Perhaps the most important recent result in cosmology is the development of a cosmological standard model that explains a remarkable variety of observational phenomena in the Universe. In this model, small initial fluctuations in density existed near the Big Bang, perhaps generated via quantum fluctuations. Between that time and the present, 13.8 billion years later, these small fluctuations developed into the rich observed structure that characterizes the current Universe: galaxies with a wide range of sizes, masses, luminosities, and appearance; groups and gigantic clusters of galaxies, and a vast, complex cosmic web of gas and galaxies connecting the largest clusters. The evolution of this cosmic structure is determined in principle by the well-known laws of gravity, hydrodynamics, and relativity; but solving these equations has been an immense challenge.
Over the past four decades Simon White, together with an exceptional group of collaborators and students, has developed N-body computer simulations as a new tool of extraordinary power, yielding fundamental insights into cosmic structure formation. The recent “Millennium simulation” captures with great precision the time evolution of cosmic structure between 10 million years after the Big Bang to the present, with over 10,000 million particles representing dark matter distributed in a cube of 2.2 billion light years on a side. In post-processing of this simulation, White, Springel and their colleagues also added models of the small-scale physical processes that govern the evolution of normal matter within the dark matter halos. The formation of stars in galaxies results from a competition between gas cooling and the ejection of matter from the galaxies through the action of supernovae and massive black holes. These semianalytic methods were first proposed by White and Frenk in 1991, and their current predictions for galaxy properties match a remarkable variety of observations, such that such simulations are beginning to approach the age-old dream of “creating the Universe in a computer”.
White has investigated and illuminated almost every aspect of the current paradigm of nonlinear structure formation. Already in 1976 he conducted numerical experiments showing that strong sub-clustering was expected during gravitational collapse, plausibly explaining the lumpy structure of many nearby clusters of galaxies. In 1978 White and Rees were the first to suggest that galaxies form by the collapse of dissipative, normal matter gas to the centres of much larger halos composed of dissipationless dark matter of unknown nature. In the early 1980s White and his collaborators Davis, Efstathiou, and Frenk carried out the most influential early numerical studies of nonlinear structure formation in a realistic cosmological model. They showed that if neutrinos have enough mass to account for most of dark matter, then their relativistic motion in the early universe would have inhibited the growth of structure, contradicting observations. This deduction set an upper limit to neutrino masses — one of the first times that cosmology produced important new constraints on the properties of elementary particles.
As an alternative to neutrinos, White and his collaborators argued that the dark matter was probably “cold”, i.e., the initial velocity dispersion of the dark-matter particles relative to the overall expansion of the Universe was negligible. Some years later, Navarro, Frenk and White showed from N-body simulations that the density profiles of halos of cold dark matter are described remarkably well by a simple two-parameter empirical law now known universally as the NFW profile. In 1993 White, Evrard, Frenk and Navarro noted that the ratio of normal to dark matter in clusters of galaxies is much larger than the ratio expected based on the relative abundances of hydrogen and helium produced in the Big Bang, and assuming a critical density Universe dominated by dark matter. Their observation provided a strong argument for a lower density of dark matter, so that the majority of the mass-energy in the Universe is in the form of a cosmological constant, now often called dark energy.
The problem of determining the origin of galaxies and other structures in the Universe has occupied cosmologists for a century. The remarkable advances in our theoretical understanding, with White as a leader over several generations of increasingly realistic modelling, will form the foundation for even more sophisticated work in the decades to come, which will finally allow a detailed understanding of how these structures came to be. These outstanding achievements make White a fitting recipient for the 2017 Shaw Prize.
Recent precision observations of the cosmic background radiation and the spatial distribution of galaxies, the distribution of intergalactic gas, and many other phenomena have verified the validity of the cosmological standard model. Powerful telescopes and surveys are probing the predictions of White and his colleagues with unprecedented accuracy, and much of the scientific motivation of billion dollar ground- and space-based telescopes that are now being planned or are under construction is to further test our understanding of cosmic structure formation and what it tells us about the age, size, geometry, content, and origin of the Universe, and in addition what it tells us about fundamental physics.
26 September 2017 Hong Kong
