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A professor at the University of California, Santa Cruz, he was a key contributor to a landmark paper that laid out how the universe came to look like it does today.

Joel Primack in 2021. The paper that he and three colleagues published in Nature in 1984 provided a coherent story of how the universe evolved over billions of years after the Big Bang.Credit...Carolyn Lagattuta/UC Santa Cruz

Jan. 17, 2026, 9:57 a.m. ET

Joel R. Primack, a physicist who helped explain how tiny quantum fluctuations at the birth of the universe led to the formation of galaxies, galactic clusters and even larger structures spanning the cosmos, died on Nov. 13 in Palo Alto, Calif. He was 80.

The cause was pancreatic cancer, said his wife, Nancy Ellen Abrams.

As a professor of physics and astrophysics at the University of California, Santa Cruz, Dr. Primack was a key contributor to a landmark paper, published in the journal Nature in 1984, that laid out how the universe came to look like it does today.

Even at the time the paper came out, astrophysicists had already realized that ordinary matter — the particles that make up stars, gas clouds, planets, people — could account for only a small fraction of the mass in the universe.

Because the outer parts of galaxies seemed to be spinning too fast, there had to be more mass that could not be observed — what scientists called dark matter — to exert the gravitational pull to keep the stars from flying apart.

Scientists did not know what dark matter could be — and decades later, they still do not. But Dr. Primack, along with two colleagues at Santa Cruz, George Blumenthal and Sandra Faber, and Martin Rees at the University of Cambridge in England, figured out key properties of dark matter that were essential for producing what is observed in the night sky.

“That became part of this whole foundation that we’ve been using now for years,” said Saul Perlmutter, an astrophysicist at the University of California, Berkeley.

The paper published in Nature provided a coherent story of how the universe evolved over billions of years after the Big Bang. During a brief moment of rapid expansion known as cosmic inflation that occurred immediately after the birth of the universe, tiny quantum fluctuations produced a bit of lumpiness in the universe. The regions that were ever so slightly denser grew into clumps of dark matter, and the gravity of the dark matter pulled clouds of ordinary matter that then collapsed into galaxies.

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The Andromeda galaxy, seen in 2023. Dr. Primack and his co-authors figured out key properties of dark matter that were essential for producing what is observed in the night sky.Credit...Pedro Puente Hoyos/EPA, via Shutterstock

For this model to work, the dark matter had to be “cold,” consisting of larger particles that moved relatively slowly, Dr. Primack and his colleagues calculated.

“It was really pretty simple pencil and paper,” Dr. Faber recalled. “That’s all that we had available.”

Although Dr. Primack was listed third among the four authors, which usually indicates a smaller role, Dr. Faber said that “he actually conceived and directed the whole thing and wrote 90 percent of it. Think of him as the orchestra director who sort of got us all together.”

Subsequent work by Dr. Primack and others, harnessing the power of supercomputers, provided much more detail.

The numerical simulations “allowed us to make specific predictions” about the structure of the dark matter concentrations, how they grew and how they merged and grew together, said James Bullock, a graduate student of Dr. Primack’s in the 1990s who is now an astrophysicist at the University of Southern California.

Those predictions, Dr. Bullock added, could then be compared with astronomical observations.

Dr. Primack also played an influential role in the world of science policy. When he was a graduate student at Stanford, his thesis adviser was Sidney Drell, who was continually flying to Washington to make recommendations to President Lyndon B. Johnson on military technology and national security matters.

In an oral history interview with the American Institute of Physics, Dr. Primack recalled: “I would keep asking Sid, ‘How come this stupid Vietnam War is going on? You’re meeting with the president frequently, serving on his science advisory committee. Can’t you tell him how dumb this war is?’”

He said he did not receive much of an answer — Dr. Drell said the discussions were classified — but Dr. Primack nonetheless was inspired to undertake similar efforts. He and other Stanford students decided to organize workshops on political and social issues, including a course on how the federal government could do a better job of managing science and technology.

One eventual outcome was that the American Association for the Advancement of Science, a group based in Washington, began offering a fellowship for scientists and engineers to advise members of Congress on policy. The program, which began in 1973, continues today.

That era was one in which scientists were increasingly “open to rethinking their relationship with their larger responsibility for science and society, saying when things were nonsense,” said Frank von Hippel, a theoretical physicist who wrote about science policy with Dr. Primack in the 1970s.

“Joel was part of that, a leader of that,” Dr. von Hippel said.

Joel Robert Primack, the oldest of three siblings, was born on July 14, 1945, in Santa Barbara, Calif., and grew up in Butte, Mont., and Granada Hill, Calif. His father, Roy, was a district manager for a chain of shoe stores, and his mother, Loretta (Zabarsky) Primack, managed the home.

In the American Institute of Physics interview, he described his parents as “not particularly intellectual,” but said they encouraged his interests in science. He graduated from Princeton with a bachelor’s degree in physics as valedictorian of the class of 1966, completed his doctorate at Stanford in 1970 and received a postdoctoral fellowship at Harvard before joining Santa Cruz as a professor in 1973.

He began his career studying particle physics — the tiniest bits of matter — but eventually grew bored of it and pivoted to astrophysics as new data offered the possibility of answering fundamental questions about the universe.

With his wife, a philosopher, Dr. Primack taught a course at Santa Cruz called “Cosmology and Culture” and wrote a book, “The View From the Center of the Universe” (2006), which explored the place of humans in the cosmos.

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Dr. Primack and his wife, Nancy Ellen Abrams, a philosopher, collaborated on a course at Santa Cruz and on a 2006 book about humans’ place in the universe.Credit...Penguin Publishing Group

In addition to his wife, whom he married in 1977, Dr. Primack is survived by a daughter, Samara Bay; and a grandson.

He retired from teaching in 2014, taking emeritus status, but his cosmological research continued. In recent years, Dr. Primack worked on the structure of early galaxies and why they were in the shape of what he called “cosmic pickles” instead of spheres or discs.

Images from the Hubble Space Telescope, which launched in 1990, first suggested the unexpected shape, and dark matter again provided an explanation. Dark matter is distributed along filaments in weblike structures spanning the cosmos. Thus, clouds of gas were attracted along the filaments, producing the elongated shapes. (Other astrophysicists have described them as bananas, cigars or surfboards instead of pickles.)

“And as the gas rotates, the stars form primarily in the pickle,” Dr. Primack said in an interview in 2020. As he predicted, the James Webb Space Telescope, launched the next year, has seen these early galaxies in greater detail.

Kenneth Chang, a science reporter at The Times, covers NASA and the solar system, and research closer to Earth.

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