AMNHA detail of an image from the American Museum of Natural History’s space show Dark Universe (2013), directed by Carter Emmart and narrated by Neil deGrasse Tyson, showing the distribution of so far unseen dark matter in the universe. It is simulated here with the use of a high-resolution algorithm at the Kavli Institute of Particle Astrophysics and Cosmology at Stanford University and the Stanford National Accelerator Laboratory.
The notion that there is more to the world than we can see was probably hardwired into our ancestors’ consciousness by natural selection. Only those in the African savannah who suspected that behind the rustling branches might lie a predator likely survived long enough to pass on their genetic information. In any case, as our mental facilities evolved further the notion of hidden realities became more formalized when human tribal groups created religions to help them make sense of the world around them, provide solace for the inequities of nature, and ultimately deal with their own mortality.
Hidden realities continued to dominate thinking as early religious myth gave way to more refined philosophical speculation. Searching for the fundamental essence underlying all matter became a common quest in Greek and Roman philosophy, while air, earth, fire, and water were sequentially dispensed with as providing such an essence. Ultimately philosophers from Empedocles to Aristotle decided that this fundamental essence must be something distinct; and Plato, through his derivation of five perfect solids, focused on a “quinta essentia,” a fifth essence. This also became known as “aether” and was imagined to comprise the fundamental essence of all space, permeating both heaven and earth. By connecting the stars and planets with terrestrial matter, this aether also motivated the ancient Alexandrian practice of astrology, which, while the aether has disappeared, unfortunately remains prevalent even today.
As religious myth and early philosophical speculation then gave way to scientific discovery and we developed machines to perceive what our eyes and ears could not, the fact that the world of our experience reflected merely a small part of a much greater whole became manifest. Light itself is just one small piece of a continuous spectrum of invisible electromagnetic waves that are filling the space around us and bombarding us at all times. When we look up at the night sky, we now realize that the seemingly dark emptiness between stars is not in fact empty. If one were to create a dime-sized hole between thumb and forefinger and hold it out at arm’s length, in that small region the largest telescopes today, like those in Chile or Hawaii, could discern literally hundreds of thousands of other galaxies like our own Milky Way.
Indeed, given the myriad recent developments in modern astronomy, it is sometimes hard to appreciate that less than one hundred years ago the entire universe, as conceived by astronomers, consisted of a single galaxy, the Milky Way, surrounded by a possibly eternal static void. We now know not only that there are at least 100 billion galaxies beyond our own in the observable universe, but also that our universe is expanding, and most recently we have discovered that the expansion is actually speeding up, for reasons we have yet to understand.
So perhaps it is not surprising to find that in the intervening century we have discovered a host of other previously invisible entities surrounding and in many cases permeating the space we occupy. Consider two cases:
1. Every cubic centimeter of space is teeming with three hundred microwave photons left over from the Big Bang explosion—particles that last interacted with matter when the universe was 300,000 years old. Yet while perhaps nothing is easier to detect than electromagnetic radiation, this background, now referred to as the cosmic microwave background radiation, remained unnoticed until 1964, when it was discovered by accident by two Bell Laboratory physicists who were using a radio telescope to search for other signals.
2. Every second over 600 billion particles called neutrinos penetrate every square centimeter of your body, traversing it, and the earth, without interaction. These neutrinos emanate from nuclear reactions deep inside the sun, the very reactions that power our star and make our lives possible. Only in the 1990s were we able to experimentally confirm that such a background of neutrinos existed and to ascertain its magnitude, through a set of observations that resulted in the awarding of one half of last year’s Nobel Prize in physics. A similar background of neutrinos, left over from the Big Bang, is predicted to exist, but to date no experiment has been sensitive enough to directly detect it.
In hindsight then, after these discoveries of invisible exotica it does not take a great leap of the imagination to wonder if there might be other undetected backgrounds out there, or even in the room in which I am typing this essay.
Needless to say, science doesn’t proceed by hindsight, but rather by insight, and the history of astronomy in the twentieth century involved a long struggle in which scientists were dragged, against their a priori prejudices, to the realization that the universe of our experience—stars, galaxies, planets, and life—is essentially an irrelevant sideshow. The important stuff is invisible, quite possibly made of some new type of matter.
Writing, as I am now, from the remote shores of frozen Antarctica, it is naturally tempting to compare our visible universe to the tip of a vast cosmic iceberg, most of which is invisible, dark, and out of sight. Tempting or not, the comparison is particularly apt. As the captain of the Titanic found out, often what you cannot see is more important than what you can.
So too this invisible background of cosmic material, which physicists, with great linguistic perspicacity, have come to call “dark matter”—a name that Lisa Randall takes issue with in her new book Dark Matter and the Dinosaurs—is now understood to have ultimately determined the dynamics of the formation of structure in the universe, including all the cosmic structures we observe with our telescopes today. In short, without this invisible background of cosmic material we would not exist.
The story of how this came about is typical of major revolutions in our understanding of nature. It involved a series of baby steps, missteps, and hard work, as well as the growing convergence of two fields of physics that on the surface couldn’t seem farther apart: particle physics, the study of the dynamics of the very small, and cosmology, the study of the dynamics of the universe on its largest scales.
The physicist and Nobel laureate Sheldon Glashow first used the example of the ancient Egyptian symbol ouroboros, a snake eating its own tail, to describe this convergence. Since Edwin Hubble’s groundbreaking discovery in 1929 that the universe is expanding, we have recognized that the entire observable universe, all 100 billion or so galaxies, each containing 100 billion or so stars, was, some 13.8 billion years ago, confined to a region that was perhaps smaller than a single atom today. If this is the case, then the initial conditions that determined the origin, makeup, and nature of the largest cosmic objects today were determined on subatomic scales. So to understand the universe on its largest scales we ultimately must push forward our understanding of the fundamental structure of matter and forces on the smallest scales.
The first suggestion that on the scale of galaxies normal matter—made up of atoms, themselves comprised of protons, neutrons, and electrons—is not all that there is came indirectly, and largely without fanfare. Fritz Zwicky was a brilliant and irascible astronomer at Caltech whose relationship with his colleagues was summarized by his description of them: “spherical bastards,” because he felt they were bastards any way you looked at them. He nevertheless made a series of discoveries in the 1930s that essentially presaged almost all the developments in cosmology over the next half-century. In 1933 he observed the velocities of individual galaxies in the distant Coma Cluster of galaxies. (Essentially all galaxies, including our own, are bound together in huge conglomerations containing hundreds or sometimes thousands of individual galaxies.) He found that the relative velocities of the galaxies in the cluster were so large that they shouldn’t have remained bound within the cluster unless there was perhaps four hundred times more mass than could be accounted for by the luminous stars in each galaxy. Although others had previously made similar observations, Zwicky was the first to suggest that this invisible matter might be some new sort of exotic material, which he called “dark matter” (dunkle Materie in German).
This evidence languished for almost forty years until the groundbreaking work of Vera Rubin and her colleague Kent Ford in the 1970s. Rubin, only the second woman to be awarded the Gold Medal of the Royal Astronomical Society, did not come by her discovery easily. She graduated with a doctorate from Georgetown (taking night classes while her husband waited in the car because she didn’t know how to drive), after being turned down by Princeton because they didn’t accept women into their graduate program at the time. Careful measurements by Rubin and Ford, and then a host of others, of stars and hot gas in our own galaxy ultimately established beyond a doubt that the outer parts of our galaxy were orbiting around the center too fast.
If the gravitational pull caused by the visible stars in our galaxy were governing their motion, their orbital speeds should fall off the farther from the center of the galaxy they are, just as the speeds of the outer planets in our solar systems do. Instead, the rotation rate appears to be constant no matter how far out one probes, well beyond the region where most of the stars and gas lie. Unless the nature of gravity changes at these distances, the only explanation is that the mass of our galaxy (and essentially all others whose rotation curves have been measured) continues to increase linearly with distance, even though the density of stars and visible matter falls off at these distances.
Fritz Zwicky appears once again in the story. In 1936 Albert Einstein published a paper pointing out that the fact that light bends in a gravitational field means that massive objects can act like “gravitational lenses,” distorting and magnifying images of objects located behind them. He felt that this phenomenon would never be observable, but within a year Zwicky wrote a paper demonstrating that not only should gravitational lensing by galaxies and clusters of galaxies be detectable, but doing so would allow us to “weigh” these systems, including both visible and dark matter—anything that contributes to the gravitational forces. Sure enough, sixty years later when astronomy had progressed to the point where this was possible, the lensing effect of gravitation was used to establish that the largest objects bound by gravitational forces in the universe today, clusters of galaxies, contain forty times more matter than can be accounted for by stars and gas.
Other techniques, including measuring the radiation from the Big Bang itself, the so-called cosmic microwave background radiation, which allows very precise measures of the amount of matter in the universe at a time when the universe was only about 300,000 years old, as well as the overall geometry of the universe, have now confirmed unequivocally and precisely that the average density of matter in the universe exceeds the density associated with stars and hot gas by the same factor of forty.
On one hand, perhaps it is not surprising that a lot of matter exists in the universe that is not visible to telescopes. Planets don’t shine, and neither do snowballs or book reviewers. However, over the past fifty years or so it has become clear that there simply isn’t enough normal matter, made of protons and neutrons, to account for all the dark matter in the universe. Careful calculations of nuclear reactions forming light elements like helium and lithium in the early universe put an upper limit on the abundance of protons and neutrons engaging in these interactions, and that limit is only about 20 percent of the total inferred density of dark matter today. This value has recently been confirmed using observations of cosmic microwave background radiation, and in fact is a factor of five to eight times as large as the abundance of visible material in the universe. So while we now know that most normal matter in the universe doesn’t shine, there is not enough to account for all the inferred dark matter.
Moreover, we also know that all or most of the inferred dark matter cannot interact as normal matter does, or galaxies wouldn’t have formed. By observing the neonatal universe, and comparing it to the universe we see today, we can calculate that normal matter simply would not have had sufficient time to collapse to form galaxies and stars over the last 13.8 billion years. What is needed is to find some new form of matter that interacts much more weakly than protons and neutrons, and that also is moving slowly enough so that it could not escape even the gentle gravitational potential present in the early universe. With this form of matter, resistance to gravitational collapse would be much smaller early on and the formation of structures could proceed apace, with normal matter falling into these nascent structures later on, forming stars and the visible parts of galaxies.
In short, what is needed is some new kind of weakly interacting elementary particle produced in the early universe—something like neutrinos, but much heavier. And this is where particle physicists began to get involved in the game. Not only do we have lots of possible candidates, but it may be possible to detect dark matter directly in the laboratory if we are clever, or perhaps produce it directly in particle accelerators like the Large Hadron Collider. With these realizations over the past thirty years or so, the race has been on to try to determine the nature of most of the matter in the universe.
The dark matter saga is sufficiently exciting and mysterious that a host of popular books have been devoted to the subject over the past twenty-five years. The most recent in this line takes a slightly different tack, however. Dark Matter and the Dinosaurs, written by a distinguished particle theorist at Harvard University, Lisa Randall, is not actually focused on dark matter per se. Nor is it focused on dinosaurs. Rather it reflects an effort to explore a possible implication for astrophysics of an idea in elementary particle physics that Randall and a colleague proposed several years ago.
In her book, Randall argues that exotic types of dark matter could alter the structure of the galaxy, and as a result a previous proposal, by Michael Rampino and colleagues, that galactic gravitational effects might result in periodic impacts of comets on earth could now become viable. Specifically, if an exotic type of dark matter forms a disk within the Milky Way galaxy, and if the sun crosses the disk every 30 million years or so in its voyage around the galaxy, the resulting extra gravitational forces might nudge comets out of the Oort cloud, the sparse cloud of trillions of objects surrounding the solar system. Some of these comets might then hit the earth and in so doing might produce devastating periodic extinctions of life, including the impact 65 million years ago that appears to have led to the demise of the dinosaurs.
The book is engaging, and written from an accessible personal perspective, which is not surprising, given the personal importance to the author of the story being told. Randall’s excitement about the areas she has studied in the process of her research is evident. The book easily flows over a diverse collection of interesting fields of science. There is a relatively brief introduction to the nature of dark matter (including an amusing digression on why the term “dark matter” is really a misnomer—“invisible matter” would be a better, if less sexy, name), followed by general discussions ranging from comets and asteroids to the nature of the galaxy, astrophysical impacts, and ultimately extinctions.
The problem, however, is whether the proposal itself warrants packaging these individual pieces together into an entire book. When scientists write popular books about science, there is an implicit mandate to present a balanced perspective of the most exciting recent developments. Because the general public does not as a whole possess the critical scientific knowledge adequate to the task of distinguishing which new scientific claims are widely supported and which are not, it is easy for a book to either knowingly or unknowingly mislead. The danger of doing this, often seen when dubious preliminary results are instead reported as exciting discoveries in the popular press, is that when they are later retracted or shown to be false the public’s trust in the scientific process, and in the dependability of results that have stood the test of time and experiment, diminishes.
Randall is not guilty of such hyperbole in this regard. She is clear about the fact that the particle physics model she has proposed is speculative, and even that the premise that extinctions of life on earth have been periodic is not necessarily generally accepted. It should also be added that there is no evidence that all major extinctions have been due to impacts from outside the earth. For example, evidence has mounted recently that the greatest mass extinction in recorded history, the so-called Permian-Triassic event about 250 million years ago, which killed more than 90 percent of species on earth, was due not to an impact but to unprecedented continuous volcanic activity in what is now Siberia over tens of thousands of years, which generated perhaps a million cubic kilometers of lava. This covered a region as large as the US, and produced unprecedented worldwide climate change and acidification of the oceans.
Yet turning what originally was a four-page paper published by Randall and her associates in a scientific journal into a four-hundred-page book for the general public suggests some significance for the claim. A quick scan of the citation record for that paper, however, reveals a total of only six citations in the year and a half or so since it was published, a very small number by standards in the field. The idea has thus far generated little scientific interest, whether or not it may be intriguing a priori, or whether it has excited interest among science journalists.
This is in stark contrast to Randall’s first book, Warped Passages (2005), which was also based on speculative work by her and collaborators—in this case the proposal that there may be large otherwise invisible extra dimensions in nature. The difference there was that the motivation for the proposal involved an attempt to solve one of the central outstanding puzzles in particle physics, namely why gravity is so weak compared to other forces in nature. Moreover, the work itself sparked considerable interest in the physics community, becoming one of the most frequently cited articles in recent years. It made sense to explain this excitement to the public, even without direct empirical evidence thus far.
Dark Matter and the Dinosaurs is reminiscent in this sense of The Life of the Cosmos(1997), by another well-established physicist, Lee Smolin, which also promoted into a book a proposal from an article that didn’t gain much traction in the scientific community—that there is a kind of cosmic natural selection process for universes, similar in operation to biological evolution. It is easy to understand how well-respected scientists like Smolin and Randall can be sufficiently excited by their ideas to want to write about them more broadly. It is just such excitement that drives theoretical physicists to devote so much energy to their work. But that doesn’t guarantee that the proposals are really ready for wide-scale publication.
Geoplanetary concerns aside, from a particle physics perspective there are reasons why this new proposal for dark matter hasn’t excited broad interest. As physicists have examined myriad ways in which exotic new microphysics might solve cosmological problems, it has become common to suggest that it is acceptable to incorporate a single “tooth fairy”—namely a single new relatively unconstrained speculation—to form the basis of a proposal. But two tooth fairies seem less convincing. In this case, it is well established that dark matter exists in a roughly spherical halo around our disk-like Milky Way sea of stars. To enable the process proposed in Randall’s book, some dark matter would have to collapse into an additional compact galactic disk. For this to happen Randall writes that there must be at least one as yet unknown additional component of dark matter. Randall also argues that this component must be able to interact in new ways that would allow it to radiate energy while still remaining undetectable to telescopes. As described near the end of the book, which is where Randall gets to the meat of her proposal, such models can be developed by creative theorists like her. But being possible and being likely are two different things.
One should always be skeptical in physics, but nevertheless it is also worth stressing that we currently have no clear understanding—merely well-motivated preconceptions—about the nature of dark matter. A priori proposals about what seems likely could easily be wrong. Skilled theoretical physicists will continue to explore new ideas, as they should, and no one can be sure where the evidence may lead us. Independent of the likelihood of Randall’s recent proposal, for over half a century the story of dark matter has established remarkable new connections between the very large and the very small that have been worth celebrating.
The story of dark matter, as it has evolved over the past fifty years, has surprised us at many turns. I fully expect that it will continue to do so in the future. Every time we open a new window on the universe, unexpected new connections arise. Like other scientists working in this area, each day I am surprised if I am not surprised.
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