2025 Gruber Cosmology Prize

The collaboration that would eventually receive the 2025 Gruber Cosmology Prize coalesced over the course of a short car ride.

In early 2009 Max Pettini, an astronomer at the University of Cambridge’s Institute of Astronomy, delivered a lecture on cosmology. Afterward he offered Ryan Cooke, a PhD candidate in the same department, a lift back to the astronomy building. Pettini was already supervising a PhD candidate and therefore wasn’t actively seeking another advisee. For his part, Cooke was already happily working with another supervisor on his own PhD project. Yet shortly after that conversation, Pettini agreed to supervise Cooke.

They bonded over common research interests and what Cooke calls “a very similar ‘attention-to-detail’ approach to science”—precisely what they would need to pursue a project that Pettini was already considering. Since the 1970s cosmologists had understood that if they could somehow determine the ratio of deuterium (an isotope of hydrogen that has one neutron and one proton in the nucleus) to hydrogen from the first few minutes  of the universe’s existence, they could infer the proportion of “regular” matter—protons and neutrons, or what physicists call baryons—in the mass-energy census of the cosmos.

The technology for making that measurement wasn’t available until the advent of supersized telescopes in the 1990s and 2000s. Even so, early efforts by several collaborations to derive the deuterium-hydrogen ratio had arrived at somewhat divergent results. Cooke and Pettini, however, followed a narrower approach to the problem—one that depended on overcoming a series of observational challenges. As Pettini says, “We were working at the edge of what’s possible.”

As was the case with earlier efforts by other astrophysicists, Cooke and Pettini would be observing quasars—extremely powerful outpourings of radiation from black holes at the centers of galaxies. The radiation from a quasar would carry information (technically, absorption lines) identifying the chemical composition of gas clouds fortuitously located in front of the quasar (as viewed from Earth). 

Then, as was not the case with many   earlier efforts, Cooke and Pettini decided to target only quasars that, by chance, intersect “near-pristine” clouds of gas—galaxies relatively free of star formation. Those systems, they reasoned, wouldn’t have undergone the evolutionary processes that produce heavier and heavier elements, so their components would still reflect the conditions that emerged from the primordial cauldron. 

Cooke and Pettini would also have to thread an observational needle by identifying the few, very rare, absorbing clouds of unprocessed gas out of a myriad of other absorbers unsuitable for this experiment. 

Finally, their targets would also have to lie at an ideal distance. The radiation they wanted to find had emerged in the highly energetic, ultraviolet portion of the electromagnetic spectrum—a regime that can’t penetrate Earth’s atmosphere. But the expansion of space would have stretched those wavelengths into the visible range when the universe was between 2 billion and 3 billion years old. (It’s 13.8 billion years old today.)

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 gas clouds. (As one Gruber nominator remarked, “the small number further demonstrates how delicate an experiment this is.”) Those results provided a precise measure of the abundance of deuterium resulting from the earliest nuclear reactions—what cosmologists call Big Bang Nucleosynthesis. That ratio in turn allowed them to calculate that baryons constitute about 5 percent of the mass-energy recipe of the universe.  (The rest is in the form of dark matter and dark energy.) 

What’s more, that 5 percent density closely matched the result that cosmologists have derived from studying the Cosmic Microwave Background, an all-sky “photo” of the earliest  photons, dating to about only 378,000 years after the Big Bang. (The principal investigators and the teams behind three previous, increasingly precise, measurements of the CMB received Gruber Cosmology Prizes in 2006, 2012, and 2018.)

For that reason, the Gruber Prize citation honors Cooke and Pettini for “bringing the light element abundances and Big Bang Nucleosynthesis (BBN) into the realm of precision cosmology.”