Dust of Dark Energy
  We introduce a novel class of field theories where energy always flows along
timelike geodesics, mimicking in that respect dust, yet which possess non-zero
pressure. This theory comprises two scalar fields, one of which is a Lagrange
multiplier enforcing a constraint between the other's field value and
derivative. We show that this system possesses no wave-like modes but retains a
single dynamical degree of freedom. Thus, the sound speed is always identically
zero on all backgrounds. In particular, cosmological perturbations reproduce
the standard behaviour for hydrodynamics with vanishing sound speed. Using all
these properties we propose a model unifying Dark Matter and Dark Energy in a
single degree of freedom. In a certain limit this model exactly reproduces the
evolution history of Lambda-CDM, while deviations away from the standard
expansion history produce a potentially measurable difference in the evolution
of structure.
