Galaxy Clusters and the Dark Universe
Observations of galaxy clusters provide powerful cosmological constraints via two main methods. The first uses measurements of the baryonic mass fraction of the largest, dynamically relaxed clusters. It provides tight constraints on the mean matter density and, like type Ia supernovae studies, traces the effects of dark energy on the expansion of the Universe. The second, independent method uses the observed distribution and evolution of cluster counts. It leads to tight constraints on the amplitude of matter fluctuations and powerful, complementary constraints on dark energy from its impact on the growth of structure. I will discuss the key systematic considerations and latest results from each method, and the prospects for improvements in the near-to-mid term.