Unified Dark Fluid with Constant Adiabatic Sound Speed: Including Entropic Perturbations

TL;DR

This paper investigates a unified dark fluid model incorporating entropic perturbations, constraining its parameters using cosmic observational data to understand its micro-scale properties and effects on cosmic evolution.

Contribution

It introduces a model with entropic perturbations and effective sound speed as free parameters, and constrains these using current cosmic data sets.

Findings

Effective sound speed is constrained to a small value around 0.00155.

Cosmic observations favor models with small effective sound speeds.

The model links the equation of state to adiabatic sound speed through a specific relation.

Abstract

In this paper, we continue to study a unified dark fluid model with a constant adiabatic sound speed but with the entropic perturbations. When the entropic perturbations are included, an effective sound speed, which reduces to the adiabatic sound speed when the entropic perturbations are zero, has to be specified as an additional free model parameter. Due to the relations between the adiabatic sound speed and equations of state (EoS) $c^2_{s,ad}(a)=w(a)-d\ln(1+w(a))/3 d\ln a$, the equation of state can be determined up to an integration constant in principle when an adiabatic sound speed is given. Then there are two degrees of freedom to describe the linear perturbations for a fluid. Its micro-scale properties are characterized by its EoS or adiabatic sound speed and an effective sound speed. We take the effective sound speed and adiabatic sound speed as free model parameters and then use the currently available cosmic observational data sets, which include type Ia supernova Union 2.1, baryon acoustic oscillation and WMAP 7-year data of cosmic background radiation, to constrain the possible entropic perturbations and the adiabatic sound speed via the Markov Chain Monte Carlo method. The results show that the cosmic observations favor a small effective sound speed $c^2_{s,eff}=0.00155_{- 0.00155}^{+ 0.000319}$ in $1\sigma$ region.