Galaxy clusters are the largest gravitationally bound structures in the universe. They began to form about 10 billion years ago when the first galaxies and their massive mantles of dark matter clumped together. Most of a cluster’s matter, however, lies between galaxies. Shocked by the assembly process and trapped by the cluster’s gravitational potential, the baryonic component of the intracluster medium (ICM) reached, and remains at, temperatures in the x-ray-emitting range of 107–108 K.
Their ages, assembly, and x-ray emission make clusters valuable probes of one of the most momentous phenomena ever discovered: The universe’s current rate of expansion is not slowing down, as one expects of a matter-dominated universe, but is speeding up. (See Physics Today, June 1998, page 17.)
The x-ray glow from a cluster’s ICM is detectable out to a redshift z of about 2. Type Ia supernovae, which featured in the...
