Abstract
JHEP 0110:012,2001 The apparent observation of dark energy poses problems for string theory. In
de Sitter space, or in quintessence models, one cannot define a gauge-invariant
S-matrix. We argue that eternal quintessence does not arise in weakly coupled
string theory, but point out that it is difficult to define an $S$-matrix even
in the presence of perturbative potentials for the moduli. The solutions of the
Fischler-Susskind equations all have Big Bang or Big Crunch Singularities. We
believe that an S-matrix (or S-vector) exists in this context but cannot be
calculated by purely perturbative methods. We study the possibility of
metastable de Sitter vacua in such weakly coupled scenarios, and conclude that
the S-matrix of the extreme weak coupling region cannot probe de Sitter
physics.
We also consider proposed explanations of the dark energy from the
perspective of string theory, and find that most are implausible. We note that
it is possible that the axion constitutes both the dark matter and the dark
energy.