An Essay on the Prize in Astronomy 2004
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. A Princeton experiment designed to detect this radiation was barely beaten to the discovery by the work of a Bell Laboratories team. The resounding success of this prediction emboldened particle physicists and cosmologists to speculate on even earlier epochs in the history of the universe, culminating in exciting and suggestive new ideas concerning the nature of the creation event itself.
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. The name “cold dark matter” derives from these attributed properties. 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, except for the concept of inflation, 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.
The consensus opinion in cosmology currently holds that we live in a Lambda Cold Dark Matter, or ΛCDM, universe, where ordinary plus dark matter constitutes perhaps 1/4 of the density required for the geometry of space to be flat, with dark energy accounting for the remaining 3/4. The theoretical attractiveness of this solution lies primarily in that spatial flatness is a prediction of the simplest forms of inflationary cosmology, although the precise combination by which the closure density is apparently achieved is one of the major scientific surprises of the past decade.
Peebles's own work on primordial nucleosynthesis forms a main underpinning of the conclusion that the amount of baryonic matter which can emit, scatter, or absorb light, is only 4 or 5% of the closure value. And the pioneering studies of the dynamics of galaxies and groups and clusters of galaxies that he initiated or inspired leads to the now accepted conclusion that cold dark matter can account for only about another 20% or so of the closure value. The conclusion that the overall mass-energy content has exactly the closure value then rests primarily with recent measurements of the angular power contained in small-amplitude anisotropies in a cosmic microwave background that itself contributes negligibly to the closure density. Combined with the other two conclusions, this finding suggests that the remainder – roughly 3/4 of the total – comes about because empty space devoid of matter and radiation can nevertheless possess nonzero energy (dark energy or cosmological constant).
This affront to common sense is believed to arise because fluctuations in the quantum vacuum can lead to unbalanced positive and negative contributions. A cosmological constant Λ (in the language of Einstein) that has an associated positive energy density and negative pressure can lead asymptotically to a universal expansion that accelerates with time. Precisely such a recent acceleration, rather than a deceleration, has been claimed by two separate astronomical teams in the measurements of the brightness of distant supernovae. This evidence, combined with numerical simulations of large-scale structure formation in the universe, has led to the ascendancy of the ΛCDM model in contemporary cosmological studies.
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. He points out that the astronomical constraints are not yet much more numerous than the assumptions of conventional ΛCDM and related theories. In his opinion such theories also do not yield a satisfactory solution for the problem of galaxy formation, nor do they explain why dwarf galaxies do not appear in large cosmic voids.
Particle physicists have joined the debate, with some claiming that a nonzero cosmological constant violates basic notions in contemporary fundamental physics. 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 constitutes 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. It is an achievement that richly deserves the award of the first Shaw Prize in Astronomy.
Astronomy Selection Committee
The Shaw Prize
7 September 2004, Hong Kong