The Elephant in the Universe, page 1
The Elephant in the Universe
OUR HUNDRED-YEAR SEARCH FOR DARK MATTER
Govert Schilling
The Belknap Press of Harvard University Press
Cambridge, Massachusetts
London, England
2022
Copyright © 2022 by Govert Schilling
Foreword copyright © 2022 by the President and Fellows of Harvard College
All rights reserved
Cover design: Henry Sene Yee
978-0-674-24899-1 (cloth)
978-0-674-27617-8 (EPUB)
978-0-674-27618-5 (PDF)
The Library of Congress has cataloged the printed edition as follows:
Names: Schilling, Govert, author.
Title: The elephant in the universe : our hundred-year search for dark matter / Govert Schilling.
Description: Cambridge, Massachusetts : The Belknap Press of Harvard University Press, 2022. | Includes bibliographical references and index.
Identifiers: LCCN 2021034391
Subjects: LCSH: Dark matter (Astronomy)—History. | Cosmology—History.
Classification: LCC QB791.3 .S32 2022 | DDC 523.1/126—dc23
LC record available at https://lccn.loc.gov/2021034391
Contents
Foreword by Avi Loeb
Introduction
PART I Ear
1 Matter, but Not as We Know It
2 Underground Phantoms
3 The Pioneers
4 The Halo Effect
5 Flattening the Curve
6 Cosmic Cartography
7 Big Bang Baryons
8 Radio Recollections
PART II Tusk
9 Into the Cold
10 Miraculous WIMPs
11 Simulating the Universe
12 The Heretics
13 Behind the Lens
14 MACHO Culture
15 The Runaway Universe
16 Pie in the Sky
17 Telltale Patterns
PART III Trunk
18 The Xenon Wars
19 Catching the Wind
20 Messengers from Outer Space
21 Delinquent Dwarfs
22 Cosmological Tension
23 Elusive Ghosts
24 Dark Crisis
25 Seeing the Invisible
Notes
Acknowledgments
Image Credits
Index
The Blind Men and the Elephant
A Hindoo Fable
It was six men of Indostan
To learning much inclined,
Who went to see the Elephant
(Though all of them were blind),
That each by observation
Might satisfy his mind.
The First approached the Elephant,
And happening to fall
Against his broad and sturdy side,
At once began to bawl:
“God bless me!—but the Elephant
Is very like a wall!”
The Second, feeling of the tusk,
Cried: “Ho!—what have we here
So very round and smooth and sharp?
To me ’t is mighty clear
This wonder of an Elephant
Is very like a spear!”
The Third approached the animal,
And happening to take
The squirming trunk within his hands,
Thus boldly up and spake:
“I see,” quoth he, “the Elephant
Is very like a snake!”
The Fourth reached out his eager hand,
And felt about the knee.
“What most this wondrous beast is like
Is mighty plain,” quoth he;
“’T is clear enough the Elephant
Is very like a tree!”
The Fifth, who chanced to touch the ear,
Said: “E’en the blindest man
Can tell what this resembles most;
Deny the fact who can,
This marvel of an Elephant
Is very like a fan!”
The Sixth no sooner had begun
About the beast to grope,
Then, seizing on the swinging tail
That fell within his scope,
“I see,” quoth he, “the Elephant
Is very like a rope!”
And so these men of Indostan
Disputed loud and long,
Each in his own opinion
Exceeding stiff and strong,
Though each was partly in the right,
And all were in the wrong!
So, oft in theologic wars
The disputants, I ween,
Rail on in utter ignorance
Of what each other mean,
And prate about an Elephant
Not one of them has seen!
John Godfrey Saxe, 1872
Foreword
AVI LOEB
The term “dark matter” is used to represent most of the matter in the universe—five times more prevalent than ordinary matter, like the atoms that make up stars and planets. But, as the name suggests, we cannot see dark matter. We infer its existence only indirectly through its gravitational influence on visible matter. In this way, dark matter encapsulates our ignorance.
Like all good mysteries, the puzzle of dark matter is enduring. It has intrigued scientists for a hundred years. Observations and scientific theories suggest that dark matter could be made of any number of hypothetical building blocks: weakly interacting massive particles, so-called axions, even atoms that do not interact with ordinary matter or light. Today there is a scientific consensus that dark matter likely came out of the fiery soup during the origins of the universe, an ocean of invisible particles with initially small random motions. Although scientists have not detected any of these invisible particles yet, they have measured the imprint of the fluctuations. Today those dark matter fluctuations are evident in the slightly varied brightness of the cosmic microwave background, the relic radiation left over from the big bang.
Lord Kelvin was the first to offer a dynamical estimate of what we now think of as dark matter. In a talk given in 1884, Kelvin theorized that there might be dark bodies in the Milky Way. Almost fifty years and many ideas later, Swiss-American astronomer Fritz Zwicky estimated that there is more mass in galaxy clusters than is visually observable. In the 1970s evidence for invisible particles was revealed through the pathbreaking work of Vera Rubin, Kent Ford, and Kenneth Freeman. They showed that the dynamics of gas and stars in galaxies imply the existence of invisible mass in a halo that extends well outside the inner region, where ordinary matter concentrates. And in 1983 Moti Milgrom proposed a theory of modified Newtonian dynamics to explain the missing-mass problem. In this alternative hypothesis of gravity, Milgrom postulated that Newton’s laws do not apply to galaxies.
Like most explorations in science, historical theories of dark matter found supporters and critics. Milgrom’s simple prescription for modified dynamics at low accelerations accounts for the nearly flat rotation curves in many galaxy halos extremely well, even after four decades of scrutiny. But the theory does not adequately account for Zwicky’s observed properties of galaxy clusters. Another possibility is that dark matter is strongly self-interacting and avoids galactic cores. And the hypotheses continue.
Throughout this book, Govert Schilling leads us on a captivating tour through the theories of dark matter and efforts to observe it, from early times to the present day. We’ll travel with him to astronomical observatories on the ground and in space and to particle detectors in underground caves and tunnels. As we circle the globe, we meet the scientists, the protagonists of the story, who have spent their careers searching for a solution to the puzzle. It is a wide-ranging cast of characters. There are towering figures in the field of dark matter research, like Jim Peebles and Jerry Ostriker. There are younger scientists, true believers, skeptics, and heretics. Through their stories, we garner an extraordinary view into the past, present, and future of one of the deepest enigmas in science.
As The Elephant in the Universe shows, the search for dark matter is a work in progress. Hence the abundance of scientific interpretations. But one day all of the pieces of the puzzle will fall into place. It is with Schilling’s stellar guidance that we join leading scientists in their crusade to understand this unknown gravitating matter, and, along the way, delight in the mysteries of our universe.
Introduction
In 1995 astronomers announced that they had developed sensitive spectrometers that made it possible to precisely measure the velocities of stars. Within a few years, I reckoned, these tools would be used to discover extrasolar planets: if the spectrometer picked up tiny, periodic perturbations in the velocity of a star, then there might be a massive planet nearby, the gravity of which was disrupting the parent star’s movement through space. I decided to start researching a new book on the hunt for exoplanets, in the hope that the breakthrough find could be described in the closing chapter.
In October of that same year, when Michel Mayor and Didier Queloz announced their discovery of 51 Pegasi b—the first confirmed planet beyond our solar system orbiting a Sun-like star—I realized I had to hurry. For most of 1996, I worked on hardly anything els
Something similar happened about twenty years later. In early 2015 I began researching a book on gravitational waves—tiny undulations of the very fabric of the universe, caused by energetic events like colliding black holes. Albert Einstein’s general theory of relativity predicted gravitational waves decades ago, and scientists have been hunting for them ever since. I knew as I started my research that advanced gravitational wave detectors would be going online in a matter of months—new versions of the Laser Interferometer Gravitational-Wave Observatory in the United States and the Virgo detector in Italy. It looked like a discovery couldn’t be more than a few years away.
In fact, the first direct observation of gravitational waves came in September 2015 and was announced to the world in February of the following year. Again I put everything aside to complete the book as soon as possible. Ripples in Spacetime was published in the summer of 2017.
So when, in early 2018, I started seriously researching a new book on dark matter, I half-jokingly told the astrophysicists and particle physicists I was interviewing that I expected a revolutionary development in the field any day now. Wouldn’t it be great if my book were the first to report on the long-awaited solution to the riddle of dark matter? The first to lay out what this mysterious stuff, said to constitute the balance of the cosmos, actually is?
Unfortunately, it didn’t happen. So here’s the spoiler: when you reach the last page of this book, you still won’t know what most of the material universe is made of. But neither do scientists. Despite decades of speculation, searching, studies, and simulations, dark matter remains one of the biggest enigmas of modern science. Still, after reading this book, you will have learned a lot about the miraculous universe we live in, and about the ways in which astronomers and physicists have teased out its secrets.
Dark matter challenges our imagination. Like some invisible glue, it is what holds the universe together and what makes it tick. Without it, galaxies would fall apart, galaxy clusters would dissolve, and space would have expanded into oblivion long ago. Dark matter is the most important stuff out there, yet we’ve only found out about it in recent decades, and no one has a clue as to its true nature.
Well, at least we’ve learned what it’s not, thanks to the work of hundreds of dedicated scientists. Dark matter is not an ocean of ultra-dim dwarf stars. It is not an all-pervading veil of murky gas in intergalactic space. Dark matter is not a population of black holes—at least not the “regular” kind that astronomers are slowly starting to explore. Dark matter isn’t even composed of the atoms and molecules that we are familiar with. It’s something weird and exotic altogether.
And it has shaped the universe we live in. Dark matter provided the scaffolding for the growth of cosmic structure. It enabled the formation of galaxy clusters, galaxies, stars, planets, and eventually people. However, despite the numerous disciplines and scientists that are involved in studying the problem, we don’t seem to be able to really solve it. There have been hints and allegations. Circumstantial evidence and wishful thinking. But so far, not a single convincing detection. No hint of dark matter’s true identity.
The story of the search for dark matter goes back to the 1930s, although the mystery wasn’t generally acknowledged until some fifty years ago, when astronomers started to wonder about the high rotational velocities of the outer parts of spiral galaxies like our own Milky Way. Before long, particle physicists got involved, as it became evident that the puzzle couldn’t be solved without invoking a completely new form of matter. And because of its pivotal role in the evolution of the universe, this new, dark matter also turned into a hot topic in cosmology, the study of the universe at the largest possible scales. Dark matter is a genuinely multidisciplinary area of research that has kept observers, theoreticians, experimentalists, and computer model builders busy for decades.
With so many people working on the problem over such a long time, it’s outright impossible to do everyone justice in a book like this. After all, The Elephant in the Universe is not a technical book, nor does it pretend to be the definitive history of the field. Instead, this book provides a broad view of dark matter research in all its bewildering variety. Personal stories of many key players give a taste of the ingenuity, perseverance, and—sometimes—stubbornness of scientists who have devoted their professional lives to solving nature’s biggest mysteries. I’ll take you, the reader, along to remote astronomical observatories and underground laboratories. We’ll attend scientific conferences and talk with Nobel laureates and postdoc researchers alike. Unfortunately, due to the COVID-19 pandemic, not all of my planned trips could be realized, and quite a number of interviews had to be conducted on the phone or via Zoom.
Our journey covers a wide range of dark matter–related topics. Although most of the twenty-five chapters could be read as stand-alone stories, I’ve arranged them in an order that presents the scope of the mystery and shows how that mystery has evolved. To set the stage, the first chapter introduces physicist James Peebles, who has been called the “father” of the popular cold dark matter (CDM) model and who was the corecipient of the 2019 Nobel Prize in Physics for his contributions to theoretical cosmology. Next, in chapter 2, a visit to the underground Gran Sasso laboratory in Italy gives a preliminary taste of the experimental approach to the dark matter riddle. Dark matter isn’t the exclusive province of computer simulations and conference papers. At this very moment, dozens of scientists all over the world are putting theory to the test in hopes of solving this puzzle.
After whetting your appetite by means of this introductory brush with theory and experiment, we travel a century back in time in chapter 3, to learn about the first indications that something was amiss in our understanding of the material contents of the universe. Much later, in the 1970s, physicists realized that galaxies like our own Milky Way cannot be stable without huge, more-or-less spherical halos of dark matter (chapter 4). Pioneers like astronomer Vera Rubin started to realize that the high spin rates of galaxies can only be explained if they contain much more than meets the eye, as described in chapter 5.
Today Rubin’s name adorns a brand-new telescope under construction. When completed, it will be one of the most powerful on Earth, an instrument central to scientists’ attempts to map the three-dimensional distribution of galaxies in space. That project is an important dimension of dark matter research and the subject of chapter 6. Then, in chapter 7, we delve into the origin of the elements, only to discover why dark matter cannot consist of ordinary atoms and molecules. The decisive role of radio astronomy in proving that dark matter really exists is the topic of chapter 8. This concludes the first part of the book, which is largely focused on astronomical research.
Part II opens with two chapters discussing the growing conviction, in the second half of the 1970s, that the mysterious stuff must be composed of relatively slow-moving (“cold”) elementary particles. Such particles fit remarkably well in the theory of supersymmetry—a promising candidate for the long-sought Theory of Everything. Thus dark matter started to play an important role in particle physics, too.
Chapter 11 details computer simulations of the evolution of the large-scale structure of the universe, which seemed to support a candidate for the contents of dark matter: weakly interacting massive particles, or WIMPs. But just as the WIMP hypothesis was emerging, some scientists began to doubt that dark matter is real. Their theory of modified Newtonian dynamics (MOND), discussed in chapter 12, claims that our understanding of gravity needs revision—dark matter hunters may be chasing a chimera after all.
In chapters 13 and 14, we encounter the powerful observational technique of gravitational lensing—the minute deflection of light by the gravity of massive objects. Gravitational lensing was recognized for its potential to rebut the MOND theory and to help scientists find an alternative dark matter candidate known as massive compact halo objects or MACHOs. Alas, the hunt for MACHOs came up all but empty. Instead, another mystery revealed itself in the late 1990s: dark energy. Scientists realized that empty space was expanding at an accelerating rate—a direct result of dark energy. That discovery, and what it might imply for the overall composition of the universe, are the topics of chapters 15 and 16.
