INTRODUCTION

“Dark matter” and “dinosaurs” are words you rarely hear together except perhaps in the playground, a fantasy gaming club, or some not-yet-released Spielberg movie. Dark matter is the elusive stuff in the Universe that interacts through gravity like ordinary matter, but that doesn’t emit or absorb light. Astronomers detect its gravitational influence, but they literally don’t see it. Dinosaurs, on the other hand . . . I doubt I need to explain dinosaurs. They were the dominant terrestrial vertebrates from 231 to 66 million years ago.

Though both dark matter and dinosaurs are independently fascinating, you might reasonably assume that this unseen physical substance and this popular biological icon are entirely unrelated. And this might well be the case. But the Universe is by definition a single entity and in principle its components interact. This book explores a speculative scenario in which my collaborators and I suggest that dark matter might ultimately (and indirectly) have been responsible for the extinction of the dinosaur.

Paleontologists, geologists, and physicists have shown that 66 million years ago, an object at least ten kilometers wide plummeted to Earth from space and destroyed the terrestrial dinosaurs, along with three-quarters of the other species on the planet. The object might have been a comet from the outer reaches of the Solar System, but no one knows why this comet was perturbed from its weakly bound, but stable, orbit.

Our proposal is that during the Sun’s passage through the midplane of the Milky Way—the stripe of stars and bright dust that you can observe in a clear night sky—the Solar System encountered a disk of dark matter that dislodged the distant object, thereby precipitating this cataclysmic impact. In our galactic vicinity, the bulk of the dark matter surrounds us in an enormous smooth and diffuse spherical halo.

The type of dark matter that triggered the dinosaurs’ demise would be distributed very differently from most of the elusive dark matter in the Universe. The additional type of dark matter would leave the halo intact, but its very different interactions would make it condense into a disk—right in the middle of the Milky Way plane. This thin region could be so dense that when the Solar System passes through it, as the Sun oscillates up and down during its orbit through our galaxy, the disk’s gravitational influence would be unusually strong. Its gravitational pull could be powerful enough to dislodge comets at the outer edge of the Solar System, where the Sun’s competing pull would be too weak to rein them back in. The errant comets would then be ejected from the Solar System or—more momentously—be redirected to hurtle toward the inner Solar System, where they might have the potential to strike the Earth.

I’ll tell you right up front that I don’t yet know if this idea is correct. It’s only an unexpected type of dark matter that would yield measurable influences on living beings (well, technically no longer living). This book is the story of our unconventional proposal about just such surprisingly influential dark matter.

But these speculative ideas—as provocative as they might be—are not this book’s primary focus. At least as important to its content as the story of the dinosaur-destroying comet are the context and the science that embrace it, which include the far better established frameworks of cosmology and the science of the Solar System. I feel very fortunate that the topics I study frequently guide my research toward big questions such as what stuff is made of, the nature of space and time, and how everything in the Universe evolved to the world we see today. In this book, I hope to share a lot of this too.

In the research that I will describe, my studies led me down a path where I started thinking more broadly about cosmology, astrophysics, geology, and even biology. The focus was still on fundamental physics. But having done more conventional particle physics all my life—the study of the building blocks of familiar matter such as the paper or screen on which you’re reading this—I’ve found it refreshing to probe into what is known—and what soon will be known—about the dark world too, as well as the implications of basic physical processes for the Solar System and for the Earth.

Dark Matter and the Dinosaurs explains our current knowledge about the Universe, the Milky Way, the Solar System, as well as what makes for a habitable zone and life on Earth. I’ll discuss dark matter and the cosmos, but I will also delve into comets, asteroids, and the emergence and extinction of life, with special focus on the object that fell to Earth to kill off the terrestrial dinosaurs—and a lot of the rest of life here. I wanted this book to convey the many incredible connections that got us here so we can more meaningfully understand what is happening now. When we think about our planet today, we might also want to better understand the context in which it developed.

When I started concentrating on the concepts underlying the ideas in this book, I was awe-struck and enchanted not only by our current knowledge of our environment—local, solar, galactic, and universal, but also by how much we ultimately hope to understand, from our random tiny perch here on Earth. I also was overwhelmed by the many connections among the phenomena that ultimately allow us to exist. To be clear, mine is not a religious viewpoint. I don’t feel the need to assign a purpose or meaning. Yet I can’t help but feel the emotions we tend to call religious as we come to understand the immensity of the universe, our past, and how it all fits together. It offers anyone some perspective when dealing with the foolishness of everyday life.

This newer research actually has made me look differently at the world and the many pieces of the Universe that created the Earth—and us. Growing up in Queens I saw the impressive buildings of New York City, but not so much of nature. What little nature I did see was cultivated into parks or lawns—retaining little of the form it took before humans arrived. Yet when you walk on a beach, you are walking on ground up creatures—or at least their protective coverings. The limestone cliffs you might see on a beach or in the countryside are composed of previously living creatures too, from millions of years in the past. Mountains arose from tectonic plates that collided, and the molten magma that drives these movements is the result of radioactive material buried near the core of the Earth. Our energy came from the Sun’s nuclear processes—though it has been transformed and stored in different ways since those initial nuclear reactions occurred. Many of the resources we use are heavier elements that came from outer space, which were deposited on the Earth’s surface by asteroids or comets. Some amino acids were deposited by meteoroids too—perhaps bringing life—or the seeds of life—to Earth. And before any of this happened, dark matter collapsed into clumps whose gravity attracted more matter—which eventually turned into galaxies, galaxy clusters, and stars like our Sun. Ordinary matter—important as it is to us— does not tell the whole story.

Although we might experience the illusion of a self-contained environment, every day at sunrise and every night when the Moon and the far more distant stars come into view, we are reminded that our planet is not alone. Stars and nebulae are further evidence that we exist in a galaxy that resides within a far larger Universe. We orbit within a Solar System where the seasons remind us further of our orientation and placement within it. Our very measurement of time in terms of days and years signifies the relevance of our surroundings.

Four inspiring lessons that I wanted to share stand out to me from the research and readings that led to this book. Close to my heart is the satisfaction of understanding how the pieces of the Universe connect in so many remarkable ways. The big lesson at the most fundamental level is that the physics of elementary particles, the physics of the cosmos, and the biology of life itself all connect—not in some New-Age sense, but in remarkable ways that are well worth understanding.

Stuff from outer space hits the Earth all the time. Yet the Earth has a love-hate relationship with its environment. The planet benefits from some of what’s around us, but much of it can be lethal. The position of our planet allows for the right temperature, the outer planets divert most incoming asteroids and comets before they strike the Earth, the distance between the Moon and the Earth stabilizes our orbit sufficiently to prevent massive temperature fluctuations, and the outer Solar System shields us from dangerous cosmic rays. Meteoroids hitting the Earth might have deposited resources critical to life, but they also affected the trajectory of life on the planet in more detrimental ways. At least one such object led to a devastating extinction 66 million years ago. Though it wiped out the land-dwelling dinosaurs, it also paved the way for the existence of larger mammals, including ourselves.

The second point—also impressive—is how recent are so many of the scientific developments that I will discuss. Perhaps people can make the following statement at any point in human history, but that does not diminish its validity: we have advanced our knowledge tremendously in the last [here insert a context-dependent number] years. For the research I will describe, that number is less than fifty. As I was doing my own research, and reading about others’, I was constantly struck by how new and deeply revolutionary so many recent discoveries have been. Human ingenuity and stubbornness have consistently emerged as scientists have tried to reconcile themselves to the often surprising and always entertaining and sometimes scary things we learned about the world. The science this book presents is part of a larger history—13.8 or 4.6 billion years according to whether you focus on the Universe or the Solar System. However, the history of human beings’ unraveling these ideas is little more than a century old.

The dinosaurs went extinct 66 million years ago, but paleontologists and geologists deduced the nature of that extinction only in the 1970s and 1980s. Once the relevant ideas had been introduced, it was a matter of decades before a community of scientists more fully evaluated them. And the timing was not entirely coincidental. The extinction’s connection to an extraterrestrial object became more credible once astronauts had landed on the Moon and seen craters up close—presenting them with detailed evidence of the dynamical nature of the Solar System.

In the last fifty years, significant advances in particle physics and cosmology have taught us about the Standard Model, which describes the basic elements of matter as we understand them today. The amount of dark matter and dark energy in the Universe too was pinned down only in the last decades of the twentieth century. Our knowledge of the Solar System also changed during the same time frame. And only in the 1990s did scientists discover the Kuiper belt objects in Pluto’s vicinity, demonstrating that Pluto is not orbiting alone. The number of planets was reduced—but only because the science you might have learned in grade school is now richer and more complex.

The third major lesson centers on the rate of change. Natural selection permits adaptation when species have time to evolve. But that adaptation won’t encompass radical changes. It is far too slow. The dinosaurs weren’t in a position to prepare for a 10-kilometer-wide meteoroid hitting the Earth. They couldn’t adapt. Those stuck on land, who were too big to bury themselves, had nowhere viable to go.

As new ideas or technologies emerge, debates over catastrophic versus gradual change have also played a big role. Key to understanding most new developments—scientific or otherwise—is the pace of the processes they describe. I frequently hear people suggest that certain developments, such as studies in genetics or advances deriving from the Internet, are unprecedentedly dramatic. But this is not entirely true. The improved understanding of disease or of the circulatory system, which dates back hundreds of years, brought about changes at least as profound as genetics does today. The introduction of written language, and later of the printing press, influenced the ways people acquired knowledge and how they thought in ways at least as significant as those that the Internet precipitated.

As with these developments, a very important factor for current change is also its rapidity—a topic that can be pertinent not only to scientific processes, but to environmental and sociological changes too. Although death by meteoroid is not likely to be a significant concern for us today, the quickening rates of changes in the environment and in extinctions likely are—and the impact could be comparable in many ways. The perhaps not-so-hidden agenda of this book is to help us better understand the amazing story of how we got here and to encourage us to use that knowledge wisely.

Even so, the fourth important lesson is the remarkable science describing the often hidden elements of our world and its development—and how much about the Universe we can hope to understand. Many people are fascinated by the idea of a multiverse—other universes not within our reach. But at least as fascinating are the many hidden worlds—both biological and physical —that we do have a chance to explore and learn more about. In Dark Matter and the Dinosaurs, I hope to convey how inspiring it can be to contemplate what we know—as well as what we might expect or hope to figure out in the future.

This book begins by explaining cosmology—the science of how the Universe has evolved to its current state. Its first part presents the Big Bang theory, cosmological inflation, and the makeup of the Universe. This section also explains what dark matter is, how we ascertained its existence, and why it is relevant to the Universe’s structure.

Dark matter constitutes 85 percent of the matter in the Universe while ordinary matter—such as that contained in stars, gas, and people—constitutes only 15 percent. Yet people are mainly preoccupied with the existence and relevance of ordinary matter—which, to be fair, interacts far more strongly.

However, as with humanity, it doesn’t make sense to focus all our attention on the small percentage that is disproportionately influential. The dominant 15 percent of matter that we can see and feel is only part of the story. I will explain dark matter’s critical role in the Universe—both for galaxies and for galaxy clusters forming out of the amorphous cosmic plasma in the early Universe—and in maintaining the stability of these structures today.

The second part of the book zooms in on the Solar System. The Solar System alone could of course be the subject of an entire book, if not of an encyclopedia. So I will focus on the constituents that might have concerned the dinosaurs—meteoroids, asteroids, and comets. This part will describe objects that we know have hit the Earth and what we anticipate might hit it in the future, as well as the sparse but not-obviously-dismissible evidence for extinctions or meteoroid strikes that occur at regularly spaced intervals of about 30 million years. This section also discusses life’s formation, as well as its destruction—reviewing what is known about the five major mass extinctions, including the devastating event that killed the dinosaurs.

The book’s third and final part integrates the ideas from the first two, starting with a discussion of models of dark matter. It explains the more familiar models for what dark matter might be, as well as the newer suggestion for dark matter interactions hinted at above.

At this point, we know only that dark matter and ordinary matter interact via gravity. Gravity’s consequences are generally so tiny that we register the influence only of enormous masses—such as that of the Earth and the Sun—and even those are pretty feeble. After all, you can pick up a paper clip with a tiny magnet, successfully competing against the gravitational influence of the entire Earth.

However, dark matter might experience other forces too. Our new model challenges people’s assumption—and prejudice—that familiar matter is unique because of the forces—electromagnetism, the weak, and the strong nuclear forces—through which it interacts. These conventional matter forces, which are much stronger than gravity, account for many of the interesting features of our world. But what if some of the dark matter experiences influential non-gravitational interactions too? If true, dark matter forces could lead to dramatic evidence of connections between elementary matter and macroscopic phenomena even deeper than the many we already know to be present.

Although everything in the Universe could in principle interact, most such interactions are far too small to readily register. Only things that affect us in a detectable way can be observed. If you have something exerting and experiencing only tiny effects, it might be right under your nose yet escape your notice. That’s presumably why individual dark matter particles—though probably all around us—have so far escaped discovery.

The third part of the book shows how thinking more broadly about dark matter—asking why the dark universe should be so simple when ours is so complicated—led us to consider some novel possibilities. Maybe a portion of the dark matter experiences is own force—dark light if you will. If most dark matter is usually relegated to the relatively uninfluential 85 percent, we could then think of the newly proposed type of dark matter as an upwardly mobile middle class—with interactions mimicking those of familiar matter. The additional interactions would affect the makeup of the galaxy and allow this portion of dark matter to affect the motion of stars and other objects in the domain of ordinary matter.

In the next five years, satellite observations will measure the galaxy’s shape, composition, and properties in greater detail than ever before—telling us a great deal about our galactic environment and testing whether or not our conjecture is true. Such observable implications make dark matter and our model legitimate science that is worthy of exploration—even if dark matter is not a building block of you and me. The consequences might include meteoroid impacts—one of which could have been the link between dark matter and the disappearance of the dinosaurs to which the book’s title alludes.

The background and concepts that connect these phenomena offers us a capacious, 3-d picture of the Universe. My goal in writing this book is to share these ideas and to encourage you to explore, appreciate, and bolster the remarkable richness of our world.