Hidden sector axions: physics and cosmology

Embedding hidden sector supergravity models in the framework of string theory leads to the appearance of axion-like degrees of freedom. Among them is the model independent axion of heterotic string theory. It has a decay constant of order of the Planck scale and could play the role of a quintessence field. In models allowing for the required $\mu$ term in the TeV range, the hidden sector dynamics leads to a vacuum energy of $(0.003 {\rm eV})^4$ via a multiple see-saw effect. A solution to the strong CP-problem is provided by an additional hidden sector pseudoscalar with properties that make it an acceptable candidate for cold dark matter of the universe.