Cooke and Pettini will equally share the $500,000 award and each will receive a gold laureate pin at a ceremony that will take place later this year. The citation honors them for “bringing the light element abundances and Big Bang Nucleosynthesis (BBN) into the realm of precision cosmology.”
BBN is a theoretical model of the nuclear reactions in the first few minutes of the universe’s expansion. One way to identify the composition within that primordial cauldron is to measure the ratio of deuterium (an isotope of hydrogen that has one neutron and one proton in the nucleus) to hydrogen. That D/H ratio correlates to the density of regular matter, or baryons, in the mass-energy recipe of the universe. (The rest is in the form of dark matter and dark energy.) Cooke and Pettini determined the D/H ratio, and then the baryon density, to an accuracy of one percent.
What’s more, their measurement of the baryon density is in excellent agreement with the percentage derived via a separate method of identifying the composition of the universe. Whereas the BBN method examines the universe when it was just a few minutes old, the alternative method looks at the universe 378,000 years later, when the first light in the universe left an all-sky lasting imprint upon space—what cosmologists call the Cosmic Microwave Background, or CMB. (The principal investigators of, and the teams behind, three successive and increasingly precise measurements of the CMB received Gruber Cosmology Prizes in 2006, 2012, and 2018.)
Cooke, a professor at Durham University’s Centre for Extragalactic Astronomy, and Pettini, a professor of observational astronomy at the University of Cambridge’s Institute of Astronomy, were capitalizing on a method that the cosmologist Thomas F. Adams proposed in 1976. Adams suggested that the absorbing clouds commonly seen in the spectra of quasars would be a promising location to measure the primordial abundance of deuterium. Quasars are supermassive black holes that emit prodigious amounts of radiation and can therefore be seen at large distances and (because the speed of light is finite) at very early times in the history of the universe. In this technique, the quasar acts simply as a background light source revealing tenuous material in front of it (as seen from Earth), unrelated to the quasar itself.
Ground-based telescopes powerful enough to observe those quasars, however, didn’t come online for another two decades. Pettini began collecting data on quasars for other research purposes at the Keck Telescopes in Hawai’i and the Very Large Telescope in Chile, but not until 2009 did he partner with Cooke, one of his PhD advisees at Cambridge, and began a project to determine the primordial D/H ratio with percent precision.
They weren’t the only astrophysicists using the quasar method of detection in an attempt to derive the D/H ratio, but previous efforts had produced divergent results. The collaboration between Cooke and Pettini, however, refined the methodology. Their research focused only on “near-pristine” clouds of gas —galaxies relatively free of stars and therefore also relatively free of the element-creating and deuterium-destroying processes attending star birth. Because these galaxies were chemically unevolved, the conditions therein would resemble the conditions in the primordial universe.
In 2018, Cooke and Pettini (with an assist from Charles Steidel, recipient of the 2010 Gruber Cosmology Prize) published the results from a sample of seven quasars, all of which agreed (within the margin of error) on the D/H ratio. That ratio in turn allowed them to calculate that baryons constitute about 5 percent of the mass-energy density of the universe—a result that not only closely matched the results from the CMB method but validated the BBN method as a tool for performing precision cosmology.
“This,” one Gruber Prize nominator wrote, “is definitely something worth shouting about from the rooftops!”
Additional Information
In addition to the cash award, each recipient will receive a gold laureate pin and a citation that reads:
The Gruber Foundation is pleased to present the 2025 Gruber Cosmology prize to Ryan Cooke and Max Pettini for bringing the light element abundances and Big Bang Nucleosynthesis (BBN) into the realm of precision cosmology. By finding and selecting the most pristine quasar absorption-line systems in the high-redshift Universe, unaltered by star formation, and by leveraging the capabilities of some of the largest ground-based telescopes, Cooke and Pettini obtained a one percent measurement of the primordial deuterium to hydrogen (D/H) ratio. This meticulous work has made possible a BBN-based determination of the baryon density of the Universe with precision comparable to that of the Cosmic Microwave Background determination, enabling important consistency tests of early-time physics between t = 1 s and t = 400,000 years.
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The Cosmology Prize honors a leading cosmologist, astronomer, astrophysicist or scientific philosopher for theoretical, analytical, conceptual or observational discoveries leading to fundamental advances in our understanding of the universe.
Laureates of the Gruber Cosmology Prize:
2024 Marcia Rieke, for pioneering work on astronomical instrumentation to reveal the breadth and details of the infrared universe
2023 Richard S. Ellis, for contributions in galaxy evolution, onset of cosmic dawn and reionization in the high redshift universe, and detection of earliest galaxies via the Hubble Ultra Deep Field study
2022 Frank Eisenhauer, designed instruments that collected evidence for a black hole at the center of our galaxy
2021 Marc Kamionkowski, Uroš Seljak, and Matias Zaldarriaga, for contributions to methods essential for studying the early universe
2020: Lars Hernquist and Volker Springel, for computer simulations that revolutionized the study of processes behind the structure of the cosmos
2019: Nicholas Kaiser and Joseph Silk, revolutionized cosmology with contributions to two of its vital components: dark matter and relic radiation from the Big Bang
2018: The Planck Team, Jean-Loup Puget and Nazzareno Mandolesi, for measuring the universe’s contents and the geometry and test inflation with unparalleled precision
2017: Sandra M. Faber, for a body of work that has helped establish many of the foundational principles underlying the modern understanding of the universe on the largest scales
2016: Rainer Weiss, Kip Thorne, Ronald Drever, and the entire LIGO team, for a first detection of gravitational waves that emanated from the collision of two black holes
2015: John Carlstrom, Jeremiah Ostriker, and Lyman Page, for their individual and collective contributions to the study of the universe on the largest scales
2014: Jaan Einasto, Kenneth Freeman, Brent Tully and Sidney van den Bergh, for pioneering contributions to the understanding of the structure and composition of the nearby Universe
2013: Viatcheslav Mukhanov and Alexei Starobinsky, for contributions to inflationary cosmology and the theory of inflationary perturbations of the metric, which changed our views on the origin of our universe and on the mechanism of formation of its structure
2012: Charles Bennett and the WMAP Team, for their exquisite measurements of anisotropies in the relic radiation from the Big Bang---the Cosmic Microwave Background
2011: Marc Davis, George Efstathiou, Carlos Frenk, Simon White, pioneering use of numerical simulations to model and interpret the large-scale distribution of matter in the Universe
2010: Charles Steidel, for his groundbreaking studies of the distant Universe
2009: Wendy Freedman, Robert Kennicutt and Jeremy Mould, for the definitive measurement of the rate of expansion of the universe, Hubble's Constant
2008: J. Richard Bond, for his pioneering contributions to our understanding of the development of structures in the universe
2007: Saul Perlmutter and Brian Schmidt and their teams: the Supernova Cosmology Project and the High-z Supernova Search Team, for independently discovering that the expansion of the universe is accelerating
2006: John Mather and the Cosmic Background Explorer (COBE) Team, for studies confirming that our universe was born in a hot Big Bang
2005: James E. Gunn, for leading the design of a silicon-based camera for the Hubble Space Telescope and developing the original concept for the Sloan Digital Sky Survey
2004: Alan Guth and Andrei Linde, for their roles in developing and refining the theory of cosmic inflation
2003: Rashid Alievich Sunyaev, for his pioneering work on the nature of the cosmic microwave background and its interaction with intervening matter
2002: Vera Rubin, for discovering that much of the universe is unseen black matter, through her studies of the rotation of spiral galaxies
2001: Martin Rees, for his extraordinary intuition in unraveling the complexities of the universe
2000: Allan R. Sandage and Phillip J. E. (Jim) Peebles, Sandage for pursuing the true values of the Hubble constant, the deceleration parameter and the age of the universe; Peebles for advancing our understanding of how energy and matter formed the rich patterns of galaxies observed today
The International Astronomical Union partners with the Foundation on the Prize and nominates the members of the Selection Advisory Board that chooses the Prize recipients. Its members are:
Jeremy Butterfield, University of Cambridge; Mihalis Dafermos, Princeton University; Luis Ho, Kavli Institute for Astronomy and Astrophysics at Peking University (Chair); Jean-Loup Puget, The National Centre for Scientific Research (CNRS); Suzanne Staggs, Princeton University; Véronique Van Elewyck, Université Paris Cité; Licia Verde, Universitat de Barcelona. Wendy Freedman of The University of Chicago and Martin Rees of The University of Cambridge also serve as special Cosmology advisors to the Foundation.
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The Gruber International Prize Program honors individuals in the fields of Cosmology, Genetics and Neuroscience, whose groundbreaking work provides new models that inspire and enable fundamental shifts in knowledge and culture. The Selection Advisory Boards choose individuals whose contributions in their respective fields advance our knowledge and potentially have a profound impact on our lives.
The Gruber Foundation was established in 1993 by the late Peter Gruber and his wife Patricia Gruber. The Foundation began its International Prize Program in 2000, with the inaugural Cosmology Prize.
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For more information on the Gruber Prizes, visit www.gruber.yale.edu, e-mail info@gruber.yale.edu or contact A. Sarah Hreha at +1 (203) 432-6231. By mail: The Gruber Foundation, Yale University, Office of International Affairs, PO Box 208320, New Haven, CT 06520.
Media materials and additional background information on the Gruber Prizes are in our online newsroom: https://gruber.yale.edu/news-media
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