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 .