Unified dark fluid with constant adiabatic sound speed and cosmic constraints

As is known, more than 90% of the energy content in the 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 the 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.