Probing Dark Matter and Dark Energy with observations of the evolution and spatial distribution of galaxy clusters
Galaxy clusters are the largest structures which have had time to form in the Universe. As that they are ideal tracers of the large-scale matter distribution and the evolution of the large-scale structure of the Universe. Since large-scale structure statistics and the growth of the matter inhomogeneities depend strongly on the cosmological model, the clustering pattern and evolution of the galaxy cluster population depends very sensitively on the nature and density of the Dark Matter (DM) and Dark Energy (DE). To exploit these properties of the galaxy cluster population we pursue four major cluster surveys focused on constraining cosmological models:
- The APEX (Atacama Pathfinder Experiment) cluster survey covering a sky area of order 200 sqd at millimeter wavelengths to detect clusters through the Sunyaev-Zel'dovich (SZ) effect and identify them through optical and near-IR observations.
- The search for distant X-ray luminous clusters in archived XMM-Newton observations, followed by their optical identification and redshift determination through an itensive observational program, mostly at ESO telescopes.
- The preparation of a new, deep, large-area X-ray survey satellite mission to be launched in ~2010, with the main goal to establish a very large X-ray cluster sample for tight constraints of the DE and DM parameters.
- The search for distant galaxy clusters in wide-area, medium depth, optical multi-colour sky surveys, such as the KIDS project that is also used by other projects in the Transregional Network.
The project teams are currently involved in the preparation of the APEX-SZ survey, in the building and operation of X-ray instrumentation as well as their application for deep and wide X-ray surveys, in the conduction of optical cluster surveys, the development and application of multi-wavelength data mining procedures, and the comprehensive modeling of the observations in the frame of various cosmological models. This involvement covers all aspects of the observational approach from instrument design to the extraction of the final modeling result, including the control of all possible systematic and statistical errors. The highly complementary, combined approach of X-ray, millimeter, and optical observations is crucial to overcome systematic uncertainties, specifically those related to the conversion from observables to cluster mass. The combined observation of cluster evolution and
their spatial clustering helps to "self-calibrate" the data sets. Using clusters to constrain cosmological models complements constraints from observations of the microwave background and of distant supernovae, thus increasing as much as possible the observational leverage within the Transregional Network. A major aim of the individual observational projects is to constrain the DE equation of state parameter to better than ~10% and its time evolution to better than ~30%. Combining these constraints we expect to decrease the uncertainties further by at least a factor of 2.