While Peebles didn’t refer to the recently proposed inflationary era, his new model fit the Zeitgeist perfectly. It invoked a massive particle that could arise from fundamental physics, connecting inner and outer space. The cold dark matter model, or CDM model for short, was adopted by a growing army of astronomers and physicists who began to work out the fine details of how galaxies would actually form. Marc Davis at Berkeley allied himself with two British astronomers, George Efstathiou and Simon White, and the Mexican astronomer Carlos Frenk to build computer models to follow the formation of individual galaxies and clusters of galaxies in virtual universes. In their simulations, this gang of four, as they became known, would follow hundreds of thousands of particles as they interacted with one another, coming together to form the large-scale structure of the universe.
While CDM was popular and eagerly adopted, too many things seemed to go wrong. In the CDM model Peebles created, the universe could be only 7 billion years old, which was far too young. Astronomers had found dense pockets of stars known as globular clusters bobbing around in galaxies. These bright concentrations of light were full of old stars that must have formed early on in the history of the universe when it was mostly full of hydrogen and helium, which meant that the globular clusters had to be at least 10 billion years old. And there was more. If the universe was primarily made up of cold dark matter, the proportion of dark matter to atoms would be roughly 25 to 1. Yet, hard as they looked, astronomers couldn’t figure out where that dark matter was. From the speed at which galaxies rotated or from the temperature of clusters of galaxies they observed they could infer how much gravity there was (the hotter they were, the more gravitational pull there had to be) and how much dark matter was necessary to generate that amount of gravity. The ratio of dark matter to atoms they kept on coming up with was closer to 6 to 1. True, the methods for weighing the dark matter were still crude and uncertain, but the deficit seemed too great to be explained by the margin of error. Almost immediately after creating the CDM model, Peebles felt compelled to give it up and look for alternative models. “There was a lot of net casting in the eighties and early nineties,” as he put it.
The gang of four didn’t fare any better. They used their computer models to create virtual universes and compared them with the real universe to see if they looked alike. They didn’t. For a start, the real universe appeared to be much more structured and complex on large scales than the fake universes. In the CDM universe, the galaxies were much more clustered on small scales but smoothed out more quickly, once you zoomed out to look at the bigger picture, than in the real universe. It was possible to alleviate some of the problems in the virtual universes by slightly fudging the results, but the truth was that Peebles’s simple model wasn’t entirely working.
Despite the fact that it conflicted with basic observations, the cold dark matter model was embraced by the majority of astronomers and physicists. It was conceptually simple and fit nicely with inflation and the evidence for dark matter in galaxies. CDM’s adherents looked for ways to further develop and somehow fix the model. One way of fixing the CDM involved resurrecting Einstein’s cosmological constant. To many, that was anathema.
The case against the cosmological constant had become stronger since Einstein first introduced it in 1917. While he had, with the discovery of the expanding universe, rapidly discarded the cosmological constant from his theory, a few of his colleagues clung to it. Both Eddington and the Abbé Lemaître chose to incorporate it in their models of the universe. Lemaître went so far as to conjecture that the cosmological constant was nothing more than the energy density of the vacuum. In 1967 Zel’dovich showed what a serious problem the cosmological constant could be. He added up the energy of all the virtual particles that would pop in and out of existence in the universe and found that the resulting energy density would look like a cosmological constant but should have a truly gigantic value. Strictly speaking, the resulting cosmological constant would be infinite, for exactly the same reasons that everything involving quantum gravity was infinite, but a little hand waving could make it finite. Even so, it was a huge number, orders of magnitude greater than any energy that had been measured in the cosmos.
Zel’dovich’s calculation showed that if there was an energy of the vacuum in the universe—and therefore a cosmological constant—it would be far too big to be compatible with observations. The only way to proceed was to assume that some as-yet-undiscovered physical mechanism intervened to make the cosmological constant equal zero. In practice cosmologists chose to ignore the cosmological constant and pretend it didn’t exist.
Yet, again and again, whenever anyone tried to resolve the problems with the cold dark matter model, the cosmological constant, known as lambda, always cropped up as one of the possible solutions. In 1984, Peebles himself found that a viable universe with cold dark matter would need lambda to make up about 80 percent of the total energy of the universe. When the gang of four—Davis, Efstathiou, Frenk, and White—tried simulating one of their universes with lambda in it, they found that many of the problems they came up against with the simple CDM scenario went away.
In 1990, George Efstathiou, then at the University of Oxford, published a paper in Nature called “The Cosmological Constant and Cold Dark Matter.” In it, Efstathiou and his collaborators compared the large-scale structure from a simulated universe, including the cosmological constant, with the real universe, this time using a catalogue with millions of galaxies that they had collected over the years. In their opening salvo, they claimed, “We argue here that the successes of the CDM theory can be retained and the new observations accommodated in a spatially flat cosmology in which as much as 80% of the critical density is provided by a positive cosmological constant,” and they proceeded to show that such a universe seemed to fit all the observational data then available. Jerry Ostriker and Paul Steinhardt, one of the fathers of the theory of inflation, published a paper in Nature in 1995 where they argued that “a universe having critical energy density and a large cosmological constant appears to be favoured.” Everything seemed to point to lambda.
While hints of lambda were appearing in large-scale structure, everyone shied away. As Jim Peebles wrote in 1984, “The problem with the choice . . . is that it does not seem plausible.” As Efstathiou and his colleagues stated in the conclusion of their paper, “A non-zero cosmological constant would have profound implications for fundamental physics.” In another paper, George Blumenthal, Avishai Dekel, and Joel Primack from Santa Cruz in California argued that having a cosmological constant “requires a seemingly implausible amount of fine-tuning of the parameters of the theory.” Indeed, as Jerry Ostriker and Paul Steinhardt wrote, the observational evidence opened up an impossible challenge: “How can we explain the non-zero value of the cosmological constant from a theoretical point of view?” The problem couldn’t be kept a dirty little secret anymore.
At the Princeton meeting in 1996, Michael Turner from the University of Chicago faced a barrage of abuse as he sparred with Richard Gott and David Spergel in defense of the cosmological constant. The observations were in his favor, but the cosmological constant remained too unpalatable for his fellow cosmologists. It was too conceptually impossible and too aesthetically unpleasing. He probably would have gotten off more easily if he had called for divine intervention. At the end of the debate, the standard, cosmological-constant-free CDM model was declared the victor. Jim Peebles watched the spectacle in fascination.
By 1996, cosmology had been transformed beyond Jim Peebles’s wildest expectations. He had started off, along with Yakov Zel’dovich, Joe Silk, and a few others, as one of the lone pioneers building up the theory of large-scale structure. Peebles had effectively made up the techniques that were used not only to theorize but also to analyze observations. Now a new generation of theorists was pushing his ideas forward with alarming ferocity while the astronomers were mapping out the universe with ever-increasing precision.
In this new era, Pe
ebles found himself in the odd position of a contrarian in a field he had helped create. He disliked the fervor with which the CDM model had been adopted by his colleagues and continuously put forth new models to compete with it. But, as his mentor, Bob Dicke, had said, good data would trump all. CDM’s supporters and Peebles were both about to be trumped.
In 1992, George Smoot, one of the principal investigators on the Cosmic Background Explorer, or COBE for short, claimed, “If you’re religious, this is like looking at God.” COBE was a satellite experiment designed to detect the relic radiation left over from the Big Bang with unprecedented precision and to map how its brightness would change as you looked in different directions in the sky. What Smoot was talking about was the first-ever measurement of the elusive ripples in the relic radiation, the small imperfections that Peebles, Silk, Novikov, and Sunyaev had for the previous twenty-five years been saying should be out there. It had been a long and almost embarrassing search. As time passed and the ripples remained invisible, the theorists had reworked their predictions, downgrading their expectations. In 1992, the COBE satellite, using a set of detectors based on Bob Dicke’s ideas, made a map of the relic radiation, and there was a collective sigh of relief. Smoot went on to win the Nobel Prize for his work on COBE.
COBE’s discovery was just the beginning. The picture it provided of the ripples in the relic light was still blurred and unfocused. The ripples needed to be brought into focus, for, as Peebles, Novikov, and Zel’dovich had shown, there should be a rich tapestry of hot and cold spots in the relic light that could be used to chart out the geometry of space. If the geometry of space was truly Euclidean, the size of the spots should subtend an angle of about 1 degree on the sky. And measuring the geometry of space was tantamount, through general relativity, to measuring the amount of energy in the whole universe. Better experiments were needed. Dozens of groups throughout the world developed instruments that could measure the relic radiation with better precision and focus. It was as if a band of intrepid explorers had set out to chart a new continent that had just been discovered. When, at the turn of the millennium, it finally all came together, a clutch of experimenters announced the discovery that the hot and cold spots indeed had an angular size of about 1 degree, and therefore the geometry of space had to be flat. The result was just as inflation had predicted and further evidence from the large-scale structure of the universe for CDM and a cosmological constant.
The final piece of data that definitively tipped the balance in favor of the cosmological constant came not from the field of large-scale structure that Peebles had so lovingly built up but from exploding supernovae in the distant universe. The first hint came in January of 1998, at the annual meeting of the American Astronomical Society, when a West Coast–based team of astronomers and physicists called the Supernova Cosmology Project claimed that there wasn’t enough gravitational pull from dark matter or atoms to rein in and slow down the expansion of the universe. In fact, the Supernova Cosmology Project was finding that expansion of the universe was quite possibly accelerating. This meant that the universe was either much emptier than previously thought or had a cosmological constant that was driving space apart.
The Supernova Cosmology Project was to some extent simply repeating what Hubble and Humason had done in the 1920s: measuring the distances and redshifts of distant objects. Instead of looking at galaxies, the observers now had to look for individual supernovae, stars that exploded with an intense burst of light as bright as a whole galaxy concentrated in a pinprick, and that could be seen at far greater distances than ever observed by Hubble and Humason. While, in spirit, the work of the Supernova Cosmology Project echoed that of Hubble and Humason, this was no longer a two-person job, but a large operation with teams spread out over three continents using many Earth-bound telescopes as well as the Hubble Space Telescope to produce their numbers. The measurement methods were difficult and had taken over a decade to perfect.
The Supernova Cosmology Project was closely followed by the High-Z Supernova Search project, which was finding similar results: tentative evidence for accelerated expansion of the universe and, therefore, a cosmological constant.
Neither team could bring themselves to announce what they saw in their data. At the AAS meeting in Washington, in January 2008, their presentations were cautious, almost painfully so. The true implication of their results was quietly discussed in the corridors and made its way into the newspapers. The day after the announcements by the supernova teams, the write-up in the Washington Post said, “The findings also appear to breathe fresh life into the theory that there is a so-called cosmological constant.” A few weeks later, Science magazine went further, publishing an article with the title “Exploding Stars Point to a Universal Repulsive Force.” In the article, the leader of the Supernova Cosmology Project, Saul Perlmutter, refused to go so far, simply commenting, “This needs more work.”
Just over a month later, the High-Z team came clean and said it: there was lambda in their data. Not only was the universe too empty of atoms and dark matter, it was full of something else that was making it accelerate. Members of the High-Z team were invited on television around the globe to explain their strange, unfathomable results to the general public. CNN announced that scientists were “stunned the universe may be accelerating,” and the leader of the High-Z, Brian Schmidt, was quoted in the New York Times as saying, “My own reaction is somewhere between amazement and horror. Amazement, because I just did not expect this result, and horror in knowing that it will likely be disbelieved by a majority of astronomers—who, like myself, are extremely skeptical of the unexpected.” The SCP rapidly followed suit with its own results. It was official: lambda was out there. For their discovery the leaders of the two teams, Saul Perlmutter, Brian Schmidt, and Adam Riess, were awarded the Nobel Prize in 2011.
For years, even decades, there had been uncertainty about the universe’s makeup, age, geometry, and basic constituents. All the different proposals had their pros and cons, and cosmology had become as much a matter of aesthetics as science, with practitioners choosing their preferred theories according to personal taste. But now the most unpalatable theory of them all, the cosmological constant, had won out. Within months, a new standard model of cosmology, known as the concordance model, or unimaginatively “Lambda CDM,” had taken root. This new model of the universe contained a cocktail of atoms, cold dark matter, and a cosmological constant. It was the universe that large-scale structure had been hinting at for a decade but that hardly anyone had been ready to embrace. Even Peebles, with his unwillingness to follow the herd, was amazed at how everything had come together. But it was the data that had done it, exactly as his mentor had said it would. Peebles had to admit, “The best explanation for what the data is telling us is a cosmological constant. Or something that looks like a cosmological constant.”
When Jim Peebles retired from teaching at Princeton, in 2000, he spent more of his time going on walks and taking pictures of wildlife. He relished the beauty and sometimes strangeness of the birds that he would stumble across on his treks, and now he had more time to do so. Instead of focusing on the patterns that galaxies traced in the sky or the ways that individual galaxies spun, he could lose himself in the surrounding beauty of woods and forests. It was this careful gaze and attention to detail that had helped him oversee the transformation of cosmology into a hard, precise science. Yet another strand of general relativity had matured and gained a life of its own. Peebles’s quiet and persistent effort, his “scribbling,” as he liked to call it, had placed the study of the large-scale structure of the universe firmly at the center of physics and astrophysics. The maverick in him had guided the field toward the bizarre model of the universe that had taken root: a universe in which 96 percent of its energy was in some dark substances, a combination of dark matter and the cosmological constant. Compared to when he had started off, almost fifty years before, it was a surreal turn of events.
The cosmological constant wa
s now universally accepted. The fundamental problem remained: the gross inconsistency with what Zel’dovich had predicted from adding up the energy of the virtual particles in the universe and the value that was actually observed, a mismatch of over a hundred orders of magnitude. But while, in the past, this inconsistency had led cosmologists to not even consider the possibility of the cosmological constant, now they embraced it. It was there, in the data, unavoidable. In their textbook on relativistic astrophysics, written in 1967, Yakov Zel’dovich and Igor Novikov had said, “After a genie is let out of the bottle . . . legend has it that the genie can be chased back in only with great difficulty.” There was truth in this analogy. Now, with the general shift toward the concordance model, the cosmological constant had to be tackled head-on.
Or maybe not. One more effort to yet again avoid the cosmological constant invoked an altogether new type of stuff that was pushing space apart. This exotic new field, particle, or substance behaved very much like a cosmological constant, but it was soon widely referred to as “dark energy.” There were, and are, high hopes for dark energy and its potential to link the successes of observational cosmology with the creativity of particle physics and the quantum. Young and old cosmologists flocked to work on the topic in droves; in one talk at a conference, a speaker put up a slide with over one hundred different models for dark energy, a testament to the creativity of the new generation of cosmologists. And yet the invention of dark energy still didn’t solve the problem that Zel’dovich had raised, that the energy of the vacuum was, in principle, far too big to be acceptable. Once again, the approach was to pretend the discrepancy wasn’t there. It would take a revolution in the quantum theory of gravity to come up with a controversial solution.
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