| They're pretty much entirely unrelated. In http://arxiv.org/abs/1507.01019 the authors lean on research into axions as extensions to the Standard Model and on Bose Einstein Condensate research in ordinary matter, and proposes a temperature (~ 1 mK) below which the light axion-like gas condenses into a superfluid. It's firmly rooted in General Relativity and is proposed as an ordinary matter field in the GR sense. The field content feels and generates curvature just like all the fields in the Standard Model. Its non-removable vacuum background is the standard four-dimensional smooth connected Lorentzian manifold. Superfluid vacuum theory (SVT) is a family of emergent gravity theories that aims to replace General Relativity by proposing that a superfluid with lattice spacing at Planck scale is part of the non-removable background and the dynamics of gravitation take place in the superfluid. At low temperatures, and at low particle energies, the superfluid reproduces GR. This approach to emergent gravity was tried with non-superfluid fluids historically, and those attempts failed in the face of detailed study of objects in our sky that are bright in extreme UV and higher frequencies. Emergent gravity theories tend to get more complicated (e.g., SVT theories have variously introduced mechanisms to suppress dissipation effects, and introduced rotation and/or phase transitions in the background superfluid) when they have to match recent successes of General Relativity. When you have to keep making substantial changes to your theory to match observables predicted using GR a century ago, it's a bad sign. |