page_title: Dark Energy Survey
section_title: Weak lensing
hierarchical_section_title: Dark Energy Survey / Weak lensing
caption_attribution_description: English: This image shows the full overview of the galaxy cluster Abell 2218 and its gravitational lenses. This image was taken by Hubble in 1999 during the Early Release Observations made immediately after the Hubble Servicing Mission 3A. Polski: Zdjęcie gromady galaktyk Abell 2218 wykonane przez Kosmiczny Teleskop Hubble'a
context_page_description: The Dark Energy Survey is a visible and near-infrared survey that aims to probe the dynamics of the expansion of the Universe and the growth of large-scale structure. The collaboration is composed of research institutions and universities from the United States, Australia, Brazil, the United Kingdom, Germany, Spain, and Switzerland.
  The survey uses the 4-meter Victor M. Blanco Telescope located at Cerro Tololo Inter-American Observatory in Chile, outfitted with the Dark Energy Camera. This camera allows for more sensitive images in the red part of the visible spectrum and in the near infrared, in comparison to previous instruments.
  DECam has one of the widest fields of view available for ground-based optical and infrared imaging. The survey has imaged 5,000 square degrees of the southern sky in a footprint that overlaps with the South Pole Telescope and Stripe 82. The survey took 758 observing nights spread over six years to complete, covering the survey footprint ten times in five photometric bands. DES officially began in August 2013 and completed its last observing session on the 9th of January 2019.
context_section_description: 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.
caption: Strong lensing in cluster Abell 2218. Credit: NASA/ESA
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