Groups and Clusters of Galaxies: Where Cosmology and Astrophysics Collide
Clusters of galaxies are the largest, most massive gravitationally bound objects in the Universe. They are also the most recent of the cosmic objects to form. According to the currently accepted models of cosmic structure formation, the number density distribution of these systems is very sensitive to the parameters describing the large-scale geometry and the expansion history of the universe For this reason, galaxy clusters are regarded as important cosmological probes. However, to use clusters as precision probes of the cosmological parameters, we need to be able to "weigh them". To do so, and do so properly is challenging. Here I will describe our effort to get a handle on the various systematics and biases that can influence the outcome, and present our mass measurements for 50 galaxy clusters comprising the "Canadian Cluster Comparison Project" or CCCP sample and the LoCuSS Cluster Sample. Using clusters as cosmological probes, however, requires many more than 50-odd clusters with known masses but this is not feasible at the present. I will discuss our ongoing effort to identify and calibrate “ asy-to-observe" proxies for the mass, focusing on the clusters' Compton Y-parameter. One remarkable upshot from this work is that cosmology suggested by clusters is in tension with Planck CMB results.