Weak lensing tomography is becoming a competitive method of constraining cosmology and dark energy. Tomography, as opposed to cosmic shear, concerns itself with the change in lensing shear with the redshift of the source (background) galaxies. This becomes particularly powerful when used on fields containing massive galaxy clusters whose mass can be constrained in multiple independent ways. The DAFT/FADA survey led by David Johnston, Jim Annis, Mel Ulmer & Doug Clowe, combines Hubble Space Telescope data with ground-based, multi-band imaging to image the fields around massive galaxy clusters and detect this tomography signal. The plot shows the expected shear strength versus source redshift for various cosmologies. Surveys such as the Dark Energy Survey will go further and be able to use this method as an independent and purely geometrical probe of dark energy.
– David Johnston