The standard interpretation of dark energy is as a cosmological constant. This implies a late-time de Sitter universe, which presents difficulties for quantum gravity theories, string/M-theory in particular. This talk will review some of these difficulties, and some past and recent attempts to circumvent them via time-dependent compactifications of the 10 or 11 dimensional effective supergravity theories of string/M-theory; the main advantage appears to be the possibility of recurrent cosmic acceleration in which the universe cycles through several de Sitter phases before approaching a late-time decelerating solution.
INFN & DFA
