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Weak gravitational lensing of background galaxies by intervening large-scale structure ('cosmic shear') provides direct information about the total mass distribution in the universe, regardless of its nature and state. Thus a measurement of cosmic shear bridges the gap between theory, which is primarily concerned with dark matter, and observation, which generally probes only luminous matter. The recent detection of coherent distortion of faint galaxies using the William Herschel Telescope in 2000 have triggered great interest in the provision of new constraints on the amount and distribution of dark matter, together with measurements of several cosmological parameters.

If intrinsic galaxy orientations are essentially random in a given survey, any coherent alignment must arise from distortion due to weak lensing. Light paths from galaxies projected close together on the sky pass through, and are gravitationally distorted by, the same dark matter concentrations. This coherent distortion contains valuable cosmological information. In particular, the variance of the distortion field measures the amplitude of density fluctuations (sim σ 8 Ω0.5m). This shear measurement is free from assumptions about Gaussianity or the mass-temperature relation, and whilst the shear-based measurement is currently comparable in precision to that from local cluster abundance, further progress is limited solely by the number of fields observed.

The validity of results from cosmic shear surveys depends sensitively on the treatment of systematic errors. A further issue arises from sample (or 'cosmic') variance, the impact of which can be limited by using numerous independent sightlines to complement panoramic imaging of a few selected areas. With these motivations in mind, a team of astronomers compared the cosmic shear observed with two independent instruments (Keck and William Herschel Telescope), using two different survey strategies.

Astronomers extended the original detection of the cosmic shear on the William Herschel Telescope by increasing the number of observed fields, with a further increase in area as a result of the larger 16×16 arcmin2 size of field with the new prime focus mosaic camera. The aim of the survey was to acquire deep (z 1) fields representing numerous independent lines of sight, sufficiently scattered to sample independent structures and thus to minimize uncertainties owing to sample variance. These lines of sight were chosen in a quasi-random fashion, without regard to the presence or absence of mass concentrations, in order to obtain a representative sample of the mass fluctuations in the universe.

The cosmic shear with both Keck and WHT was measured at a signal-to-noise of 5.1, finding an amplitude of the matter power spectrum of σ 8m/0.3)0.68= 0.97 ± 0.13, with 0.14 <Ωm< 0.65, including all contributions to the 68 per cent confidence level uncertainty: statistical noise, sample variance, covariance between angular bins, systematic effects and redshift uncertainty. A measurement of this quantity from cosmic shear is cosmologically valuable, as it represents a direct measure of the amplitude of mass fluctuations.

Determination of parameters
Constraints on the joint distribution of Ωm and σ 8 for the combination of Keck and WHT measurements. [ GIF ]

These results for Keck and WHT are consistent with each other, strengthening confidence in control of systematics. The joint results are also consistent with other recent cosmic shear measurements. They also agree with the old cluster abundance normalization. However, they cannot rule out lower cluster-abundance normalization which has been derived recently. This discrepancy, if confirmed, could arise from unknown systematics in either the cluster or cosmic shear methods. For the cluster method, further studies would be needed to understand the difference between the observed mass-temperature relation and that found in numerical simulations. It is important to understand the origin of the discrepancy between cosmic shear and cluster abundance methods. If this is not explained by such systematics, it could point towards a failure of the standard ΛCDM paradigm, and therefore have important consequences for cosmology.

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Last modified: 13 December 2010