Constraining the dark fluid

TL;DR

This paper uses current cosmological data to directly constrain a unified dark fluid, demonstrating that it cannot be described by a single constant equation of state and supporting the existence of separate dark matter and dark energy components.

Contribution

It provides a model-independent analysis constraining the dark fluid's equation of state using observational data, covering various parameterizations and scenarios.

Findings

Data cannot be fit by a dark fluid with a single constant equation of state.

Results support the standard model with separate dark matter and dark energy.

Constraints are obtained using polynomial parameterizations of the dark fluid equation of state.

Abstract

Cosmological observations are normally fit under the assumption that the dark sector can be decomposed into dark matter and dark energy components. However, as long as the probes remain purely gravitational, there is no unique decomposition and observations can only constrain a single dark fluid; this is known as the dark degeneracy. We use observations to directly constrain this dark fluid in a model-independent way, demonstrating in particular that the data cannot be fit by a dark fluid with a single constant equation of state. Parameterizing the dark fluid equation of state by a variety of polynomials in the scale factor $a$, we use current kinematical data to constrain the parameters. While the simplest interpretation of the dark fluid remains that it is comprised of separate dark matter and cosmological constant contributions, our results cover other model types including unified dark energy/matter scenarios.