Modified Chaplygin Gas as a Unified Dark Matter and Dark Energy Model
  and Cosmic Constraints
  A modified Chaplygin gas model (MCG),
$\rho_{MCG}/\rho_{MCG0}=[B_{s}+(1-B_{s})a^{-3(1+B)(1+\alpha)}]^{1/(1+\alpha)}$,
as a unified dark matter model and dark energy model is constrained by using
current available cosmic observational data points which include type Ia
supernovae, baryon acoustic oscillation and the seventh year full WMAP data
points. As a contrast to the consideration in the literatures, we {\it do not}
separate the MCG into two components, i.e. dark mater and dark energy
component, but we take it as a whole energy component-a unified dark sector. By
using Markov Chain Monte Carlo method, a tight constraint is obtained: $\alpha=
0.000727_{- 0.00140- 0.00234}^{+ 0.00142+ 0.00391}$, $B=0.000777_{- 0.000302-
0.000697}^{+ 0.000201+ 0.000915}$ and $B_s= 0.782_{- 0.0162- 0.0329}^{+ 0.0163+
0.0307}$ .}
