Epilogue

THE TRUMPET fanfare began, and then the procession. Up the center aisle in a Cambridge University combination room—what's called a common room everywhere else—the leaders of the two supernova teams that discovered evidence for dark energy marched in a line of dignitaries. Across a courtyard were the rooms Newton had occupied as a student. Nearby was the observatory where Eddington had plotted the eclipse expedition that validated Einstein's general relativity. At many of the scientific conferences the setting didn't matter, but it did on this occasion: the conferring of the 2007 Gruber Prize in Cosmology. Ten years after noticing something strange in the supernova data, Saul Perlmutter and Brian Schmidt, as well as the entirety of the High-z and SCP collaborations, were beginning to go down in history.

They already had posterity. Whenever the discovery of evidence for cosmic acceleration appeared in a peer-reviewed journal, it would forevermore be accompanied by two citations: Riess, A. G., et al. 1998, AJ, 116, 1009; Perlmutter, S., et al. 1999, ApJ, 517, 565. But for these recipients of the Gruber Prize in Cosmology, the award ceremony at Cambridge wasn't only about posterity. It was about history, and history was something else. History was posterity in motion.

Schmidt had reached a compromise with the Gruber Foundation regarding the recognition of Adam Riess: The honor would go to everyone—all fifty-one members of the High-z and SCP collaborations. The two teams would split the $500,000 prize; Schmidt and Perlmutter would each get half of his team's $250,000, and the remaining $125,000 would be divided among the team members. After taxes, the individual awards would amount to maybe $2,000 each, but thirty-five members paid their way to the ceremony in Cambridge. It was probably the first time that so many of them had been in one place, and it might be the last. Perhaps fittingly for a commemoration of a universe that was mostly missing, even an absence was present: Schmidt and Perlmutter included in their joint lecture a PowerPoint slide that recognized the often-overlooked contribution of the Chilean supernova search in the early 1990s—including the name and photograph of Jose Maza, the mentor to Mario Hamuy who in 1995 withdrew from the program.

"Our teams, certainly in the U.S., were known for sort of squabbling a bit," Schmidt had said at a press conference in London the day before the awards ceremony. "The accelerating universe was the first thing that our teams ever agreed on," he added, and Perlmutter, standing beside him, laughed. The two of them had extensively discussed in advance how to present a united front, and they had collaborated closely on the choreography of the weekend. For the lecture they gave the day after the ceremony, they worked out a tag-team routine; they took turns narrating the history of modern cosmology, sometimes finishing each other's sentences.

The history of modern cosmology. A history of something that would have been philosophically laughable to Jim Peebles in 1964, or professionally risky to Michael Turner in 1978, or physically dubious to any number of scientists pre-COBE. Perlmutter and Schmidt were themselves as young as the universe—the one that popped into existence over the course of a phone call in 1965, when theory met observation and a phenomenon in the heavens matched a calculation on paper. Yet already that cosmology had become a commonplace.

When the anniversary year arrived, the celebrations, not surprisingly, often reflected on the past. "Let's just pause for a second and think how amazingly lucky we are, the stage cosmology has reached," John Peacock, from the University of Edinburgh, said at the spring 2008 Space Telescope Science Institute symposium, "A Decade of Dark Energy." "A poor soldier who died in the trenches in 1914 knew as much about the universe as a caveman." That infantryman lived in a cosmos that was as vast as the stars, but no vaster, and stood still. In the past century, however, our knowledge had grown from one island universe to hundreds of billions of galaxies, from eternally repetitive motions in space to structural evolution over time. And now we even had one more more: darkness.

For this reason, the celebrations also often looked not only at how far we'd come but at how far we had to go. "It's not often," STScI director Matt Mountain said at the same meeting, "that astrophysics challenges modern or fundamental physics. Perhaps in the last four hundred years you can count maybe on one hand, perhaps on two, when these instances have occurred. Well, the discovery of the acceleration of the universe a decade ago has handed this generation just one of those opportunities."

Since the invention of the telescope four centuries earlier, astronomers had been able to figure out the workings of the universe simply by observing the heavens and applying some math, and vice versa. Take the discovery of moons, planets, stars, and galaxies, apply Newton's laws, and you have a universe that runs like clockwork. Take Einstein's modifications of Newton, apply the discovery of cosmic expansion, and you get the Big Bang universe—what Saul Perlmutter once called "a ridiculously simple, intentionally cartoonish picture."

He was sitting in George Smoot's office on the Berkeley campus. Three days earlier Smoot had learned that he had won the 2006 Nobel Prize in Physics for his work on COBE. Bearded, booming, eyes pinwheeling from adrenaline and lack of sleep, Smoot leaned back in his chair. Perlmutter leaned forward in his.

"Time and time again," Smoot shouted, "the universe has turned out to be really simple."

Perlmutter nodded eagerly. "It's like, why are we able to understand the universe at our level?"

"Right. Exactly. It's a universe for beginners! The Universe for Dummies! We're just incredibly lucky that that first try has matched so well."

Would our luck hold? Scientists liked to say that what physics needed was "the next Einstein." But if we took seriously the once-a-millennium quality of the dark-universe revolution—and we had every reason to think we should—then the analogy was inexact. Einstein was our Copernicus, finding the equations that might or might not represent the real—or "real"—universe. The discoverers of dark matter and dark energy were our Galileo, making the observations that validated this universe, though it turned out to be far more elaborately mysterious than we had ever imagined. What science needed now wasn't the next Einstein but the next Newton—someone (or someones, or some collaboration, or some generations-long cathedral of a theory) to codify the math of this new universe. To unite the physics of the very big with the physics of the very small, just as Newton had united the physics of the celestial with the physics of the terrestrial. To take the observations and make sense of our universe all over again in ways that we couldn't begin to imagine, but that would define our physics and philosophy—our civilization—for centuries to come.

It was this prospect that led cosmologists to regard these originally disturbing discoveries of a universe beyond our senses with fascination and optimism, to view a seeming human limitation as a source of intellectual liberation. "The really hard problems are great," Mike Turner said, "because we know they'll require a crazy new idea." Or as an astronomer told his colleagues at the McMaster conference on dark energy, "If you put the timeline of the history of science before me and I could choose any time and field, this is where I'd want to be."

So: Let there be dark. Let there be doubt, even amid the certainty. Especially amid the certainties—the pieces of evidence that in one generation transformed cosmology from metaphysics to physics, from speculation to science.

In early 2010, the WMAP seven-year results arrived bearing the latest refinements of the numbers that define our universe. It was 13.75 billion years old. Its Hubble constant was 70.4, and its equation of state (w) -0.98, or, within the margin of error, -1.0. And it was flat, consisting of 72.8 percent dark energy, 22.7 percent dark matter, and 4.56 percent baryonic matter (the stuff of us)—an exquisitely precise accounting of the depth of our ignorance. How the story would end remained a mystery, for now and possibly forever. The astronomers who set out to write the final chapter in the history of the universe had to content themselves instead with a more modest conclusion: To be continued.

"In a very real sense," Vera Rubin once wrote, "astronomy begins anew." In 1992, the Department of Terrestrial Magnetism moved her to an office in a new building. The photograph of Andromeda came with her, and they promised to put it on her ceiling, but nobody ever got around to it. She didn't care: The world changes. Besides, the other office had a low ceiling. There, M31 seemed close enough to touch. In her new office, it would have been out of reach. "The joy and fun of understanding the universe," she continued in that essay, "we bequeath to our grandchildren—and to their grandchildren."

"I have this three-year-old daughter at home," Perlmutter said now, sitting in Smoot's office, "and we're just at that stage where she's asking us, 'Why?' It's pretty obvious that she knows it's a bit of a game. She knows that whatever we say, she can then say, 'Yes, but—why?'" He laughed. "I have the impression that most people don't realize that what got physicists into physics usually is not the desire to understand what we already know but the desire to catch the universe in the act of doing really bizarre things. We love the fact that our ordinary intuitions about the world can be fooled, and that the world can just act strangely, and you can just go out and make it good over and over again. 'Do that again! Do that again!'"

Smoot agreed. "They're always testing the limits. And that's what we're doing. We're babies in the universe, and we're testing what the limits are."

If our luck did hold, and another Newton did come along, and the universe turned out once again to be simple in ways we couldn't have previously imagined, then Saul Perlmutter's daughter or Vera Rubin's grandchildren's grandchildren would not be seeing the same sky that they did, because they would not be thinking of it in the same way. They would see the same stars, and they would marvel at the hundreds of billions of galaxies other than our own. But they would sense the dark, too. And to them that darkness would represent a path toward knowledge—toward the kinds of discoveries that we all once called, with understandable innocence, the light.