratory settings, and nobody knows how they behave at ultracold temperatures and ultrasparse densities. Do they even BEC? However, if they did exist and have the properties to form a Bose gas etc etc, and their cosmological participation isn't forbidden by higher-energy completions of the standard model, then there is a nice property here: it's just relativistic quantum fields on curved spacetime. Relativity (specifically in the sense that we can relate physical coincidences in arbitrary systems of coordinates) is baked in from the start. And you would get the empirical MOND relation in galaxy dynamics. But it's certianly not a MONDian theory in spirit. There is no modified gravity to be seen here. |
There is no such thing as "purity" in physics. The whole reason most people work on MOND is because it provides a simple effective theory for not only describing observations with fewer parameters than cold dark matter, but predicting them more effectively. Every MONDian paper I've read has effectively been saying, "stop ignoring this obvious relation in preference to unnecessarily complicated and ad-hoc DM that provides no predictive power". If a superfluid or other dark matter theory derived MONDian dynamics, then pretty much everyone would be happy with that.