The 2021 Gruber Cosmology Prize recognizes Marc Kamionkowski, Johns Hopkins University, Uroš Seljak of the University of California at Berkeley and Lawrence Berkeley National Laboratory, and Matias Zaldarriaga of the Institute for Advanced Study, for their contributions to methods essential for studying the early universe.
Kamionkowski, Seljak, and Zaldarriaga will equally share the $500,000 award, and each will receive a gold medal at a ceremony that will take place in August at the 24th International Conference on Particle Physics and Cosmology (COSMO'21) conference series at the University of Illinois and online. The citation honors contributions to cosmology that "have been essential to the development of this field of research over the last twenty-five years."
The award is given through the Gruber Foundation, which was established at Yale to honor and encourage excellence in the fields of cosmology, genetics, neuroscience, justice, and women's rights.
Many of the techniques developed by these three physicists focus on the cosmic microwave background (CMB), the relic radiation that dates to the era of decoupling -- when atoms and radiation emerged from the developing universe's primordial plasma and went their separate ways. The atoms remain in the form of the matter in the universe, and the radiation remains in the form of a background "noise" that pervades space in every direction, providing an image of the universe when it was only 379,000 years old. That same "baby picture" of the universe, however, serves as an observational limit.
As a key part of this work, Kamionkowski and, independently but simultaneously, Seljak and Zaldarriaga found a mathematical means to use the radiation from the CMB to infer what's on the other side, all the way back to the first fraction of a fraction of a second of the universe's existence. That method is polarization -- the degree to which an oscillating wave, bouncing up and down relative to the direction of travel, diverges from a strictly perpendicular orientation.
The two papers appeared side by side in the journal Physical Review Letters in 1997. (Kamionkowski shared authorship with Albert Stebbins, from Fermilab, and Arthur Kosowsky, of the University of Pittsburgh.) The impact of the papers was seismic, the reverberations enduring.
By observing polarization in the CMB, cosmologists can match theoretical predictions of early-universe properties using data that would otherwise be inaccessible. Among the observatories that have used polarization to make those measurements are the WMAP and Planck satellites (whose principal investigators and teams received the Gruber Cosmology Prize in 2012 and 2018, respectively). Those measurements have allowed cosmologists to determine that the universe is 13.8 billion years old and comprises roughly 5% ordinary matter, 26% dark matter, and 69% dark energy.
Yet as powerful as polarization has proven to be in probing the early universe, it is now inspiring a new generation of research programs that will detect -- or not -- the final piece in that reigning cosmological model: inflation, a theoretical moment at the very beginning of the universe's existence when space would have undergone an expansion of almost unfathomable proportions. (The theorists who independently created that idea, Alan Guth and Andrei Linde, received the 2004 Gruber Cosmology Prize.) The two 1997 papers -- by Kamionkowski, and Seljak and Zaldarriaga -- identified a signature in the CMB polarization that would render a verdict on the existence of primordial gravitational waves, a key prediction of inflationary theory.
Although the 1997 papers are two of the seminal works by Kamionkowski, Seljak, and Zaldarriaga, the 2021 Gruber Prize also recognizes their career-long contributions to cosmology.
Even before writing their own 1997 paper, Seljak and Zaldarriaga had already created a code that made the study of certain aspects of the CMB thousands of times faster -- in fact, they named it CMBfast. And because they made the code available for free, it dominated CMB research for years to come. (Today it endures as the foundation for every code analyzing CMB polarization.) Over the decades, singly and together, they have continued to perform influential mathematical analyses and to create new theoretical interpretations in such areas as inflation, gravitational waves, and the use of general relativistic effects (weak lensing) on light from distant sources to infer the characteristics of the dark matter in individual galaxy halos as well as in the large-scale structure of galactic formation.
Kamionkowski is also well known for his work on cosmological-parameter determination and parity-breaking effects in the CMB, and he has similarly over the years made important subsequent contributions to the study of CMB polarization. He also has significant work in multiple other areas of cosmology, including dark matter, inflation, the acceleration of the expansion of the universe, cosmic phase transitions, and gravitational waves.
As for those two 1997 papers: In the words of one nominator to this year's Gruber Prize, "The significance of this work for cosmology cannot be overstated."
For more information, visit The Gruber Foundation's website, send e-mail to info@gruber.yale.edu; contact A. Sarah Hreha at 203-432-6231; or write to: The Gruber Foundation, Yale University, Office of Development, PO Box 2038, New Haven, CT 06521.
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