Following the Universe from Dark to Light

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AST 0206299CenThe formation and evolution of cosmic structures, including galaxies, clusters of galaxies and large-scale structure, are fundamental problems of modern cosmology. While the Big Bang picture of how the Universe began is generally accepted, the details of how and when structures developed and formed are in hot debate. On scales larger than clusters of galaxies (10 million light years across) it is generally believed that gravity is the dominant force and structures grow from small initial seeds planted in the very early universe ( 1 second since the Big Bang), because gravity is attractive and unstable. However, on smaller scales gas hydrodynamics, microphysics (i.e., forces on atomic scales) and star formation processes play progressively more important roles in shaping and determining when galaxies form and how they evolve; the picture at these smaller scales is much less clear. The radiation field, especially, the ultra-violet radiation that is capable of ionizing atomic hydrogen must play a very important role in these processes. In particular, it determines when atomic hydrogen is fully ionized and the universe is cleared of hydrogen "fog" and becomes transparent to distant sources. This epoch has been characterized as the "end of the dark ages". Since radiation controls the evolution of the thermodynamic state of the gas, a fraction of which is subsequently incorporated into stars, it is essential to include radiation hydrodynamics in full detail in cosmological computer simulations in order to provide answers to many of the important questions in modern cosmology. This is a formidable task to tackle even with the fastest supercomputers today. Dr. Cen and his collaborators have developed an efficient way to compute this complicated situation and begun to carry out a program that should help provide some clues about how the radiation field and the matter evolve with time.***