The High-Energy Universe of Galaxy Clusters

The past two decades have seen dramatic advances in our knowledge of the structure and evolution of the universe, the direct result of investments in observational facilities and in the high-performance computing used to analyze the resulting data and to calculate predictions of theory using simulations. While some of these developments have supported earlier ideas, others have raised fascinating new questions. It appears, for example, that about 73 percent of the universe consists of something about which we know almost nothing: dark energy.

Dark energy is characterized by its repulsive interaction with gravity, which for the past six billion years has caused the expansion rate of the universe to accelerate. Our best current hope for learning more about dark energy comes from the growth and evolution of galaxies and clusters of galaxies, and the use of bright events (such as supernovae) to measure distances to these objects. Clusters of galaxies are of particular interest, because they are the most recent objects to have formed and thus bear the marks of accelerated expansion especially well.

More than 35 percent of high-mass clusters are strongly associated with diffuse radio emission (“radio halos”), and this area of exploration offers a new and exciting method of testing cosmology. Recent advances in sensitive low-frequency radio surveys, combined with existing and planned antenna arrays worldwide, will over the next fifteen years dramatically increase the number of known radio halos and generate an enormous amount of new data. Now is the time to plan for what these surveys will detect. Professor Ricker’s focus during his Center appointment will be computer simulations of galaxy cluster radio halos.

By the fall of 2012, his research group expects to have assembled a complete set of the program modules needed to simulate galaxy clusters together with their diffuse radio emission, with unmatched physical fidelity. The simulations—the largest of their type to date—will establish theoretical predictions crucial to the interpretation of future data and potentially provide a new method for probing the dark energy content of the universe. The work will also provide a database of simulated clusters for study in other wavebands.