Kaiser and Silk will divide the $500,000 award, and each will receive a gold medal at a ceremony that will take place on June 28 at the CosmoGold conference at the Institut d’Astrophysique de Paris in Paris, France.
Although the two theorists have worked mostly independently of each other, their results are complementary in the two major areas of research for which they are receiving the Gruber Prize.
The first concerns the infant universe. In the 1920s astronomers found compelling evidence that the universe is expanding. The obvious question: expanding from what? Run the “film” of an expanding universe backward and you might arrive at a primordial fireball—an origin event that later acquired the nickname “big bang,” and one that would still be “echoing” in the form of relic radiation suffusing all of space. That theoretical possibility became physical reality in 1965 with the report of the astronomical detection of the cooled fossil relic of the primordial fireball radiation.
But that discovery presented a further challenge—one that Joseph Silk immediately seized, providing a potential method of investigation that, in turn, Nick Kaiser made possible.
The cosmic microwave background, or CMB, that is a baby picture of the universe cannot be entirely uniform; if it were, the early universe would not have contained the nascent features (small fluctuations in the average density) that, over the course of 13 billion years, would mature into the universe that astronomers observe today. In 1967 and 1968, Silk calculated that density fluctuations below a critical size corresponding to the mass of a typical galaxy would have dissipated due to diffusion of the primordial radiation. Confirmation of that prediction – now called ‘Silk damping’ – would have to wait however until space observatories could provide views of the CMB at a sufficient level of subtlety.
In the meantime, ongoing observations of the corresponding large-scale structure—the universe as we know it—were subject to conflicting interpretations, at least until Kaiser, in a series of papers beginning in 1984, provided the statistical tools that would allow astronomers to separate the “noise” from the data.
The so-called DEFW collaboration (Marc Davis, George Efstathiou, Carlos Frenk, and Simon D. M. White, who shared the Gruber Prize for Cosmology in 2011) exploited that methodology to map large-scale structures, while succeeding generations of space observatories provided the evidence for the matching primordial fluctuations at greater and greater levels of precision—first the Cosmic Background Explorer in 1992 (Gruber Prize, 2006), then the Wilkinson Microwave Anisotropy Probe (Gruber Prize, 2012), and finally the Planck satellite observatory (Gruber Prize, 2018).
The second area of cosmology that this year’s Gruber Prize honors is the study of dark matter—the so-far unidentifiable matter that cosmologists can detect indirectly through its gravitational influence on matter that they can observe directly. The DEFW collaboration used Kaiser’s statistical methodology to determine both the distribution of dark matter in the universe and its non-relativistic nature (that is, moving at a velocity not approaching the speed of light). Kaiser also devised a statistical methodology to detect dark matter distribution through “weak lensing”—an effect predicted by Einstein’s general theory of relativity in which a foreground galaxy magnifies the light from a background galaxy, thereby providing a measure of the mass of both. Today weak lensing is one of cosmology’s most prevalent tools.
In 1984 Silk proposed exploring the identity of dark-matter particles through their possible self-annihilations into particles that we can identify (photons, positrons, and antiprotons), a strategy that continues to drive research around the world.
Both Kaiser and Silk are currently affiliated with institutions in Paris, Kaiser as a professor at the École Normale Supérieure and Silk as an emeritus professor and a research scientist at the Institut d’Astrophysique de Paris (in addition to a one-quarter appointment at The Johns Hopkins University). Each has made any number of other significant contributions to their field, but their work on the CMB and dark matter has truly revolutionized our understanding of the universe.
INSU press release of June,27 2019 : http://www.insu.cnrs.fr/fr/cnrsinfo/remise-du-prix-gruber-de-cosmologie-2019-nicholas-kaiser-et-joseph-silk
Press article in Libération of June 26, 2019 : https://www.liberation.fr/debats/2019/06/26/univers-cosmique-de-situation_1736389