One of the biggest questions in cosmology today is the cause of the accelerated expansion of the universe. Is this surprising property of the cosmos an indication of a cosmological constant? Is a new form of energy, often called dark energy, or a new, as yet unobserved particle/field responsible? Or is our theory of gravity, Einstein's general theory of relativity, incomplete? With the help of a wide variety of observational techniques, cosmologists today are trying to answer these questions.
Research activities in the Observational Cosmology Group focus on exploiting the weak gravitational lensing effect, potentially the most accurate observational method for studying accelerated expansion. Large parts of this research take place within the framework of the German Centre for Cosmological Lensing (GCCL ), founded in 2019 with the Max Planck/Humboldt Research Award money by Visiting Professor Catherine Heymans (University of Edinburgh). Masses in the universe, such as galaxies or galaxy clusters, influence the propagation of light and thus act similarly to optical lenses. This property leads to tiny distortions in the images of background galaxies that we take with large telescopes. Measuring these distortions in combination with a measurement of the distance (redshift) of the background galaxies then allows us to measure masses in the universe, i.e. to weigh cosmic objects.
These measurements are taken with large-format cameras on special wide-field telescopes. Since the effect is very small, large areas of the sky must be imaged in order to have enough galaxies (many millions) available for a significant measurement. Currently, the working group mainly uses data from the European projects KiDS (Kilo-Degree Survey; http://kids.strw.leidenuniv.nl/) and VIKING (VISTA Kilo-Degree Infrared Galaxy Survey). In the future, data from the LSST (Large Synoptic Survey Telescope) large telescope as well as the ESA/NASA Euclid satellite mission will be used.