DUET - The Dark Universe Exploration Telescope
  DUET, a proposed NASA Midex class mission, will produce a catalog of ~20,000 X-ray selected clusters and groups of galaxies suitable for detailed cosmological investigations. We present the cosmological constraints achievable by DUET, the functional requirements of the survey, and briefly describe an instrument/mission design capable of achieving these requirements. The advantages of an X-ray survey for constraining the nature and amount of dark energy and dark matter are emphasized. Constraints on cosmological parameters are precise and complementary to other approaches. The density of dark energy, dark matter, and neutrinos will be measured with a statistical uncertainty of 0.5%, 0.5%, and 0.2% of the critical density and the dark energy equation of state can be measured to +/- 0.2 if dark energy and dark matter contribute ~2/3 and ~1/3 of the critical density as currently believed. DUET is a multi-national collaboration led by GSFC with substantial contributions from groups in the US, Europe, and Japan. 
Keith Jahoda (for the DUET collaboration)