The number of mass clusters and their distribution in redshift are very sensitive to the density of matter Omega_m and the equation of state of dark energy w. Using weak lens gravitational tomography one can detect clusters of dark matter, weigh them, image their projected mass distribution, and determine their 3-D location. The degeneracy curve in the Omega_m - w plane is nearly orthogonal to that from CMB or SN measurements. Thus, a combination of CMB data with weak lens tomography of clusters can yield precision measurements of Omega_m and w, independently of the SN observations. The Large Synoptic Survey Telescope (LSST) will repeatedly survey 30,000 square degrees of the sky in multiple wavelengths. LSST will create a 3-D tomographic assay of mass overdensities back to half the age of the universe by measuring the shear and color-redshift of billions of high redshift galaxies. By simultaneously measuring several functions of cosmic shear and mass cluster abundance, LSST will provide a number of independent constraints on the dark energy density and the equation of state.
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