A Quintessential Axion
  The model independent axion of string theory has a decay constant of order of the Planck scale. We explore the properties of this quintessence candidate (quintaxion) in the scheme of hidden sector supergravity breakdown. In models allowing for a reasonable $\mu$ term, the hidden sector dynamics may lead to an almost flat potential responsible for the vacuum energy of $(0.003 {\rm eV})^4$. A solution to the strong CP-problem is provided by an additional hidden sector pseudoscalar (QCD axion) with properties that make it an acceptable candidate for cold dark matter of the universe. 
Jihn E. Kim, Hans Peter Nilles (Bonn University)