We assume that dark matter and dark energy satisfy the unified equation of state: $p=B(z)\rho$, with $p=p_{ dE}$, $\rho=\rho_{ dm}+\rho_{ dE}$, where the pressure of dark matter $p_{ dm}=0$ has been taken into account. A special function $B=-\frac{ A}{ (1+z)^{ \alpha}}$ is presented, which can well describe the evolution of the universe. In this model, the universe will end up with a Big Rip. By further simple analysis, we know other choices of the function $B$ can also describe the universe but lead to a different doomsday.
Categories: astro-ph