Large structures in our Universe have formed hierarchically, with small objects attracting one another gravitationally and merging to form larger objects. Clusters of galaxies are the largest such structures that have had time to form since the Big Bang, being typically several million light years in extent and with masses typically one quintillion times that of our Sun. In essence, the most massive clusters are never "finished" forming, but instead have been growing steadily over the entire history of the Universe. As a result, clusters' internal structure and population statistics (number as a function of mass and time) reflect the history of the Universe's expansion, and the action of gravity over large distances and long time scales. By searching for distant clusters and measuring their properties, we can learn about the different types of mass and energy which drive the dynamics of the Universe as a whole, including dark energy, dark matter, and neutrinos.

The thermodynamic state of the hot gas that fills clusters of galaxies reflects both the cosmological formation of clusters and the action of astrophysical processes within them. For example, the relatively dense gas in the centers of many clusters appears to be exquisitely balanced between radiative cooling and heating through some combination of feedback from active galactic nuclei and turbulent motions. Cluster atmospheres also retain metals (read: elements heavier than helium) that are expelled from stars near the end of their lives and then stripped from their host galaxies by ram pressure, providing a record of star formation over cosmic time.

Astronomical facilities are expensive, and so the data they produce is precious. Extracting every last bit of information from them that we can, and compensating for inevitable biases (e.g., the most distant source known is necessarily one of the brightest at that distance from us, and therefore not representative) requires careful statistical modeling and advanced inference techniques.