Large galaxy surveys provide detailed information about the large-scale structure of the Universe, which, in turn, helps understand its geometry, composition, evolution and fate. Particularly relevant is the “weak gravitational lensing” effect, by which the measured shape and orientation of distant galaxies are slightly distorted by the gravitational pull of the masses between them and us, thus providing a measurement of the distribution of the intervening matter, be it visible or dark.
The Dark Energy Survey (DES) is one such galaxy survey, an international collaboration of 400 scientists from 25 institutions in 7 countries that has surveyed an eighth of the sky using DECam, a 570-megapixel camera installed at the 4-meter Blanco telescope in the Cerro Tololo Inter-American Observatory in Chile. The IFAE group was responsible for the design and production of most of the read-out electronics for the 74 CCDs in DECam.
Using half of its final data set, DES has measured the shape and orientation of over 100 million distant galaxies (, led by an IFAE PhD student), together with their distances to us (, led by two IFAE PhD students). Combining these with the measurement of the positions of a sample of 11 million nearby galaxies, DES has produced stringent constraints on the mean matter density in the Universe and its level of inhomogeneity (; figure). In the figure, we compare the determination of these quantities by DES in the current Universe with the extrapolation to the current Universe of the measurements performed by Planck, an ESA satellite, in the early Universe, as probed by the Cosmic Microwave Background radiation, a relic from the Big Bang. This extrapolation assumes the prevailing Lambda Cold Dark Matter (/\CDM) cosmological model, with Einstein’s cosmological constant as dark energy. The agreement between the DES and Planck results is fair, but not perfect, which might (or not) point to deficiencies in the /\CDM model. More data are needed to elucidate this issue.