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Analysis of the Dark Universe through Lensing Tomography

Final Report Summary - ADULT (Analysis of the Dark Universe through Lensing Tomography)

Weak lensing by large-scale structure, or cosmic shear, remains one of the most promising techniques to study the nature of dark matter and dark energy. The successful measurement of this signal requires accurate measurements of the shapes of faint, distant galaxies. Thanks to ADULT we have improved our understanding of the main challenges in preparation for Euclid, ESA’s space mission to study dark energy, and have applied our findings to observations of massive clusters of galaxies and data from the KiloDegree Survey.
Unbiased estimates of the cosmological parameters rely on a correct interpretation of the signal. Two astrophysical sources of bias are important to consider, and thanks to ADULT we have made progress on both aspects. First, intrinsic alignments of galaxies lead to an additional signal, which needs to be modelled. We have examined the predicted signal using state-of-the-art numerical hydrodynamic simulations and observationally constrained the alignments of galaxies in clusters of galaxies. Moreover, we contributed actively to the development of the PAUCam Survey, which will greatly improve observational constraints. The second source of bias is the modification of the distribution of matter due to baryonic feedback processes. We have shown how this can be accounted for and our study of galaxy groups provides additional direct constraints. Moreover our observations of the lensing signal around galaxies provide additional constraints on the predictions from hydrodynamic simulations.
Our advances have resulted in the best determination of cluster masses using weak gravitational lensing, which have been used by the Planck team to derive cosmological parameters based on their cluster counts. However, the highlight of the project is the analysis of 450 square degrees of imaging data from the KiloDegree Survey (KiDS), which has yielded the best constraints on cosmological parameters from weak lensing. Interestingly, the results are in mild tension with measurements from the cosmic microwave background by Planck. Reduction of the statistical and systematic uncertainties is ongoing, but it is clear that cosmic shear is already a competitive technique to study the dark universe. The KiDS data provide an important test bed in preparation of the analysis of Euclid data. ADULT has been essential to achieve the current results, but its legacy will continue as we continue to improve our models of intrinsic alignments and baryon physics in preparation of the launch of Euclid in 2020.