The Dark Energy Survey

The Dark Energy Survey

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Gravitational Lensing

A Hubble Space Telescope image of galaxy cluster Abell 2218 showing strong lensing. The arches around the bright galaxy on the left side of the image are actually light from objects behind that galaxy cluster. The tremendous gravity of that cluster actually bends the light around its center. (Credit: NASA, A. Fruchter and the ERO Team)
Like galaxy clusters, gravitational lensing probes the nature of dark energy through both the growth of structure in the universe as a function of time and the dependence of distances on the expansion rate. How large structures grew in the history of the universe can tell us a lot about the interplay between gravity and dark energy. However, most of this structure is made up of dark matter, which can not be detected by standard astronomical means.

To solve this problem, DES will use the force of gravity as a detector. All matter in the universe is affected by gravity, and gravitational lensing is caused by the effect of gravity on light itself. Light from distant galaxies is bent as it passes by massive objects. In some cases, the bending of the light is so significant that multiple images of the galaxy are actually formed. This is referred to as strong gravitational lensing. In other cases, this bending is small, so that 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 galaxy is referred to as cosmic shear and can amount to a typical stretching of an image on the order of 2 percent. Because not all galaxies are perfectly circular and because we are looking at different galaxies from different angles, this small effect is impossible to disentangle for an individual galaxy. Fortunately, weak lensing by gravity affects many galaxies in the same part of the sky in the same way. By studying a large number of galaxies in a small patch of sky and looking for alignments in their slight distortion, astronomers can detect this effect.

DES will create a catalog of over 300 million galaxies looking for these shear affects. The effect of lensing will depend upon how clumped the distribution of dark matter is and upon the distances to the lensing structure and the object being lensed. This method will allow DES to simultaneously probe the growth of structure and the expansion of the universe over time.