An X-ray Galaxy Cluster Survey for Investigations of Dark Energy
  The amount and nature of dark energy (DE) can be tightly constrained by measuring the spatial correlation features and evolution of a sample of ~ 100,000 galaxy clusters over the redshift range 0<z < 1.5. Such an X-ray survey will discover all collapsed structures with mass above 3.5e14 solar masss at redshifts z<2 (i.e. the full range where such objects are expected) in the high Galactic latitude sky. Above this mass threshold the tight correlations between X-ray observables and mass allow direct interpretation of the data. We describe the constraints on Dark Energy that can be inferred from such a survey, using powerful self-calibration techniques to relate the X-ray observables (luminosity and temperature) to the underlying mass. 
Z. Haiman, S. Allen, N. Bahcall, M. Bautz, H. Boehringer, S. Borgani,   G. Bryan, B. Cabrera, C. Canizares, O. Citterio, A. ...