Physicist James Peebles on the Charm of the Unknown

The esteemed Princeton professor has spent a lifetime chasing ideas about the birth of the universe. And now, he has a Nobel Prize to show for it.

Princeton’s James Peebles’s theories laid the groundwork for what the Nobel committee called our understanding of the universe’s history since the Big Bang. Photo by Frank Veronsky

He drifted through high school. By his own admission, he entered college in a state of mind that could best be described as “out to lunch.” He decided to major in engineering because he liked to take things apart and put them back together again, but he changed his major when something else struck him as potentially more interesting. 

Parents of underachievers, take heart: The slacker in question is James Peebles, Albert Einstein Professor of Science, Emeritus at Princeton and a winner of the 2019 Nobel Prize in Physics. The scientific community, not to mention the world at large, owes him a debt of gratitude for his decision to switch majors—to physics, as it happens. In doing so, Peebles, now 84, set in motion events that would lead him to transform the field of cosmology and create a theoretical framework for what the Nobel committee described as “the foundation of our modern understanding of the universe’s history, from the Big Bang to the present day.”

The universe might never have gained its prime historian but for the guidance of Robert Dicke, Peebles’s advisor at Princeton, which the Winnipeg, Canada, native attended after graduating from the University of Manitoba.

It was Dicke’s suggestion that Peebles go into the then fledgling field of cosmology, the study of the genesis and evolution of the universe. Peebles had his doubts. The level of evidence in the field at the time, he says, “was so meager that I thought I might be chasing after wrong ideas.” 

Still, Peebles was intrigued by the notion of a universe that, at its birth, was spectacularly hot and dense and then proceeded to expand and cool, as Georges Lemaitre had posited in 1931 in what became known as the Big Bang theory. For someone intrigued by the inner workings of things, the idea was intellectual catnip. “It led me to think of thermonuclear reactions,” Peebles says. What’s more, the scientific puzzles facing him “would be fun to work out.” 

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That was in the early 1960s, when the Big Bang theory wasn’t universally accepted and, in fact, hadn’t yet been proven to any degree. Under Dicke’s leadership, Peebles was part of a group of Princeton researchers seeking hard evidence of the Big Bang in the form of radiation that should, if the theory was correct, permeate the universe. They were in the process of constructing instruments to detect that cosmic afterglow when a pair of researchers at Bell Labs, Arno Penzias and Robert Wilson, stumbled upon the so-called cosmic background radiation (CBR). The Princeton group may have been scooped, but Peebles would take Wilson and Penzias’s discovery and ride it to the beginnings of space and time, and to the farthest reaches of the expanding universe.

In 1970, Peebles and a graduate student, Jer Yu, predicted a set of temperature fluctuations for the CBR that should they ever be verified, would stand as even stronger proof that the universe had begun in a super-hot, super-dense state. He was well aware that the prediction might be flawed or even out-and-out wrong.  But for Peebles, the process of asking questions appears to be nearly as exciting as arriving at answers. 

“I did occasionally pause to think I might be barking up the wrong tree,” he says, “but that didn’t bother me too much.”

In the early 1990s, in yet another of Peebles’s experiments, the satellite known as COBE, or Cosmic Background Explorer, finally yielded an accurate measurement of the spectrum of the cosmic background radiation, indicating that Peebles’s calculations had been correct.

He remembers the moment he learned the results. “Before the formal announcement,” he says, “my friend David Wilkinson drew me aside and pulled out of his pocket a piece of paper and showed it to me.” On the paper, says Peebles, was COBE’s confirming measurement, “and it was magnificent.”

* * *

Between that prediction and its vindication, Peebles moved on to another critical chapter in the universe’s history, the formation of galaxies. In 1982, he published a paper theorizing that galaxies coalesced in cocoons of dark matter, the mysterious substance that’s believed to comprise 85 percent of the universe. He wasn’t entirely sold on the theory, and when it became increasingly popular, he began to suggest it might not hold water. When in 1998 the theory was shown to be, as Peebles says, “bang on,” he was pleased—and a little surprised.

“For a decade or so, I’d been pestering people with the thought that that theory wasn’t the only possibility, and I’d ask why they weren’t paying attention to other possibilities,” he remembers. “Now I had to withdraw that rude question.”

In fact, Peebles’s willingness to be wrong is likely one of the things that garnered him his reputation as a brilliant theoretician—that, and the fact that he was so often right. Robert Wilson, the researcher who’d bested Dicke, Peebles and others at the discovery of the CBR, calls him “the person I would most trust for solid information from the theory side.” 

Indeed, Peebles’s research provided the measurements that moved cosmology from speculation to hard science, as Sean Carroll, a theoretical physicist at Caltech, recently observed on Twitter. “More than any other person,” Carroll tweeted, “he made physical cosmology into a quantitative science.”  

As a teacher and author, Peebles also affected generations of students. He didn’t regard teaching as a chore that pulled him away from research, but as a way to hone his understanding of the subjects he taught. “Teaching teaches not only the students,” he says, “it also teaches the teacher, and physics is so full of complications that you can’t in your career have thought of everything.”

He also found that the challenge of explaining the science to students helped his own comprehension of it. Paraphasing Samuel Johnson’s famous observation, Peebles says “nothing concentrates the mind like the prospect of being hanged,” equating Johnson’s noose with “being asked a question you can’t answer.”  

* * *

In June, Princeton University Press will publish Peebles’s memoir of a life in science. In early November, that book sat in the center of his Princeton-office desk in the form of a mountain of paper. “I’ve read through this stupid pile of edited manuscript several times, page by page,” he said, eyeing it warily as if it might suddenly develop a voice and demand more readings. 

Although he may not love the process of creating a book, Peebles clearly relishes the science it describes. “I love physics because it’s neat and it’s orderly in its own peculiar way,” he says. He wants the rest of us to love physics, too, and in fact, all of science—or if not love it, then at least afford it the respect it deserves. 

“There is one fact that I think the public needs to know about science,” he says. “We often say that our science is objective and accurate, but we don’t often say that our science is incomplete—that although the established parts of natural science are very well tested and the evidence makes a compelling case for things being as they’ve been described, there nevertheless are open questions that we cannot answer.”

Two of those open questions involve the nature of dark matter and its cohort, dark energy, which physicists believe could be responsible for the fact that the universe isn’t just expanding, but expanding at an ever-increasing rate. Peebles notes that the mass and energy we can readily observe—the stuff that makes up and enlivens you and me, the rivers and trees, and all the distant and expanding galaxies—is characterized by “a rich, rich complexity.” He’s betting that same complexity will extend to the dark sector as well. 

It’s a pretty good bet. If you take our present theories of how galaxies formed and you factor in what we surmise about dark mass and energy, you end up with something impressively close to what we see in the universe. And yet that result differs, Peebles explains, “in what seem to be fascinating ways, as if there’s something happening in the dark sector that we don’t quite understand.” Typically, that inconsistency—along with all the questions it raises—doesn’t faze him at all.  “To me,” he says, “it’s a really charming notion.” 

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