The Dark Energy Survey - Weak Lensing

Weak Lensing

Gravitational lensing occurs when light from distant sources such as quasars or galaxies is bent by the gravitational field of a massive object. An image of Abell 2218, a cluster of galaxies, shows how the intervening matter field distorts the light from background galaxies. The effect around the cluster is so strong, that multiple images of the source galaxy are seen as arcs. This is referred to as strong gravitational lensing.

The history of growth of large scale structure (LSS) can give us a handle on the interplay between gravity and dark energy. However, most of this structure is made up of dark matter, which cannot be detected by standard astronomical means. The cosmological gravitational field can also bend the light from distant sources but in this case the images of galaxies are distorted, stretched and magnified, in small amounts. This is referred to as weak gravitational lensing.

This small distortion of the image of a galaxies referred to as cosmic shear and can amount to a typical stretching of an image on the order of 2 percent. The effect is too small to be measured for an individual galaxy. Fortunately, the same matter density field affects many galaxies in the same part of the sky and by studying a large number of galaxies in the same area of the sky, astronomers can look for alignments in cosmic shear statistically.

This is achieved by measuring the shear-shear correlation function, a two point function, or its Fourier Transform, the shear power spectrum. DES will measure the shear power spectrum as a function of photometric redshift. Another statistic that can be used is the angular correlation function between the foreground galaxy positions and the shear of the source galaxy, the so-called galaxy-shear correlation.

Since shear is sensitive to the matter density field, which is dominated by dark matter, it is less sensitive to baryonic effects, although such effects may have a large enough contribution to the evolution of the matter power spectrum at small scales, such that we can no longer distinguish between predictions of interesting dark energy models. The calculation of the non-linear matter power spectrum poses another challenge for weak lensing measurements and must include baryonic effects.

DES will be able to probe Dark Energy because cosmic shear measurements are sensitive to the evolution of the matter power spectrum (linear growth of structure) and the distance-redshift relation (expansion history and geometry). Higher order statistics such as the three-point function or the bispectrum, which combine measurements of the CMBR anisotropy and galaxy distribution will be able to break degeneracies between geometry, growth of structure and spatial curvature.