The 4 Percent Universe
Dark Matter, Dark Energy, and The Race to Discover the Rest of Reality
Richard Panek
Table of Contents
Title Page
Table of Contents
...
Copyright
Dedication
Epigraph
Contents
Acknowledgments
Prologue
PART I
1. Let There Be Light
2. What's Out There
3. Choosing Halos
PART II
4. Getting in the Game
5. Staying in the Game
6. The Game
PART III
7. The Flat Universe Society
8. Hello, Lambda
9. The Tooth Fairy Twice
PART IV
10. The Curse of the Bambino
11. The Thing
12. Must Come Down
Epilogue
Notes
Works Cited
Index
Footnotes
HOUGHTON MIFFLIN HARCOURT
Boston New York
2011
Copyright © 2011 by Richard Panek
All rights reserved
For information about permission to reproduce selections from this book, write to
Permissions, Houghton Mifflin Harcourt Publishing Company, 215 Park Avenue South,
New York, New York 10003.
www.hmhbooks.com
Library of Congress Cataloging-in-Publication Data
Panek, Richard.
The four-percent universe : dark matter, dark energy, and the race to discover the rest of
reality / Richard Panek.
p. cm.
Includes bibliographical references and index.
ISBN 978-0-618-98244-8
1. Cosmology. 2. Physics. 3. Astrophysics. I. Title.
QB981.P257 2010
523.1—dc22 2010025838
Book design by Lisa Diercks
This book is typeset in Bulmer, Hydra Text, and Belizio.
Printed in the United States of America
DOC 10 9 8 7 6 5 4 3 2 1
Some passages in this book appeared, usually in different form, in Discover, the New York
Times Magazine, Sky & Telescope, and Smithsonian.
Portions of chapter 11 are based on work supported by the National Science Foundation
under Grant No. 0739893. Any opinions, findings, and conclusions or recommendations
expressed in this material are those of the author and do not necessarily reflect the views of
the National Science Foundation.
For Meg, with love
"I know," said Nick.
"You don't know," said his father.
—Ernest Hemingway
Contents
Acknowledgments [>]
Prologue [>]
PART I. MORE THAN MEETS THE EYE
1. Let There Be Light [>]
2. What's Out There [>]
3. Choosing Halos [>]
PART II. LO AND BEHOLD
4. Getting in the Game [>]
5. Staying in the Game [>]
6. The Game [>]
PART III. THE FACE OF THE DEEP
7. The Flat Universe Society [>]
8. Hello, Lambda [>]
9. The Tooth Fairy Twice [>]
PART IV. LESS THAN MEETS THE EYE
10. The Curse of the Bambino [>]
11. The Thing [>]
12. Must Come Down [>]
Epilogue [>]
Notes [>]
Works Cited [>]
Index [>]
Acknowledgments
THE AUTHOR EXPRESSES deep appreciation to Amanda Cook for her extraordinary editorial guidance as well as her genuine passion for the dark side of the universe; Henry Dunow, who, with his usual wisdom, made the match of editor and author; Katya Rice for her expert eye; Katherine Bouton for taking a chance on science and assigning an article on this subject; the John Simon Guggenheim Memorial Foundation, the National Science Foundation's Antarctic Artists and Writers Program, and the New York Foundation for the Arts for their generous and essential support; and Gabriel and Charlie (who claim to know what dark matter and dark energy are but refuse to tell their father, who loves them anyway).
Prologue
THE TIME HAD come to look inside the box. On November 5, 2009, scientists at sixteen institutions around the world took their seats before their computer screens and waited for the show to begin: two software programs being run by two graduate students—one at the University of Minnesota, the other at the California Institute of Technology—simultaneously. For fifteen minutes the two scripts would sort through data that had been collecting far underground in a long-abandoned iron mine in northern Minnesota. Over the past year, thirty ultrasensitive detectors—deep-freeze cavities the size of refrigerators, shielded from stray cosmic rays by half a mile of bedrock and snug blankets of lead, their interiors cooled almost to absolute zero, each interior harboring a heart of germanium atoms—had been looking for a particular piece of the universe. The data from that search had sped from the detectors to offsite computers, where, following the protocol of a blind analysis, it remained in a "box," out of sight. Just after 9 A.M. Central Time, the "unblinding party" began.
Jodi Cooley watched on the screen in her office at Southern Methodist University. As the coordinator of data analysis for the experiment, she had made sure that researchers wrote the two scripts separately using two independent approaches, so as to further ensure against bias. She had also arranged for all the collaborators on the project—physicists at Stanford, Berkeley, Brown; in Florida, Texas, Ohio, Switzerland—to be sitting at their computers at the same time. Together they would watch the evidence as it popped up on their screens, one plot per detector, two versions of each plot.
After a few moments, plots began appearing. Nothing. Nothing. Nothing.
Then, three or four minutes into the run, a detection appeared—on the same plots in both programs. A dot on a graph. A dot within a narrow, desirable band. A band where all the other dots weren't falling.
A few minutes later another pair of dots on another pair of plots appeared within the same narrow band.
And a few minutes later the programs had run their course. That was it, then. Two detections.
"Wow," Cooley thought.
Wow, as in: They had actually seen something, when they had expected to get the same result as the previous peek inside a "box" of different data nearly two years earlier—nothing.
Wow, as in: If you're going to get detections, two is a frustrating number—statistically tantalizing but not sufficient to claim a discovery.
But mostly Wow, as in: They might have gotten the first glimpse of dark matter—a piece of our universe that until recently we hadn't even known to look for, because until recently we hadn't realized that our universe was almost entirely missing.
It wouldn't be the first time that the vast majority of the universe turned out to be hidden to us. In 1610 Galileo announced to the world that by observing the heavens through a new instrument—what we would call a telescope—he had discovered that the universe consists of more than meets the eye. The five hundred copies of the pamphlet announcing his results sold out immediately; when a package containing a copy arrived in Florence, a crowd quickly gathered around the recipient and demanded to hear every word. For as long as members of our species had been lying on our backs, looking up at the night sky, we had assumed that what we saw was all there was. But then Galileo found mountains on the Moon, satellites of Jupiter, hundreds of stars. Suddenly we had a new universe to explore, one to which astronomers would add, over the nex
t four centuries, new moons around other planets, new planets around our Sun, hundreds of planets around other stars, a hundred billion stars in our galaxy, hundreds of billions of galaxies beyond our own.
By the first decade of the twenty-first century, however, astronomers had concluded that even this extravagant census of the universe might be as out-of-date as the five-planet cosmos that Galileo inherited from the ancients. The new universe consists of only a minuscule fraction of what we had always assumed it did—the material that makes up you and me and my laptop and all those moons and planets and stars and galaxies. The rest—the overwhelming majority of the universe—is ... who knows?
"Dark," cosmologists call it, in what could go down in history as the ultimate semantic surrender. This is not "dark" as in distant or invisible. This is not "dark" as in black holes or deep space. This is "dark" as in unknown for now, and possibly forever: 23 percent something mysterious that they call dark matter, 73 percent something even more mysterious that they call dark energy. Which leaves only 4 percent the stuff of us. As one theorist likes to say at public lectures, "We're just a bit of pollution." Get rid of us and of everything else we've ever thought of as the universe, and very little would change. "We're completely irrelevant," he adds, cheerfully.
All well and good. Astronomy is full of homo sapiens-humbling insights. But these lessons in insignificance had always been at least somewhat ameliorated by a deeper understanding of the universe. The more we could observe, the more we would know. But what about the less we could observe? What happens to our understanding of the universe then? What currently unimaginable repercussions would this limitation, and our ability to overcome it or not, have for our laws of physics and our philosophy—our twin frames of reference for our relationship to the universe?
Astronomers are finding out. The "ultimate Copernican revolution," as they often call it, is taking place right now. It's happening in underground mines, where ultrasensitive detectors wait for the ping of a hypothetical particle that might already have arrived or might never come, and it's happening in ivory towers, where coffee-break conversations conjure multiverses out of espresso steam. It's happening at the South Pole, where telescopes monitor the relic radiation from the Big Bang; in Stockholm, where Nobelists have already begun to receive recognition for their encounters with the dark side; on the laptops of postdocs around the world, as they observe the real-time self-annihilations of stars, billions of light-years distant, from the comfort of a living room couch. It's happening in healthy collaborations and, the universe being the intrinsically Darwinian place it is, in career-threatening competitions.
The astronomers who have found themselves leading this revolution didn't set out to do so. Like Galileo, they had no reason to expect that they would discover new phenomena. They weren't looking for dark matter. They weren't looking for dark energy. And when they found the evidence for dark matter and dark energy, they didn't believe it. But as more and better evidence accumulated, they and their peers reached a consensus that the universe we thought we knew, for as long as civilization had been looking at the night sky, is only a shadow of what's out there. That we have been blind to the actual universe because it consists of less than meets the eye. And that that universe is our universe—one we are only beginning to explore.
It's 1610 all over again.
PART I
More Than Meets the Eye
1. Let There Be Light
IN THE BEGINNING—which is to say, 1965—the universe was simple. It came into being one noontime early that year over the course of a telephone conversation. Jim Peebles was sitting in the office of his mentor and frequent collaborator, the Princeton physicist Robert Dicke, along with two other colleagues. The phone rang; Dicke took the call. Dicke helped run a research firm on the side, and he himself held dozens of patents. During these weekly brown-bag lunches in his office, he sometimes got phone calls that were full of esoteric and technical vocabulary that Peebles didn't recognize. This call, though, contained esoteric and technical vocabulary that Peebles knew intimately—concepts the four physicists had been discussing that very afternoon. Cold load, for instance: a device that would help calibrate the horn antenna—another term Peebles overheard—that they would be using to try to detect a specific signal from space. The three physicists grew quiet and looked at Dicke. Dicke thanked the caller and hung up, then turned to his colleagues and said, "Well, boys, we've been scooped."
The caller was an astronomer at the Bell Telephone Laboratories who had collected some curious data but had no idea what it meant. Peebles and Dicke had developed a curious idea but had no data to support it. The other two physicists at lunch had been building an antenna to detect a signal that would offer support for the curious idea, but now, Dicke said, a pair of astronomers at Bell Labs had probably found it first—without knowing what they'd done.
The mood in Dicke's office was not one of deflation or disappointment. If the four of them had in fact been scooped, they had also been vindicated. If the caller was right, then they too were right, or at least heading in a potentially profitable scientific direction. If nothing else, they could take some comfort from the possibility that they were the first persons in the history of the world to understand the history of the universe.
But before reaching any conclusions, they would have to check the data for themselves. Dicke and two of the other Princeton physicists soon drove the thirty miles to Holmdel Township, New Jersey, home of the Bell Labs research center. The Bell Labs astronomers—Arno Penzias, who had placed the call to Dicke, and his collaborator Robert Wilson—took them to see the antenna. It was a horn-shaped instrument, as big as a boxcar, by the side of a private road at the top of Crawford Hill, the highest point for miles around. After the five of them had squeezed into the control cab, their elbows brushing vacuum tubes and instrument panels, the Bell Labs astronomers explained the physics to the physicists.
Bell Labs had built the antenna in 1960 to receive coast-to-coast signals bouncing off the Echo communications satellite, a highly reflective balloon 100 feet in diameter. When the Echo mission ended, the antenna was used on the Telstar satellite. When that mission ended, Penzias and Wilson appropriated the antenna to study radio waves from the fringes of our Milky Way galaxy. The measurements would have to be much more sensitive than they were for Echo, so Penzias had built a cold load, an instrument that emitted a specific signal that he and Wilson could compare with measurements from the antenna to make sure it wasn't detecting any excess noise. And the cold load worked, just not in the way they'd hoped. Aside from the unavoidable rattling of electrons in the atmosphere and within the instrument itself, Penzias and Wilson were left with a persistent, inexplicable hiss.
For much of the past year they had been trying to determine the source of the noise. They pointed the antenna at New York City, less than fifty miles away. The radio static was negligible. They pointed the antenna at every other location on the horizon. The same. They checked the signal from the stars to see whether it differed from what they'd already factored into their calculations. Nope. The phases of the Moon? Temperature changes in the atmosphere over the course of a year? No and no. That spring they had turned their attention back to the antenna itself. They put tape over the aluminum rivets in the antenna—nothing—and took apart the throat of the horn and put it back together again—nothing—and even scraped away the droppings from a pair of pigeons that had taken up residence within the horn. (They caught the pigeons and mailed them to the Bell Labs site in Whippany, New Jersey, more than forty miles away; the birds turned out to be homing pigeons and were back in the horn within days.) Still nothing—nothing but the noise.
The five scientists repaired to a conference room on Crawford Hill, and now the physicists explained the astronomy to the astronomers. Dicke started writing on a blackboard. If the Big Bang interpretation of the history of the universe was correct, Dicke said, then the cosmos emerged in an unfathomably condensed, obscenely hot explosion of energ
y. Everything that would ever be in the universe was there then, rushing outward on a shock wave of space itself, and continuing to rush outward until it evolved into the universe we see today. And as the universe expanded, it cooled. One member of the Princeton collaboration—Jim Peebles, the colleague who wasn't present—had calculated what that initial level of energy would have been, and then he had calculated what the current level of energy, after billions of years of expansion and cooling, should be. That remnant energy—assuming it existed; assuming the Big Bang theory was right—would be measurable. And now, apparently, Penzias and Wilson had measured it. Their antenna was picking up an echo, all right, but this time the source wasn't a radio broadcast from the West Coast. It was the birth of the universe.
Penzias and Wilson listened politely. Dicke himself didn't entirely believe what he was saying—not yet. He and the two other Princeton physicists satisfied themselves that Penzias and Wilson had run a clean experiment, then drove back to Princeton and told Peebles what they had learned. Peebles didn't entirely believe what he was hearing, either. He was cautious; but then, he was always cautious. The four collaborators agreed that scientific results require corroboration, a second opinion—in this case, their own. They would finish constructing their antenna on the roof of Princeton's Guyot Hall and see if it got the same reading as the Bell Labs antenna. Even if it did, they knew they would still have to proceed with caution. It's not often, after all, that you get to discover a new vision of the universe.
The American writer Flannery O'Connor once said that every story has "a beginning, a middle, and an end, though not necessarily in that order." By the 1960s, scientists wanting to tell the story of the universe—cosmologists, by definition—could proceed under the assumption that they were in possession of the middle of the narrative. They had the latest version of one of civilization's most enduring characters, the universe—in this case, an expanding one. Now they could ask themselves: How did Our Hero get here?
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