  The recent observations of type Ia supernovae strongly support that the
universe is accelerating now and decelerated in the recent past. This may be
the evidence of the breakdown of the standard Friedmann equation. Instead of a
linear function of the matter density, we consider a general function of the
matter density to modify the Freidmann equation. We propose a new model which
explains the recent acceleration and the past deceleration. Furthermore, the
new model also gives a decelerated universe in the future. The new model gives
$\Omega_{m0}=0.46$ and $z_T=0.44$.
