Axionic dark energy and a composite QCD axion
  We discuss the idea that the model-independent (MI) axion of string theory is
the source of quintessential dark energy. The scenario is completed with a
composite QCD axion from hidden sector squark condensation that could serve as
dark matter candidate. The mechanism relies on the fact that the hidden sector
anomaly contribution to the composite axion is much smaller than the QCD
anomaly term. This intuitively surprising scenario is based on the fact that
below the hidden sector scale $\Lambda_h$ there are many light hidden sector
quarks. Simply, by counting engineering dimensions the hidden sector instanton
potential can be made negligible compared to the QCD anomaly term.
