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 195
0s, 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.
In 1979, Stephen Hawking, along with a South African relativist named Werner Israel, put together a survey of relativity to celebrate Einstein’s centenary. They brought together the leading researchers in cosmology, black holes, and quantum gravity. Bob Dicke and Jim Peebles contributed an essay titled “The Big Bang Cosmology—Enigmas and Nostrums.” It was a short essay. In a few pages, Dicke and Peebles laid out what they believed to be some fundamental problems in an incredibly successful theory.
So what was wrong? For a start, the universe seems far too smooth. Although there had been attempts to come up with an explanation in the past, Dicke and Peebles couldn’t to their satisfaction identify one that worked. And there was more. Why does the geometry of space, as opposed to that of spacetime, look so simple? The geometry of space seems to have no overall curvature, and the rules of high-school-level Euclidean geometry apply. Such rules as Parallel lines never intersect and The sum of the angles of a triangle is 180 degrees seem invariably true. A universe with no spatial curvature is allowed in general relativity, but it is a very special case. Einstein’s equations predict that the evolution of the universe is likely to push the curvature away from zero incredibly quickly. So, if the universe seems to have almost no curvature today, it must have had even less curvature in the past. The universe we live in is extremely unlikely. Finally, the galaxies and structures built up of galaxies spanning the heavens must have come from somewhere. Conditions had to be perfectly tuned for the universe to look as it does today. At the Big Bang, the tendency of the universe to expand had to be just enough to compensate for the pull of gravity and prevent the whole of spacetime from collapsing into itself, yet not so extreme that spacetime would fly apart in an empty void. Their article boiled down to a simple question: What happened in the very beginning?
Dicke and Peebles’s article was followed by another short essay by Yakov Zel’dovich. In his article, Zel’dovich pondered the very early universe following the line of reasoning that the Abbé Lemaître had first taken when discussing his primordial atom. There was a whole plethora of interesting phenomena at play in the hot early universe that could impact its evolution and affect how it evolved into what we see today. Zel’dovich urged the community of particle physicists and relativists to figure out what these effects would be.
Dicke and Peebles’s and Zel’dovich’s papers were prescient. Just one year later, cosmology would be turned on its head by a simple proposal for how the early universe evolved. The idea had been floating around in an unformed way, but it took Alan Guth, a postdoc at the Stanford Linear Accelerator Center, to come up with the essence of cosmic inflation. Guth realized that in some grand unified theories—theories that attempted to unify the electromagnetic, weak, and strong forces into one overarching force—the universe could be trapped in a state in which the energy of one of the fields was incredibly high and dominated everything else. In that state, the universe would be driven to expand rapidly, or inflate, as Guth dubbed it. Although Guth’s original idea turned out to be flawed—if the universe was trapped in such a state, there was no way of getting out of it—new ways of making the universe inflate were quickly proposed by others.
The idea of an inflating universe, or inflation, opened up a new avenue in cosmology, revealing a new period in the universe’s past that could be explored. Now there was a theory that predicted exactly how the universe should be when structure started to form, and it seemed to address the problems raised by Dicke and Peebles. For a start, the theory of inflation pushed space to almost instantaneously have no curvature. Imagine taking a round balloon that you can hold in your hands and using a giant pump to blow it up so quickly that it almost instantly becomes the size of the Earth. From your perspective, the piece of balloon in front of you would now look pretty flat. Inflation would also drive the universe toward a tremendously smooth, pristine state. Any large lumps or voids that would naturally pepper the landscape of spacetime would have been pushed far out into the distance, invisible to our gaze. Inflation also brought with it a way to kick-start the growth of structure in the very early universe. During the period of intense inflation, the microscopic quantum fluctuations in the fabric of spacetime would be stretched and imprinted onto the largest scales.
Inflation, as astrophysicists in Chicago succinctly put it, established the link between “inner space and outer space.” Inner space was the world of the quantum and the fundamental forces, and outer space encompassed the cosmos, where general relativity came into its own. And so, the program of research that Peebles had been developing over the previous decade, along with the work of Zel’dovich, Silk, and others, took on a new purpose: the large-scale structure of the universe, the distribution of galaxies, and relic light should hold the clues that link inner and outer space. People began to take notice.
In 1982, Peebles tried to construct a new universe. The old model he’d developed with Jer Yu, made of atoms and radiation, wasn’t working out. When he compared the results of his model to the surveys of galaxies that had been mapped out in the sky, they didn’t match. Reality simply didn’t agree with his elegant calculation. Not only that, in the previous decade, galaxies themselves seemed to have become a whole lot more complicated. A strange picture was emerging of what was going on inside them.
The American astronomer Vera Rubin had found that galaxies seemed to spin far too quickly for their own good, like manic Catherine wheels held together by a mysterious force. Rubin focused her telescope on the Andromeda Galaxy, a swirl of stars and gas spinning at hundreds of kilometers per second. At least that’s how it appears if you look at it with a telescope. There was much more light at the center where all the stars are concentrated, so Rubin expected that most of the gravitational pull keeping the galaxy together would come from its central core. But as she looked at nuggets of stars farther and farther away from the center of the galaxy, she found they were moving far too quickly. In fact, the stars were speeding around so quickly that Rubin simply couldn’t understand how the gravitational pull of the galaxy’s center could rein them in. It was as if the Earth suddenly doubled or tripled the speed of its orbit around the sun. Unless the sun somehow increased its gravitational pull, the Earth would simply fly out of the sun’s orbit and shoot off into space. Something else, big and invisible, was holding the outer stars in their orbits.
Fritz Zwicky observed a similar phenomenon in the 1930s, but his results were ignored for almost forty years. Zwicky had looked at the Coma cluster of galaxies and added up the total amount of mass he could see there. He had t
hen measured the speed with which the galaxies were moving around inside the cluster and found that they were moving far too quickly. As he said in a paper he published in Switzerland in 1937, “The density of luminous matter in Coma must be minuscule compared with the density in some sort of dark matter.”
Jim Peebles was coming up against his own problems with galaxies. With a young collaborator from Princeton, Jerry Ostriker, he set about building simple computer models for how galaxies formed, representing them as a bunch of particles pulling each other through gravity and spinning around in a spiral. But whenever he set his models spinning, the galaxies would disintegrate. A blob would form at the center that would stretch out through the arms and tear the galaxy apart. Ostriker and Peebles tried to stabilize their models by immersing their spinning particles in a ball of invisible mass. This sphere of stuff—a halo, they called it—would bolster the gravity keeping the galaxy together. The halo had to be dark (that is, invisible) so as not to be detected by telescopes. Paradoxically, the model showed that this dark matter had to be much more abundant than the atoms that were seen in stars. In the late 1970s, Sandra Faber, working at Santa Cruz in California, and Jay Gallagher, working in Illinois, wrote a review in which they collated the odd findings that astronomers were getting when looking at galaxies and that Peebles and his colleagues were discovering when simulating them. They concluded that “we think it likely that the discovery of invisible matter will endure as one of the major conclusions of modern astronomy.”
In 1982, when Peebles began building a new model of the universe, he decided to include atoms and dark matter. In fact, he assumed that almost all of the universe was made up of a mysterious form of matter composed of heavy particles, invisible to us because it didn’t interact with light. Peebles’s cold dark matter model was simple, and it enabled him to predict what the distribution of galaxies looked like and how large the ripples in the relic radiation would be. This approach would prove to have a momentous impact on the development of cosmology, but as Peebles recalled, “I didn’t take it at all seriously . . . I wrote it down because it was simple and it could fit the observations.”
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