The most stringent cosmological constraints come from the combination of measurements of the galaxy clustering in a flux-limited sample of nearby galaxies, the shear-shear correlations in the weak-lensing sample of 100 million distant galaxies, and the tangential shear of the galaxies in the weak-lensing sample around the galaxies in the nearby sample (galaxy-galaxy lensing). This combination maximizes the constraining power of the data, in particular by breaking the degeneracy between the galaxy bias of the nearby sample and the amplitude of the fluctuations in the underlying matter distribution. The result of the overall combination of the three two-point correlation functions is presented in Fig. 1, which shows the 68% and 95% allowed contours in the S8 – Ωm plane. Here S8 measures the inhomogeneity of the matter distribution now: it is related to the standard deviation of the matter density fluctuations in spheres of 8 Mpc/h radius, while Ωm is the fraction of matter in the total matter-energy of the Universe now. In Fig. 1, we can see that the DES results are in fair agreement with those from the Planck satellite.
Planck measures the tiny inhomogeneity of the early Universe at redshift ~1100, and extrapolates to the current epoch assuming the ΛCDM cosmology. The agreement between DES and Planck is, then, a test of the validity of ΛCDM.
Due to the weak gravitational lensing effect, the observed size, shape and orientation of distant galaxies are slightly distorted by the gravitational pull of the masses between them and us. Then, the statistical properties of a large set of images of distant galaxies can be studied to determine the location of the intervening (mostly dark) matter to produce mass maps as the one shown in the figure, produced by the Dark Energy Survey analyzing over 100 million distant galaxies corresponding to the data from their first three years of observations.
Brighter areas in the map contain more (mostly dark) matter, while darker areas contain less. The green circles correspond to visible galaxy clusters, also observed by DES. The inset shows a zoom into the randomly chosen small cyan area in the main map. One can clearly see that areas with more dark matter are rich in galaxy clusters, while areas with less dark matter are also relatively empty of ordinary matter. This is by far the largest dark-matter map produced to date. From N. Jeffrey, M. Gatti et al. (DES Collaboration) 2021, “Dark Energy Survey Year 3 results: curved-sky weak lensing mass map reconstruction,” MNRAS 505, 4626. At the time, Marco Gatti was a PhD student at IFAE.