  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.
