Probing dark energy with the shear-ratio geometric test
  We adapt the Jain--Taylor (2003) shear-ratio geometric lensing method to measure the dark energy equation of state, and its time derivative from dark matter haloes in cosmologies with arbitrary spatial curvature. The full shear-ratio covariance matrix is calculated for lensed sources, including the intervening large-scale structure and photometric redshift errors as additional sources of noise, and a maximum likelihood method for applying the test is presented. Combining with the expected results from the CMB we design an optimal survey for probing dark energy. A targeted survey imaging 60 of the largest clusters in a hemisphere with 5-band optical photometric redshifts to a median galaxy depth of zm=0.9 could measure w0 to a marginal 1-sigma error of $\Delta$w0=0.5. We marginalize over all other parameters including wa, where the equation of state is parameterized in terms of scale factor a as w(a)=w0+wa(1-a). For higher accuracy a large-scale photometric redshift survey is required. Such a near-future 5-band survey covering 10,000 square degrees to z_m=0.7 could measure w0 to $\Delta$w0=0.075 and $\Delta$wa=0.33. A stronger combined constraint is measured at a pivot redshift zp=0.27 of $\Delta$w(zp)=0.0298. We compare and combine the geometric test with the cosmological and dark energy parameters measured from planned Baryon Acoustic Oscillation (BAO) and supernova Type Ia experiments, and find that the geometric test results combine with a significant reduction in errors due to different degeneracies. A combination of geometric lensing, CMB and BAO experiments could achieve a pivot redshift constraint of $\Delta$w(zp)=0.020 at zp=0.62. Simple relations are presented that show how our lensing results can be scaled to other telescope classes and survey parameters. 
A.N. Taylor, T.D. Kitching, D.J. Bacon, A.F. Heavens