Cross-correlation Tomography: Measuring Dark Energy Evolution with Weak Lensing

A cross-correlation technique of lensing tomography is presented to measure the evolution of dark energy in the universe. The variation of the weak lensing shear with redshift around massive foreground objects like bright galaxies and clusters depends solely on ratios of angular diameter distances. Use of the massive foreground halos allow us to compare relatively high, linear shear values in the same part of the sky, thus largely eliminating the dominant source of systematic error in cosmological weak lensing measurements. The statistic we use does not rely on knowledge of the foreground mass distribution and is only shot-noise limited. We estimate the constraints that deep lensing surveys with photometric redshifts can provide on the dark energy density Omega, the equation of state parameter w and its redshift derivative w’. The accuracies on w and w’ are: sigma(w) ~ 0.02 fsky^{-1/2} and sigma(w’) ~ 0.05 fsky^{-1/2}, where fsky is the fraction of sky covered by the survey and sigma(Omega)=0.03 is assumed in the marginalization. Combining our cross-correlation method with standard lensing tomography, which has complementary degeneracies, will allow measurement of the dark energy parameters with significantly better accuracy.