  In order to depict the transition from deceleration to acceleration expansion
of the universe we use a power-law expansion scale factor, $a\sim
t^{n_0+bt^m}$, with $n_0$, $b$ and $m$ three parameters determined by $H_0$,
$q_0$ and $z_T$. For the spatially flat, isotropic and homogeneous universe,
such a scale factor leads to the results that the dark energy density is slowly
changing currently, and predicts the equation of state $w_X$ changes from
$w_X>-1$ to $w_X<-1$.
