Constraint on the generalized Chaplygin gas as an unified dark fluid model after Planck 2015

The generalized Chaplygin gas could be considered as the unified dark fluid model because it might describe the past decelerating matter dominated era and at present time it provides an accelerating expansion of the Universe. In this paper, we employed the Planck 2015 cosmic microwave background anisotropy, type-Ia supernovae, observed Hubble parameter data sets to measure the full parameter space of the generalized Chaplygin gas as an unified dark matter and dark energy model. The model parameters $B_s$ and $\alpha$ determine the evolutional history of this unified dark fluid model by influencing the energy density ${\rho }_{GCG}={\rho }_{GCG0}{[B_s+(1-B_s)a^{-3(1+\alpha )}]}^{1∕(1+\alpha )}$. We assume the pure adiabatic perturbation of unified generalized Chaplygin gas. In the light of Markov Chain Monte Carlo method, we found that $B_s=0.759_{-0.032-0.046}^{+0.020+0.051}$ and $\alpha =0.0801_{-0.0801-0.0801}^{+0.0208+0.1087}$ at $2\sigma$ level. The model parameter $\alpha$ is very close to zero, the nature of GCG model is very similar to cosmological standard model $\Lambda$CDM.