The Shaw Prize in Astronomy for 2004 is being awarded to P. James E. Peebles for a lifetime of groundbreaking contributions to cosmology and astrophysics. Between 1963 and 1985 he laid the foundations for almost all modern investigations in the field, theoretical and observational. Independently of some earlier pioneering work, Peebles realized that the observed abundance of light element isotopes, particularly hydrogen, deuterium, and helium-4, required a hot big bang, which would result today in a directly observable cosmic microwave background.

Peebles also made the first accurate calculation of the crucial transition of the universe from an ionized to a neutral state, and he introduced the formalism for computing the structure imprinted on the cosmic microwave background at recombination. He popularized the notion of massive dark-matter haloes surrounding spiral galaxies, and he was the first to make detailed and elaborate predictions for the large-scale clustering in the cosmos based on the hypothesis that this dark matter consists of elementary particles with relatively low random velocities that interact only weakly with ordinary matter and not at all with light except through gravitation. Many of his seminal ideas have been confirmed, decades after they were first proposed, by observations of the high-redshift universe, in particular by the recent measurements of the angular power-spectrum and polarization of the fluctuations of the cosmic microwave background. His two books on physical cosmology and the large-scale structure of the universe defined to a large extent the language and the scientific agenda of almost all of the work in the field today. His leadership and vision have been critical for turning cosmology from a loosely constrained study in pure theory into the high-accuracy science that it has become at the beginning of the twenty-first century.

During the past decade and a half, Peebles has been vocal in cautioning against a rush to judgment concerning whether the important cosmological parameters are now known to high precision.

Independent of how the debate will finally end, history will probably record that the past four decades firmly established the following major tenets of modern astrophysics: that our universe expanded from a considerably hotter and denser state, now known as the hot big bang; that this expansion is basically described by the relativistic Friedmann-Lemaître equations; that only the very lightest elements emerged from the hot big bang; that the matter which physicists, chemists, and biologists ordinarily study in terrestrial laboratories constitute only a minor fraction of the overall mass-energy content of the cosmos; that a relict radiation from the hot big bang now bathes every point of space; that small fluctuations in this radiation trace the distribution of matter perturbations during the epoch when the universe underwent a transition from being ionized to being neutral and from being optically opaque to being optically transparent; and that the small enhancements of matter grew gravitationally after the decoupling of matter and radiation to become the galaxies and clusters of galaxies that now populate the astronomical universe. This is a remarkable and extraordinary painting of the modern scientific understanding of the creation and evolution of the cosmos, and the handiwork of Jim Peebles can be seen in every brushstroke.