The Laureate: Why Jim Peebles *62 Got the Nobel Prize

When the phone rang at 5 a.m., Jim Peebles was already awake, but still in his nightclothes.

“Are you Phillip James Edwin Peebles?” asked the voice on the other end. Hearing an affirmative, the voice said, “We are awarding you the Nobel Prize in Physics. Do you accept this prize?”

An easy question for a man who has spent his life trying to answer hard ones.

“Who was that?” asked his wife of 61 years, Alison. He told her he’d won the Nobel. 

“Good God,” she said. 

What followed from that Oct. 8 phone call was a tumultuous, joyous, demanding period for a man comfortably retired for nearly two decades.

“This has been craziest week of my life,” Peebles said when I visited him in his Jadwin Hall office eight days after he got the call. “You understand this is disruptive of our usually tranquil, quiet life.”

When you’re 84 years old and have been on emeritus status since 2000, a Nobel Prize arrives with supernova intensity. Peebles, the Albert Einstein Professor of Science, emeritus, has won many awards — including the Shaw Prize and the Crafoord Prize, two of the world’s biggest. But the Nobel is many orders of magnitude larger in its repercussions. He was immediately bombarded with interview requests, invitations to give lectures, and emails from former students, some of whom had been out of contact for decades. 

When I first asked to interview him, he demurred — “I’m totally swamped.” But I pressed the case and he obliged, granting 90 freewheeling minutes in which we covered a territory spanning more than 60 years of life in Princeton and roughly 13 billion years of cosmic evolution. 

The decision to give Peebles a Nobel (he shared it with two astronomers who discovered the first planet around a distant sun-like star) was met with what appears to be universal, table-thumping acclaim across the broader physics community. 

“It was so obvious that he should get it,” said Princeton physics professor Jo Dunkley. “He’s done everything in cosmology.”

The Nobel committee doesn’t give lifetime achievement awards — officially the prizes go for specific discoveries — but Peebles assumes the honor reflects his long tenure in the field. “I have been working in this subject since 1964, 55 years, and during that time I’ve given endless lectures all over the world,” he said. “I think pure longevity might be at the root of the whole thing.”

“He essentially invented modern cosmology. He was there at the beginning,” said Princeton astrophysicist David Spergel ’82. Spergel played a key role in NASA’s Wilkinson Microwave Anisotropy Probe — a space-based instrument, named after Peebles’ Princeton contemporary, physicist David Wilkinson, that examined the cosmic microwave background radiation. When it came time to present the results, Spergel told me, he had a simple message for everyone: “The thing we learned is that Jim Peebles is right.”

Physicist Michael Turner, who recently retired from the University of Chicago, said Peebles “took cosmology from a time when it was the province of astronomers, and maybe there were five or 10 of them around the world, to the present where cosmology is an activity done by thousands of physicists and astronomers. He really has been the dominant figure in the field.”

Peebles still conducts research — his chalkboard is loaded with equations dealing with galaxy formation — but he doesn’t publish as much as he used to. He’s a sage now, a sounding board. He’s the guy in the physics department who plays the role of skeptic when someone comes up with a shiny but overly fragile new theory.

“My children remember him as the guy who’s like Dumbledore,” said Princeton physicist Bill Jones. “I think that’s very apt. He’s in every way brilliant and benevolent.”

When Peebles arrived at Jadwin Hall that Tuesday morning, freshly laureated, his colleagues had already gathered, and greeted him with sustained applause. Peebles had to make an impromptu speech. 

“It is astounding to think that nature operates by rules that we can discover, by trial and error, of course,” he said. He spoke of Einstein’s general theory of relativity, which is central to cosmology but remained largely untested when Peebles first began pondering the mysteries of the universe. In fact there was only one precision test back then, involving the orbit of the planet Mercury. But theorists believed the theory should apply at cosmological scale. 

“That used to haunt me. Why in the world would you imagine that a theory that is tested on the scale of the solar system would apply to the universe? It is absolutely startling, yet true, very well checked,” he said.

And then he said something very Peebles-like about the standard model of cosmology — the consensus narrative of the origin, composition, and evolution of the universe — which he played a key role in developing. 

“I’m still hoping it’s wrong,” he said.

Meaning: He hopes that the work of cosmologists is not complete, that there will be new discoveries, new theories, a deeper understanding of the nature of the universe. 

“Just the variety of theories we’ve discovered so far, and their incompleteness, leads me to wonder if there is such a thing as a complete theory,” he told me. “Or is there just a succession of approximations all the way down?”

When I visited Peebles, he’d begun to settle down after his wild and wonderful Nobel week. He’d had lunch alone that day, in the faculty dining room at Prospect House, his only companion The New York Times. Early that morning he had walked to work, as always, as he lives just a mile away, east of campus. At one point during our interview he got a call from his wife, Alison, who was concerned that a pelting rain had arrived. He told her not to worry; he could still walk home. “I have an umbrella,” he assured her. I’m reporting this trivial detail only to dispel any notion that winning a Nobel Prize means you start traveling everywhere by limo.

When he walks to work, he said, his mind wanders. I asked him if that’s good or bad. Good, he said: “I think it’s important that your mind wanders. It’s kind of like being asleep. Odd things occur to you.”

“Just the variety of theories we’ve discovered so far, and their incompleteness, leads me to wonder if there is such a thing as a complete theory. Or is there just a succession of approximations all the way down?”

— James Peebles *62

The universe is the ultimate odd thing: inconceivably more immense than we knew a century ago, and shockingly dynamic, the whole thing expanding at an accelerating rate. For a theorist strolling to work, the most entrancing fact of the universe is that it obeys mathematical equations. As Peebles said on Nobel day: Nature operates by rules we can discover. It was Newton who realized that the fall of the apple from the tree was governed by the same force that guides planets around the sun. A mortal mind, just a few pounds of flesh, can crack cosmic codes. 

When I asked Peebles if he was humbled by the vastness of the universe in comparison to his own tiny slice of spacetime, he said no — to the contrary, it’s empowering. In the old days his colleague Rubby Sherr *38 always used to say to him, “Isn’t that neat?” And that’s how Peebles thinks of the universe. It’s really neat! 

He was born in Manitoba, Canada. His father worked for a grain exchange and his mother was a homemaker. Peebles remembers that his father was good with his hands and liked to build things. “I always loved to watch him do it, and I loved to build things on my own. I was never exposed as a kid to any real science,” he told novelist and physicist Alan Lightman ’70.

So he seemed on a path to be an engineer. Astronomy? Astrophysics? Cosmology? He wasn’t even interested in the stars.

 “I remember admiring the starry skies over the plains of southern Manitoba, and aurora, but took no particular interest in the names of planets or stars or constellations. I still admire the sky, and my career has fed on what astronomers are doing, but I still cannot get interested in where the planets are,” he wrote in an essay titled “Seeing Cosmology Grow,” published in 2012 in the Annual Review of Astronomy and Astrophysics.

He went to college at the University of Manitoba, where he met Alison, who was studying microbiology. As he tells the story, his career path was externally determined. An influential professor at the university, Kenneth Standing, simply informed Peebles that he would go to Princeton to study physics as a graduate student. And that’s what Peebles did. He and Alison married and then arrived in Princeton in the fall of 1958. He would receive his doctorate from Princeton in 1962 under the guidance of physics professor Robert Dicke ’39. 

Dicke embraced the big bang theory, which held that the universe had emerged from a hot, dense state billions of years ago, expanding and cooling. The theory was not universally accepted. The great British physicist Fred Hoyle, for example, promoted a “steady-state” universe. Hoyle coined the term “big bang,” which was meant to be derisive.

“It’s a terribly inappropriate term,” Peebles told me. “A ‘bang’ connotes an event in a position in spacetime. The theory describes not an event, but the evolution of the universe as it expands from high density and temperature to the present. There was not a position as you would associate with a ‘bang.’ The universe everywhere was expanding. ‘Big’ is OK. ‘Bang’ is silly.”

But the name stuck and lost its patina of derision.

One implication of the theory fascinated Dicke: The universe should be saturated with radiation. It should be detectable and measurable. What scientists believe today is that the infant, expanding, cooling universe had a transitional stage in which photons (light) decoupled from matter and the cosmos became transparent. Dicke believed it would be possible to find the cosmic microwave background radiation — the CMB.

Peebles works in his office in Jadwin Hall days after winning the Nobel Prize.

Photo: Ricardo Barros

“Bob’s invitation to think about theoretical consequences of finding or not finding this radiation set my career,” Peebles wrote in his 2012 essay. “I was uneasy about the slight empirical basis for cosmology, but could think of a few interesting things to work out, which led to others, which led to the realization that this was fertile ground.”

What happened next is perhaps the most famous moment in the history of Princeton physics. 

During World War II, while at MIT, Dicke had invented a radiation detector called a Dicke radiometer. He had designed it as a tool to help win a war, but two decades later he thought it could be used to detect the universe’s “fossil” radiation. Dicke tapped Princeton colleagues David Wilkinson and Peter Roll to be the experimentalists, and they set up a contraption inside an unused pigeon coop that was then on top of Guyot Hall. The fourth member of the team was Peebles, serving as the theorist. 

Peebles was aware that scientists at other institutions might also be searching for the background radiation, and he asked Wilkinson if it would be acceptable to mention the Princeton effort at a symposium in Maryland. No problem, Peebles was told: We’re so far ahead of everyone, no one will catch us. So Peebles let the cat out of the bag. 

Unbeknownst to the Princeton scientists, two researchers at Bell Labs in Holmdel, N.J., Arno Penzias and Robert Wilson, were also using a Dicke radiometer, but at a different wavelength and in hope of studying radiation from within our own galaxy. There was mysterious noise in their data, coming from all points of the sky. They tried to account for its origin and could not. Then Penzias and Wilson learned of the Princeton effort. Suddenly it all became clear. They called Dicke.

Peebles was in the room when the phone rang, and remembers Dicke speaking for just a few minutes, repeating what he was hearing from the Bell Labs team. Then Dicke hung up the phone and said, “Well, boys, we’ve been scooped.”

They could have been crushed by the news. They weren’t. Peebles said the team was relieved to learn that they weren’t on a wild goose chase. There was something there, something detectable. And the faint radiation offered the first observational support of the theory of a hot early universe. 

The research by Dicke, Wilkinson, Roll, and Peebles was well along when Penzias and Wilson announced that they’d found the CMB. As a result, both teams published papers in the same journal at the same time, referencing each other. It was a double-whammy event, theory and observation arriving arm in arm. The discovery became the cornerstone of what Peebles would spend his career doing — crafting theories about the origin, composition, and evolution of the universe that could be pushed through the fine filter of experiment and observation. 

“ I think it’s important that your mind wanders. It’s kind of like being asleep. Odd things occur to you.”

— James Peebles *62

Bitterness over the “scoop” came much later. In 1978, the Nobel committee awarded its physics prize to Penzias and Wilson, but not to Dicke. It seemed odd, even capricious. 

“Three was a no-brainer: Penzias, Wilson, Dicke,” Peebles told me.

Why didn’t they give it to Dicke?

“I think they just blew it.”

In his essay, Peebles recalls teaching a graduate course in cosmology in 1969 as his colleague, the great physicist John Wheeler, sat in on the lectures and took notes. Then he’d give Peebles the notes at the end of the lecture, urging him to write a book collecting all his wisdom. And so Peebles did, authoring Physical Cosmology, a textbook for this young, emerging field.

Many of his students went on to careers in science. Andrew Appel ’81, now a Princeton professor of computer science, took Peebles’ class on quantum mechanics for two semesters in his junior year and persuaded Peebles to become his thesis adviser. Appel asked Peebles to suggest a topic involving computer programming. Peebles told him to find a way to “make his many-galaxy gravity simulations go faster,” Appel recalls. “It was a great experience. In his relaxed and low-key way, he told me the physics I needed to know and let me figure out the computer science for myself.”

Spergel says Peebles taught him quantum mechanics, statistical mechanics, and thermodynamics, and was renowned for his dry sense of humor. “We recorded his jokes and tried to duplicate his style,” Spergel said.

When not in the classroom, Peebles played a key role in assembling the standard model of cosmology. The standard model is generally referred to as λCDM, with the Greek letter lambda out front. 

Lambda is the “cosmological constant,” which has a bizarre history. Albert Einstein’s original 1915 theory of general relativity implied that the universe should be either contracting or expanding, while he assumed that the universe is static. In a moment of desperation in 1917, he plopped lambda into his equations — a fudge factor, basically, which counteracted gravity and stabilized the cosmos. 

 But the universe is not, in fact, static. In 1929, astronomer Edwin Hubble announced that galaxies are moving away from one another, and their recessional velocity increases with distance. There followed many decades of debate, often acrimonious, about the rate of the cosmic expansion — the Hubble Constant — and even today there are contradictory measurements that have roiled the field. It’s not clear if there’s an error in one of the measurements or if there needs to be a major revision in the consensus theory of how the cosmos evolved. 

But however that gets resolved, the existence of some kind of repulsive gravity — it’s often labeled dark energy — seems to have passed observational tests and is central to the reigning theory of cosmic evolution.

Then there’s the CDM: cold dark matter. It’s what galaxies are mostly made of, as opposed to ordinary matter, which is a small component. The cold dark matter emits no radiation (it’s cold and dark) and has a ghostly presence, its existence inferred from its gravitational effects. 

The standard model posits that only about 5 percent of the universe is in the form of ordinary matter. Roughly 25 percent or so is cold dark matter. The rest, about 70 percent, is dark energy. 

The standard model came together in chunks, over the course of many years. Peebles showed me an influential paper he published in 1982 that discussed the role of dark matter in shaping galaxies. Two years later he said there has to be a lambda — a cosmological constant — to reconcile everything observed about the universe. (Spergel provided an example: Without the lambda, galaxies would be moving much faster.)

Peebles didn’t go around touting the standard model as gospel truth. To the contrary, he kept looking for alternatives.

“I was very uneasy that people were accepting lambda CDM so readily, and I spent much of my time inventing alternative viable theories. I was startled by the late 1990s at how well lambda CDM was working. And I’m still startled,” he said.

Even as a Nobel laureate, Peebles is unpretentious. He doesn’t default to jargon or give the impression that his interlocutor should know more about physics. 

When we talked about the Dicke oversight, he said something that resonated: “The Nobel committee members are human beings.” 

Peebles is familiar with human frailty and tragedy: His daughter Margaret Ellen Peebles ’84, a writer, died a year ago after a long struggle with alcoholism. He and Alison have two surviving daughters, Lesley ’82 and Marion. 

The Nobel Prize comes not only with a substantial monetary award (his share of the prize will be about $910,000) but also the requirement to give a speech in Stockholm. Peebles had to prepare that even as he proofed the pages of a book he’s written on the history of cosmology, aimed at the scientific community and to be published by Princeton University Press.

I asked him why he was honored this year and not years ago. He didn’t venture a guess. Another quirky thing about this Nobel was that he shared it with two observational astronomers. Isn’t that apples and oranges — a theorist and two star-gazers? He doesn’t think so. All three people in this Nobel clutch are explorers, looking into deep space, he said. “I feel comfortable with that connection,” he said.

It was 5:30 and raining hard as Peebles headed outside to go home, leaving his nice blue blazer, which he described as a rare element of his wardrobe, safe and dry in his office packed with a lifetime’s ideas and a chalkboard full of galactic mass density equations. As we walked down the stairs — even in his 85th year Peebles doesn’t take an elevator — the Dicke matter burbled up again, and Peebles said his mentor was never bitter about being denied the Nobel. But Peebles said he — Peebles — was bitter about it for a long time. 

I suggested he bring it up in his speech in Stockholm, graciously. 

“It’s awkward,” he said. 

After a firm handshake, and with a warm smile, he headed out into the rain with his sensible umbrella. 

Joel Achenbach ’82 is a reporter at The Washington Post.