Measuring the Growth of Structure with Spectroscopically Identified   Groups and Clusters
  Number counts of galaxy clusters offer a very promising probe of the Dark Energy (DE) equation-of-state parameter, $w$. The basic goal is to measure abundances of these objects as a function of redshift, compare this to a theoretical prediction, and infer the values of cosmological parameters. Various teams have proposed such a measurement, including the South Pole Telescope, the Dark Energy Survey and the Red-Sequence Cluster Survey. The specific study discussed here detects clusters and smaller galaxy groups in the three-dimensional distribution of galaxies inferred from a large spectroscopic redshift survey. This method allows the abundance, $N$, of groups and clusters to be measured as a function of \emph{velocity dispersion}, as well as of redshift, permitting a more sensitive test of cosmology. 
M. Davis, B. F. Gerke, A. L. Coil, M. C. Cooper, R. Yan, J. A. Newman,   S. M. Faber, D. Koo, P. Guhathakurta