Dust of dark energy

Abstract

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