RYAN COOKE AND MAX PETTINI RECEIVE $500,000 GRUBER COSMOLOGY PRIZE

The 2025 Gruber Cosmology Prize recognizes Ryan Cooke and Max Pettini both for their determination of a key value in the composition of the universe moments after it came into existence and for perfecting the method that allowed them to make that measurement.

Image: Photo of Ryan Cooke (left) and Max Pettini (right) the 2025 Gruber Cosmology Prize Winners. Credit: IAU / The Gruber Foundation

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 honours 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 capitalising 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 assistance 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!"