  If the potential of a scalar field, Phi, which currently provides the ``dark
energy'' of the universe, has a negative minimum -M_0^4, then
quantum-mechanical fluctuations could nucleate a bubble of Phi at a negative
value of the potential. The bubble would expand at the speed of light. Given
that no such bubble enveloped us in the past, we find that any minimum in
V(Phi) must be separated from the current Phi value by more than min{1.5M_0,
0.21M_Pl}, where M_Pl is the Planck mass. We also show that vacuum decay
renders a cyclic or ekpyrotic universe with M_0^4 > 10^{-10}M_Pl^4, untenable.
