A Global View on The Search for de-Sitter Vacua in (type IIA) String
  Theory
  The search for classically stable Type IIA de-Sitter vacua typically starts
with an ansatz that gives Anti-de-Sitter supersymmetric vacua and then raises
the cosmological constant by modifying the compactification. As one raises the
cosmological constant, the couplings typically destabilize the classically
stable vacuum, so the probability that this approach will lead to a classically
stable de-Sitter vacuum is Gaussianly suppressed. This suggests that
classically stable de-Sitter vacua in string theory (at least in the Type IIA
region), especially those with relatively high cosmological constants, are very
rare. The probability that a typical de-Sitter extremum is classically stable
(i.e., tachyon-free) is argued to be Gaussianly suppressed as a function of the
number of moduli.
