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 20; 000 galaxies per field, with properly calibrated and optimally weighted shear estimates. These were averaged in cells of sizes ranging from 1 0 :875 to 30 0 to obtain estimates of the cosmic shear varianceh 2 i, 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< 10 0 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 0 cells of h 2 i =( 0:28 1:84) 10 -5 . Subject headings: Cosmology: observations — dark matter — gravitational lensing — large-scale structure of Universe — galaxies: photometry