Detection of weak gravitational lensing by large-scale structure

We report a detection of the coherent distortion of faint galaxies arising from gravitational lensing by foreground structures. This 'cosmic shear' is potentially the most direct measure of the mass power spectrum, as it is u naffected by poorly justified assumptions made concerning the biasing of th e distribution. Our detection is based on an initial imaging study of 14 se parated 8 x 16 arcmin(2) fields observed in good, homogeneous conditions wi th the prime focus EEV-CCD camera of the 4.2-m William Herschel Telescope. We detect an rms shear of 1.6 per cent in 8 x 8 arcmin(2) cells, with a sig nificance of 3.4 sigma. We carefully justify this detection by quantifying various systematic effects and carrying out extensive simulations of the re covery of the shear signal from artificial images defined according to meas ured instrument characteristics. We. also verify our detection by computing the crosscorrelation between the shear in adjacent cells. Including (Gauss ian) cosmic variance, we measure the shear variance to be (0.016)(2) +/- (0 .012)(2) +/- (0.006)(2), where these 1 sigma errors correspond to statistic al and systematic uncertainties, respectively. Our measurements are consist ent with the predictions of cluster-normalized cold dark matter (CDM) model s (within 1 sigma) but a Cosmic Background Explorer normalized standard col d dark matter model is ruled out at the 3.0 sigma level. For the currently favoured Lambda CDM model (with Omega (m) = 0.3), our measurement provides a normalization of the mass power spectrum of sigmas = 1.5 +/- 0.5, fully c onsistent with that derived from cluster abundances. Our result demonstrate s that groundbased telescopes can, with adequate care, be used to constrain the mass power spectrum on various scales. The present results are limited mainly by cosmic variance, which can be overcome in the near future with m ore observations.