Large-Scale Cosmic Shear Measurements

We present estimates of the gravitational lensing shear variance obtained from images taken at the CFHT using the UH8K CCD mosaic camera. Six fields were observed for a total of 1 hour each in V and I, resulting in catalogs containing approximately 20,000$galaxies per field, with properly calibrated and optimally weighted shear estimates. These were averaged in cells of sizes ranging from 1'.875 to 30' to obtain estimates of the cosmic shear variance$<\bar{\gamma}^2>$, with uncertainty estimated from the scatter among the estimates for the 6 fields. Our most reliable estimator for cosmic shear is provided by the cross-correlation of the shear measured in the two passbands. At scales$\lsim 10'$the results are in good agreement with those of van Waerbeke et al. 2000, Bacon et al. 2000 and Wittman et al. 2000 and with currently fashionable cosmological models. At larger scales the shear variance falls below the theoretical predictions, and on the largest scales we find a null detection of shear variance averaged in 30' cells of$<\bar{\gamma}^2 > = (0.28 \pm 1.84) \times 10^{-5}$.