Theory of Dark Matter Superfluidity
  We propose a novel theory of dark matter (DM) superfluidity that matches the
successes of the LambdaCDM model on cosmological scales while simultaneously
reproducing the MOdified Newtonian Dynamics (MOND) phenomenology on galactic
scales. The DM and MOND components have a common origin, representing different
phases of a single underlying substance. DM consists of axion-like particles
with mass of order eV and strong self-interactions. The condensate has a
polytropic equation of state P~rho^3 giving rise to a superfluid core within
galaxies. Instead of behaving as individual collisionless particles, the DM
superfluid is more aptly described as collective excitations. Superfluid
phonons, in particular, are assumed to be governed by a MOND-like effective
action and mediate a MONDian acceleration between baryonic matter particles.
Our framework naturally distinguishes between galaxies (where MOND is
successful) and galaxy clusters (where MOND is not): due to the higher velocity
dispersion in clusters, and correspondingly higher temperature, the DM in
clusters is either in a mixture of superfluid and normal phase, or fully in the
normal phase. The rich and well-studied physics of superfluidity leads to a
number of observational signatures: array of low-density vortices in galaxies,
merger dynamics that depend on the infall velocity vs phonon sound speed;
distinct mass peaks in bullet-like cluster mergers, corresponding to superfluid
and normal components; interference patters in super-critical mergers.
Remarkably, the superfluid phonon effective theory is strikingly similar to
that of the unitary Fermi gas, which has attracted much excitement in the cold
atom community in recent years. The critical temperature for DM superfluidity
is of order mK, comparable to known cold atom Bose-Einstein condensates.
