I am an observational cosmologist with weak gravitational lensing as primary field of expertise. Weak lensing describes subtle image distortions that get imprinted onto the shapes of distant galaxies when their light bundles pass through the gravitational field of foreground structures. These distortions can be estimated statistically by studying the shapes of galaxies in high-resolution images. The strength of the distortion depends on geometry and the mass distribution of the foreground object acting as a lens, but not on the nature of this foreground object. This makes weak lensing a powerful tool to investigate the invisible components of the Universe. In particular, it allows us to study the large-scale distribution of matter, which is dominated by the invisible dark matter. Its gravitational growth over time also depends on the expansion history of the Universe and the way gravity acts on matter on large scales. Therefore, gravitational lensing also provides us with a powerful tool to study the properties of the mysterious dark energy, which is believed to cause the accelerated expansion of the Universe.


One approach to study cosmology via weak lensing is called "cosmological weak lensing", where one constrains the coherent lensing distortions between galaxy pairs as function of their separation. I led two such studies using observations from the Hubble Space Telescope (HST, Schrabback et al. 2007Schrabback et al. 2010). In particular the latter analysis allowed us to obtain independent evidence for the accelerated expansion of the Universe from weak lensing. Given its small field of view, HST surveys however only cover small fractions of the sky, leading to large statistical uncertainties. Thanks to its much bigger field of view, a much more powerful cosmological weak lensing survey will be conducted by ESA's Euclid mission. I spend a major fraction of my research time preparing for the Euclid weak lensing measurements. In particular, I am co-leading the Organisational Unit SHEAR within the Euclid Consortium, which is tasked with the development of algorithms for highly accurate measurements of galaxy shapes in the Euclid data (e.g. Tewes et al. 2019). Thanks to their greater depth, higher resolution, and multi-band coverage HST observations of representative galaxy samples provide a key resource to train and calibrate these algorithms (e.g. Er et al. 2018), and to constrain statistical galaxy properties that are relevant for weak lensing measurements (see e.g. Euclid Collaboration 2019 and Appendix A in Schrabback et al. 2018a; ). 

ESA/ HST press release:  Hubble confirms cosmic acceleration with weak lensing
credit: NASA, ESA, P. Simon, T. Schrabback (University of Bonn, Leiden Observatory)


Current (pre-Euclid) cosmological weak lensing surveys either lack the area or the depth to provide really competitive constraints on dark energy properties. We can bypass this limitation by obtaining deep weak lensing data in the areas of the sky that contain the densest peaks in the cosmic large-scale structure, as they feature the strongest weak lensing signal and provide the strongest sensitivity to cosmological parameters. Modern galaxy cluster surveys allow us to robustly find these regions on the sky, but in order to assign the correct mass scale and enable the robust comparison of the measured cluster number counts to cosmological predictions we must add the weak lensing data. 
Dark energy primary affects the redshift-dependent growth in the cluster mass function. In order to constrain its properties we therefore need to conduct this experiment over a wide range in redshift or cosmological look-back time. Suitable cluster samples, which extend to high redshifts and benefit from a well-characterized selection function, are now in place, see e.g. the Sunyaev–Zel'dovich (SZ) survey conducted by the South Pole Telescope (SPT-SZ, Bleem et al. 2015, 2019). What has been lacking is the weak lensing complement to calibrate the cluster mass scale, especially at high redshifts. At lower redshifts such cluster weak lensing measurements are possible with optical ground-based (seeing-limited) images (e.g. Dietrich et al. 2019; Stern et al. 2019). 
For clusters at higher redshifts (z>~0.7) deeper images with better resolution are needed in order to resolve the typically small and faint background (z>~1.5) galaxies carrying most of the weak lensing signal. In Schrabback et al. (2018a) we presented the first such study for an SZ-selected sample of high-redshift clusters, using mosaic HST observations for 13 distant clusters from SPT. These measurements provide a key input to the latest cluster scaling relation (Dietrich et al. 2019) and cosmology (Bocquet et al. 2019) results from SPT. 
In order to further tighten the resulting dark energy constraints we have to enlarge the sample of distant clusters with deep weak lensing data. As one step in this direction I am currently analyzing new HST observations obtained by me as Principal Investigator (P.I., HST programs 13412, 13493, 14677, and 15883 adding to 77 observed orbits) together with the AIfA research group I am leading.
Analyzing  ESO/VLT observations obtained by me as P.I., we showed in Schrabback et al. (2018b) that deep VLT/HAWK-I Ks images observed under good seeing conditions provide a similarly effective alternative to HST for weak lensing measurements of massive  galaxy clusters at redshifts 0.7<~z<~1.1. This is thanks to the larger field of view of HAWK-I (removing the need for time-consuming mosaics), the sharper images (0.35" FWHM of the PSF) that are obtained from the ground in the Near Infrared compared to the optical, a very effective photometric selection of background galaxies, and rounder intrinsic galaxy shapes in the Ks-band. Currently, VLT is conducting a new observing campaign (programs 0101.A-0892, 0102.A-0054, 60.A-9473, and 0104.A-0025, adding to 107.4h, all with P.I. Schrabback), which will allow us to exploit this new route to tighten cluster dark energy constraints.
The German/Russian X-ray space mission eROSITA has recently started its all-sky survey and will soon provide another large galaxy cluster survey aiming to provide improved constraints on dark energy properties. I am one of the coordinators of a joint galaxy cluster science working group of the eROSITA_DE consortium and the Hyper Suprime-Cam weak lensing survey, which will provide key contributions for the eROSITA cluster mass calibration.  

ESO/HST picture of the weekHAWK-I and Hubble Explore a Cluster with the Mass of two Quadrillion Suns (credit: ESO & ESA/Hubble & NASA). See also the press release of Bonn University and cover picture of Astronomy & Astrophysics. 

3. Studying the dark matter environment of galaxies

Weak lensing can also be used to statistically probe the dark matter environment of galaxies. In Schrabback et al. (2015) we used ground-based weak lensing data from the CFHTLenS survey to probe not only the masses of galaxy dark matter halos, but also their shapes. N-body simulations predict that dark matter halos should approximately appear elliptical in projection. Likewise, the projected light distribution of lens galaxies appears approximately elliptical. Assuming galaxies and their halos are aligned, we expect to measure a stronger weak lensing shear signal at a given radius close to the lens major axis than close to the minor axis. We have conducted this measurement split for lenses according to stellar mass and color, comparing the measured signal to predictions that we derived using the Millennium Simulation. If dark matter halos were as elliptical as predicted and perfectly aligned with the visible light distribution, we would expect a much stronger signal than what we have measured. However, our derived constraints are in perfect agreement with model predictions once we fold in current models for the expected misalignment between the orientations of galaxies and their host dark matter halos.