  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.
