A New Unified Dark Fluid Model and Its Cosmic Constraint

In this paper, we propose a new unified dark fluid (UDF) model with equation of state (EoS) w(a) = -alpha/(beta a(-n) + 1), which includes the generalized Chaplygin gas model (gGg) as its special case, where alpha, beta and n are three positive numbers. It is clear that this model reduces to the gCg model with EoS w(a) = -B-s/(B-s + (1 - B-s)a(-3(1+ a))), when alpha = 1, beta = (1 -B-s) / B-s and n = 3(1 + alpha). By combination the cold dark matter and the cosmological constant, one can coin a EoS of unified dark fluid in the form of w(a) = -1/(1 + (1 - Omega(Lambda))a(-3)/Omega(Lambda)). With this observations, our proposed EoS provides a possible deviation from Lambda CDM model when the model parameters alpha and n deviate from 1 and 3 respectively. By using the currently available cosmic observations from type Ia supernovae (SN Ia) Union2.1, baryon acoustic oscillation (BAO) and cosmic microwave background radiation (CMB), we test the viability of this model and detect the possible devotion from the Lambda CDM model. The results show that the new UDF model fits the cosmic observation as well as that of the Lambda CDM model and no deviation is found from the Lambda CDM model in 3s confidence level. However, our new UDF model can give a non-zero sound speed, as a contrast, which is zero for the Lambda CDM model. We expect the large structure formation information can distinct the new UDF model from the Lambda CDM model.