Part I: Discussion of recent papers
Part II: Blackboard presentation by Bob McElrath:
We show that any cosmological relic with small couplings to itself
and baryons becomes a superfluid shortly after decoupling, due to
the broadening of its wave packet, and lack of any elastic
scattering. The dynamics of a superfluid are given by the
excitation spectrum of bound state quasi-particles, rather than the
center of mass motion of constituent particles. If this relic is a
fermion with a repulsive interaction mediated by a heavy boson, such
as the $Z^0$ with neutrinos or dark matter, a theory of gravity
emerges spontaneously. In this theory the vierbein is a Goldstone
boson associated with the spontaneous breaking of Lorentz invariance
due to the presence of a physical background superfluid.