3.7. Gravitational lensing
Images of background galaxies get distorted due to the gravitational effect of mass fluctuations along the line of sight. Deep gravitational potential wells such as galaxy clusters generate `strong lensing', i.e., arcs and arclets, while more moderate fluctuations give rise to `weak lensing'. Weak lensing is now widely used to measure the mass power spectrum in random regions of the sky (see Ref.  for recent reviews). As the signal is weak, the CCD frame of deformed galaxy shapes (`shear map') is analyzed statistically to measure the power spectrum, higher moments, and cosmological parameters.
The shear measurements are mainly sensitive to the combination of m and the amplitude 8. There are various systematic effects in the interpretation of weak lensing, e.g., due to atmospheric distortions during observations, the redshift distribution of the background galaxies, intrinsic correlation of galaxy shapes, and non-linear modeling uncertainties. Hoekstra et al.  derived the result 8 m0.52 = 0.46-0.07+0.05 (95% confidence level), assuming a CDM model. Other recent results are summarized in Ref. . For a m = 0.3, = 0.7 cosmology, different groups derived normalizations 8 over a wide range, indicating that the systematic errors are still larger than some of the quoted error bars.