Campus, Varian 355
Galaxy clusters—the largest nodes on the cosmic web of dark matter—compress the mass of 1015 Suns into a volume of order ~Mpc3. Their extreme gravitational influence gives rise to several unique observational signatures, such as X-ray emission from diffuse baryonic matter that is heated and ionized by the cluster’s gravitational squeeze, and the prominent weak lensing signal imprinted upon the shapes of background galaxy images as light travels through a cluster’s gravitational field. These and other measurable cluster properties can be used to obtain cluster masses and constrain cosmology. Clusters are powerful dark energy probes, both as tracers of cosmic structure growth and the universe's geometry. The former experiment counts the number of clusters in mass and redshift bins using large astronomical surveys like the forthcoming Large Synoptic Survey Telescope (LSST). The latter is based upon the gas mass fraction fgas=Mgas/Mtotal of clusters. Leveraging the fact that clusters are so large they contain a representative sample of the universe, the mass fraction of intracluster gas, the dominant baryonic component of clusters, can be related to the cosmic baryon fraction (Ωb / Ωm) using a small correction factor from hydrodynamic simulations. Since the absolute fgas of massive clusters is redshift-independent, while its measured apparent fgas depends on distance, it serves as a standard quantity to constrain dark energy. The key challenge for both experiments is measuring cluster masses accurately.