The Perfect Theory (28 page)

When we look out at the universe, we see an elaborate tapestry of light, with galaxies clumped into clusters, filaments, and walls, leaving large voids of emptiness. It is rich, full of information and complexity. Where does this large-scale structure of the universe come from? This was the most pressing question for the conference attendees, for the answer was still completely up for grabs, and the conference organizers dedicated a full afternoon to the topic. J. Richard Gott, a tall, gangly astronomer from Princeton with a deep and slow southern drawl, stood up and defended common sense. At a first glance the universe looks very empty, so Gott proposed a universe almost completely devoid of matter that slowly evolved to form a tapestry of galaxies and clusters of galaxies that would populate the night sky. Another young and energetic astronomer from Princeton named David Spergel proposed that the universe is not at all empty, but rather full of an invisible, dark form of matter. Spergel's dark matter would be made up of some fundamental particle unaccounted for in the standard model of particle physics that had not yet been observed in any experiment. But it was the final speaker, Michael Turner, a sharp-witted theoretical cosmologist from Chicago, who made the most outlandish proposal of the afternoon: Why not assume that the universe is permeated by the energy of a cosmological constant? In Turner's universe, about two-thirds of the overall energy would be accounted for by the constant Einstein had so firmly rejected almost seventy years before. The crowd was not impressed with Turner's proposal.
Anything but a cosmological constant
—it was Einstein's biggest blunder.

Chairing the gladiatorial combat between the universes was Phillip James (Jim) Peebles, then the Albert Einstein Professor of Science at Princeton University. A tall, slim man with a thoughtful face lifted from a portrait by Modigliani, Peebles was the consummate gentleman, courteously moderating the debate. While he was careful to the keep the conversation on track, he would sometimes chuckle with almost childish glee at the jibes and comments being thrown across the stage. The Critical Dialogues meeting was partly organized to celebrate Peebles's sixtieth birthday, a fitting tribute. For the previous three decades, Peebles had been the prime architect of the theory of large-scale structure of the universe at the heart of modern cosmology.

In the early 1970s, Jim Peebles published a slim volume,
Physical Cosmology,
a summary of a set of graduate lectures he gave at Princeton in 1969. John Wheeler had attended, taken notes, and, according to Peebles, bullied him into publishing the lectures. In the introduction to
Physical Cosmology,
Peebles briefly mentioned the cosmological constant, saying that
“the cosmological constant Λ [the Greek capital letter “lambda,” which is the mathematical symbol for the cosmological constant] is seldom mentioned in these notes.” For Peebles, the constant was an unnecessary complication, “the dirty little secret” of cosmology. Everyone knew that the mathematics allowed for it, but because it made the physics too bizarre and troublesome, everyone pretended it wasn't there. Now, a quarter of a century later, despite being reviled by the majority of Peebles's colleagues, the cosmological constant was about to make a comeback. It would do so with a vengeance.

 

When Jim Peebles arrived in Princeton in 1958, fresh out of engineering school at the University of Manitoba, he found John Wheeler and his crew chipping away at black holes and the final state. Wheeler was not the only acolyte of general relativity at Princeton; there was also Robert Dicke. Like Wheeler, in the mid-1950s, Dicke realized what dire straits Einstein's theory was in, with little or no progress being made in testing it. He created his own gravity group at Princeton, where general relativity could be discussed and, most important, measured and tested.
“Rather quickly in my career I got into orbit around Bob and into doing things that were exciting,” Peebles says. He joined Dicke's team as a PhD student and, after graduating, focused his research on testing gravity physics. He would stay in Princeton for the next fifty years.

In the 1960s, Peebles recalls, cosmology was still “a limited subject—a subject, as it used to be advertised, with two or three numbers,” and, Peebles says, “A science with two or three numbers always seemed to me to be pretty dismal.” There were few people actively working in the field, and very little research was under way. This suited Peebles just fine. He could devote himself privately and quietly to tackling problems that took his fancy at his own pace. Having completed his PhD on quantum physics, from then on Peebles devoted himself to fleshing out cosmology. He started with what his colleagues at Princeton called the “primeval fireball,” working out what actually happened to atoms and nuclei in the very early universe when it was hot and dense. He worked like a craftsman. Shut away in his office, he filled page after page with handwritten equations, slowly going over his calculations and honing his approach.

Peebles's mentor took a different approach. As Peebles recalls, “To him physics was certainly theory but it had to lead to an experiment that could be done in the near future,” so Dicke had his team look for the relic radiation left over from the primeval fireball. They developed a new form of detector that could scan the sky from the roof of the physics building, but they didn't find the radiation in time. One Tuesday in late 1964, Dicke's team was sitting in his office for their weekly meeting when the phone rang. Dicke picked up the phone and spoke to someone for a few minutes. “We've been scooped,” he said when he put the phone down. Arno Penzias had just called to tell him that, with Robert Wilson at Bell Labs, he may have just found evidence for the relic radiation. Within months Dicke and his team had confirmed the result from Bell Labs, but it was too late: Penzias and Wilson would go on to win the Nobel Prize on their own.

To Peebles, there was something wrong with the picture of the cosmos that appeared in 1960s physics textbooks. At the time, there were two completely different topics. On the one hand there was the history and evolution of the universe, the story that Friedmann and Lemaître had told. It explained how space, time, and matter evolved on the largest possible scales. On the other hand there was the stuff the astronomers looked at, galaxies and clusters of galaxies. While these galaxies are part of the universe, their presence seemed almost superficial and unconnected to the fundamental development and structure of the universe, like rich, colorful swirls of light painted on spacetime. It was true that galaxies told us a lot about the universe, such as how fast the universe was expanding and how much stuff it actually contained. But, looking up at the sky, Peebles felt that there had to be more to galaxies—he was convinced they must play a key role in the evolution and large-scale structure of the universe, and surely their own origin must be connected to it as well. They couldn't have appeared out of nothing, great blobs of light, gas, and stars dropped into spacetime as an afterthought. This meant that galaxies must also play a role in Einstein's general theory of relativity. The question was how. This was a perfect challenge for Peebles:
a difficult, open problem that hardly anyone wanted to work on.

The role of gravity in individual galaxy formation is obvious. A collection of matter collapses under the pull of its own gravity. If there's enough matter, and it has enough kinetic energy to avoid collapsing below a certain point, the resulting blob becomes a galaxy, reined in by its own gravitational pull. What was less clear when Peebles approached the topic was how the gravitational effects in individual galaxy formation related to gravity's role in the expansion of the universe as a whole. The Abbé Lemaître had pointed out that there must be a connection, and the Russian theorist George Gamow had mused on how galaxies would form in an expanding universe, but neither could provide a proper calculation to back up their speculations. In 1946, Evgeny Lifshitz, one of Lev Landau's disciples, had taken Einstein's field equations and attempted to link what happened on the scale of the universe with the much smaller scale of individual galaxies. His result hinted at how the large-scale structure of the universe would emerge—small ripples in spacetime would evolve and grow, following his equations, and galaxies would end up forming and clustering in regions of high curvature to create the large structures
that can be observed today.

When Peebles worked out how atoms and light would have behaved in the early universe, he realized this new understanding of the hot early universe might explain how galaxies formed shortly after the Big Bang. When Peebles put in some rough estimates for the age of the universe, the density of atoms, and the temperature of the relic radiation, he found that collapsed structures
could
form with masses between a billion and hundreds of thousands of billions times that of the sun, just like the Milky Way. As Gamow had previously surmised, the early universe appeared to be an ideal breeding ground for galaxies.

As Peebles continued to figure out the details of how galaxies formed, he was not alone. A young PhD student at Harvard named Joseph Silk argued that the collapsing blobs that would ultimately form galaxies should also leave an imprint on the primeval fireball—a faint patchwork of hot and cold regions in the relic radiation that had recently been discovered by Penzias and Wilson. Silk's results were echoed by Rainer Sachs and his student Arthur Wolfe at Austin, who found that even on the largest scales, the relic radiation would be affected by the gravitational collapse of all the matter in the universe. Yakov Zel'dovich's team in the Soviet Union was also finding the same thing. Their results indicated that by looking at the ripples in the relic radiation left over from when the universe was a few hundred thousand years old, it would be possible to see the first moments that led to the formation of galaxies. In a scattered and disjointed way, Gamow and Peebles's physical cosmology was beginning to bear fruit.

Peebles wanted to explain the expansion of the universe—the hot beginning, the primeval fireball, the atoms, the gravitational collapse—in terms of basic textbook physics, combining general relativity, thermodynamics, and the laws of light. With a PhD student from Hong Kong named Jer Yu, Peebles wrote out the complete set of equations that would allow him to work through the evolution of the universe from the earliest moments after the Big Bang until today. Peebles's universe starts off in a smooth, hot state with a very small set of ripples disturbing the primordial slush of gas and light. As these disturbances evolve, they encounter pressure from the messy, sticky plasma of free electrons and protons. The universe vibrates with waves like a rippling pond until the moment electrons and protons combine to form hydrogen and helium. Then the next stage begins: atoms and molecules start to clump together, collapsing under the pull of gravity, creating nuggets of mass and light scattered throughout spacetime. These are the galaxies and clusters of galaxies that emerged from the hot Big Bang.

In Peebles and Yu's universe, the way that galaxies are scattered in space to form the large-scale structure of the universe should carry with it the memory of the universe's hot beginning. The relic radiation left over from the Big Bang, which Penzias and Wilson had measured to have a temperature of just 3 degrees Kelvin, should carry an echo of the small ripples that seeded the formation of galaxies. By solving the equations of the universe in one consistent, coherent whole, Peebles and Yu found a new, powerful way of studying Einstein's theory of general relativity: look at how galaxies are distributed in space to form the large-scale structure of the universe and use it to discover how spacetime began and evolved.

It was a powerful, compelling narrative, but Peebles and Yu's results were met with silence.
“No one paid any attention to our paper,” recalls Peebles. In bringing together the different areas of physics, Peebles and Yu had wandered into an intellectual no man's land. Their work wasn't strictly astronomy, nor was it general relativity or fundamental physics. The lack of response was fine by Peebles. He continued working on the universe, occasionally roping in the odd student or young collaborator, but for the most part quietly and peacefully calculating away on his own.

Now that Peebles had a model of the universe, he needed to look at some data to see if he was on the right track. In the early 1950s, the French astronomer Gérard de Vaucouleurs, based at the University of Texas, had looked at a particular catalogue of over a thousand galaxies, the Shapley-Ames Catalogue, and found a “stream of galaxies” stretching across the sky, bigger than any cluster, more like a “supercluster” or “supergalaxy.” His work was not well received. Walter Baade, a Caltech astronomer, dismissed the result, saying, “We have no evidence for the existence of a Super Galaxy,” as did Fritz Zwicky, who simply asserted,
“Superclustering is nonexistent.” Peebles was skeptical about de Vaucouleurs's result, but as one of his students recalls, Peebles would echo the view of his mentor, Bob Dicke, that “good observations are worth more than another mediocre theory.” So he set out on a quest to map out the large-scale structure himself, with his protégés, sometimes with surprising results. When Marc Davis and John Huchra, both young researchers at Harvard, found that indeed there were immense structures in the far crisper surveys of galaxies they were producing, Peebles was “flabbergasted.” As he acknowledged, “I wrote some pretty vitriolic papers with examples in the past of how astronomers had been misled by just this tendency . . . to pick patterns out of noise. It was clear you needed a pattern forming mechanism.” But with time, he realized that galaxies were indeed arranged in a vast tapestry of walls, filaments, and clusters, what became known as the cosmic web. The large-scale structure that Peebles had predicted in his computer models was beginning to emerge in the real world.