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 $w(a)=-B_s/(B_s+(1-B_s)a^{-3(1+\alpha)})$, 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
$3\sigma$ 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.
