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Phillip James E. Peebles is the world's foremost theoretical cosmologist. He has led our quest to understand how the universe evolved from a hot, dense, nearly uniform state just after the Big Bang to the highly structured cosmos that we see today. Peebles's research reveals the physics of how the contents of this universe — matter and energy — develop with time into the rich patterns of galaxies, clustering of galaxies and radiation backgrounds now observed by astronomers.
Peebles described how the light elements in the periodic table formed by fusion in the hot early Big Bang universe. His calculations with Robert Dicke showed that a universe which began with a Big Bang should be filled with heat radiation which would be detectable at microwave frequencies. His collaborators at Princeton built a detector for this radiation. When this radio signal was accidentally discovered by Arno Penzias and Robert Wilson of Bell Laboratories (for which they were awarded the Nobel Prize), Peebles and Dicke provided the correct interpretation: they had seen radiation emitted from the epoch when hydrogen first formed. Peebles showed how the universe makes this crucial transition from a hot sea of free electrons and protons. He further realized that the microwave background radiation should hold the imprint of the small fluctuations that would later grow into galaxies and other structures. This began a three-decades long search for small ripples in the radiation background, a search culminating in their discovery by the Cosmic Background Explorer satellite in 1992.
Peebles developed a theory that describes how galaxies, clusters of galaxies and super clusters of galaxies form by gravity from tiny fluctuations in the primordial universe. Thus, he explained how the universe evolves from a nearly uniform state to one in which much of space appears empty and most of the matter is clustered in or near galaxies. With Jeremiah Ostriker, he proposed that disk galaxies like our Milky Way would not be stable without large halos of dark matter. Peebles made detailed calculations of what a universe filled with such collisionless, cold matter would look like. When this cold dark matter model required revision, he suggested investigation of models with a positive, possibly time-variable, cosmological constant — an energy density of seemingly empty space.
To complement his theoretical contributions, Peebles developed powerful statistical methods that quantify the observed clustering and motions of matter in the cosmos. His analyses of galaxy surveys as a test for his structure formation theory opened a new era in observational and theoretical cosmology, motivating new three-dimensional surveys of up to a million galaxies and computer simulations to compare competing theoretical models.
In this rapidly developing discipline, Peebles's books, particularly The Large-Scale Structure of the Universe and Principles of Physical Cosmology, remain the standards of reference. Entering the fifth decade of his contributions to cosmology, Peebles continues to innovate, inspire, and challenge others in the field.
By Michael S. Vogeley, Department of Physics, Drexel University, Philadelphia | 
