Why do you need such accurate measurements of the shear power spectrum?
Outside of the huge and rare mass overdensities of rich clusters of galaxies, the mass contrast is much lower, giving rise to shear values below one percent. Here, in the vast volume between the rich clusters, is where most of the dark matter and dark energy of the universe exists. Current observations suggest that most of the energy density of the universe is in some unknown form. Weak gravitational lens “cosmic shear” as a function of distance will measure both the geometry of the universe and the growth rate of structure two related probes of the equation of state of this dark energy. It is necessary to measure the weak gravitational lens induced shear of billions of galaxies over a large area and control systematic shear errors at the 0.0001 level.
