Dust of Dark Energy

TL;DR

This paper proposes a new scalar field theory where energy flows along timelike geodesics with zero sound speed, unifying dark matter and dark energy, and reproducing Lambda-CDM evolution with potential observable deviations.

Contribution

It introduces a novel two-scalar field model with a Lagrange multiplier that unifies dark matter and dark energy, featuring no wave modes and zero sound speed.

Findings

Model reproduces Lambda-CDM expansion history

Deviations can lead to measurable structure formation differences

System has a single dynamical degree of freedom

Abstract

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.