  In a broad class of dark energy models, the universe may collapse within a
finite time t_c. Here we study a representative model of dark energy with a
linear potential, V(\phi)=V_0(1+\alpha\phi). This model is the simplest
doomsday model, in which the universe collapses rather quickly after it stops
expanding. Observational data from type Ia supernovae (SNe Ia), cosmic
microwave background anisotropy (CMB), and large scale structure (LSS) are
complementary in constraining dark energy models. Using the new SN Ia data
(Riess sample), the CMB data from WMAP, and the LSS data from 2dF, we find that
the collapse time of the universe is t_c > 42 (24) gigayears from today at 68%
(95%) confidence.
