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