Why are galaxy clusters worthy of our attention? For cosmologists, clusters provide unique signatures of the evolution of the universe. The overall growth of structure in the universe is regulated by a few key parameters, such as the fraction of mass in baryons compared to dark matter and the overall pressure and nature of dark energy. The number density of the most massive clusters and their distribution in redshift (time) provide very sensitive probes of such parameters. And for dynamicists pondering how gravitationally bound structures form and come into equilibrium, clusters offer a critical laboratory. They are the largest bound structures that currently exist, illuminating the transition from the linear infall regime to the non-linear relaxation stage. The diffuse cluster plasma offers even more rich opportunities for astrophysicists. It allows us to explore a system with a full census of its baryons, unlike the situation for galaxies and less massive groups of galaxies. Therefore, for issues such as the evolution of galaxies in clusters, we can trace the effects of shocks and ram pressure on the formations of stars and galaxies from the observed intracluster medium, without worrying about what we've missed.
And this “collisionless” high-beta (low magnetic field) plasma itself offers bountiful opportunities (to put a positive spin on our ignorance!) to explore interesting physics, e.g., turbulent cascades, ion-electron equilibrium, amplification of magnetic fields and the coupling of cosmic rays to MHD structures. Interested readers are commended to the review by Andrei Bykov et al. [4]