Unified Dark Fluid with Constant Adiabatic Sound Speed and Cosmic Constraints

TL;DR

This paper investigates a unified dark fluid model with constant adiabatic sound speed, constrained by cosmic observations, suggesting very small sound speeds are favored to match data and potentially unify dark matter and dark energy.

Contribution

It introduces a unified dark fluid model with constant sound speed and constrains it using current cosmological data, highlighting the preference for very small sound speeds.

Findings

Small adiabatic sound speeds are favored by observational data.

Values of α less than 10^{-5} are necessary to match SDSS matter power spectrum.

The model provides a unified description of dark matter and dark energy.

Abstract

As is known above 90% of the energy content in Universe is made of unknown dark component. Usually this dark fluid is separated into two parts: dark matter and dark energy. However, it may be a mixture of these two energy components, or just one exotic unknown fluid. This property is dubbed as dark degeneracy. With this motivation, in this paper, a unified dark fluid having constant adiabatic sound speed $c_s^2=\alpha$, which is in the range $[0,1]$, is studied. At first, via the energy conservation equation, its energy density, $\rho_d/\rho_{d0}=(1-B_s)+B_s a^{-3(1+\alpha)}$ where $B_s$ is related to integration constant from energy conservation equation as another model parameter, is presented. Then by using Markov Chain Monte Carlo method with currently available cosmic observational data sets which include type Ia supernova Union 2, baryon acoustic oscillation and WMAP 7-year data of cosmic background radiation, we show that small values of $\alpha$ are favored in this unified dark fluid model. Furthermore, we show that smaller values of $\alpha<10^{-5}$ are required to match matter (baryon) power spectrum from SDSS DR7.