The Perfect Theory (30 page)

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, Peebles 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 was 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.

The rise of physical cosmology in the past forty years transformed the way we look at spacetime and the universe. In mining general relativity on the grandest of scales and carefully teasing out the large-scale properties of the universe, Jim Peebles and his contemporaries opened up a completely new window on reality. Allied with the stupendous successes in mapping out the distribution of galaxies and the relic radiation, their work has revealed a bizarre universe, full of exotic substances that remain poorly understood. It is a far cry from the cosmology of the 1960s, a “pretty dismal” science, as Peebles called it, with just three numbers. Modern cosmology has been one of the great successes of Einstein's general theory of relativity and modern science as a whole, raising as many questions about the universe as it answers.