Origins: Fourteen Billion Years of Cosmic Evolution (5 page)

By then, matter in large regions of the universe had already begun to coalesce. Where matter accumulates, gravity grows stronger, enabling more and more matter to gather. Those matter-rich regions seeded the formation of galaxy superclusters, while other regions remained relatively empty. The photons that last scattered off electrons within the coalescing regions developed a different, slightly cooler spectrum as they climbed out of the strengthening gravity field, which robbed them of a bit of energy.

The CBR indeed shows spots that are slightly hotter or slightly cooler than average, typically by about one hundred-thousandth of a degree. These hot and cool spots mark the earliest structures in the cosmos, the first clumping together of matter. We know what matter looks like today because we see galaxies, galaxy clusters, and galaxy superclusters. To figure out how those systems arose, we probe the cosmic background radiation, a remarkable relic from the remote past, still filling the entire universe. Studying the patterns in the CBR amounts to a kind of cosmic phrenology: we can read the bumps on the “skull” of the youthful universe and from them deduce behavior not only for an infant but also for a grown-up.

By adding other observations of the local and the distant universe, astronomers can determine all kinds of fundamental cosmic properties from the CBR. Compare the distribution of sizes and temperatures of the slightly warmer and cooler areas, for instance, and we can infer the strength of gravity in the early universe, and thus how quickly matter accumulated. From that we can then deduce how much ordinary matter, dark matter, and dark energy the universe comprises (the percentages are 4, 23, and 73, respectively). From there, it’s easy to tell whether or not the universe will expand forever, and whether or not the expansion will slow down or speed up as time passes.

Ordinary matter is what everyone is made of. It exerts gravity and can absorb, emit, and otherwise interact with light. Dark matter, as we’ll see in Chapter 4, is a substance of unknown nature that produces gravity but does not interact with light in any known way. And dark energy, as we’ll see in Chapter 5, induces an acceleration of the cosmic expansion, forcing the universe to expand more rapidly than it otherwise would. The phrenology exam now says that cosmologists understand how the early universe behaved, but that most of the universe, then and now, consists of stuff they’re clueless about.

Profound areas of ignorance notwithstanding, today, as never before, cosmology has an anchor. The CBR carries the imprint of a portal through which all of us once passed.

The discovery of
the cosmic microwave background added new precision to cosmology by verifying the conclusion, originally derived from observations of distant galaxies, that the universe has been expanding for billions of years. It was the accurate and detailed map of the CBR—a map first made for small patches of the sky using balloon-borne instruments and a telescope at the South Pole, and then for the entire sky by a satellite called the Wilkinson Microwave Anisotropy Probe (WMAP)—that secured cosmology’s place at the table of experimental science. We shall hear much more from WMAP, whose first results appeared in 2003, before our cosmological tale is done.

Cosmologists have plenty of ego: how else could they have the audacity to deduce what brought the universe into being? But the new era of observational cosmology may call for a more modest, less freewheeling stance among its practitioners. Each new observation, each morsel of data, can be good or bad for your theories. On the one hand, the observations provide a basic foundation for cosmology, a foundation that so many other sciences can take for granted because they achieve rich streams of laboratory observations. On the other hand, new data will almost certainly dispatch some of the tall tales that theorists dreamed up when they lacked the observations that would give them thumbs up or down.

No science achieves maturity without precision data. Cosmology has now become precision science.

CHAPTER 4

Let There Be Dark

G
ravity, the most familiar of nature’s forces, offers us simultaneously the best and the least understood phenomena in nature. It took the mind of Isaac Newton, the millennium’s most brilliant and influential, to realize that gravity’s mysterious “action at a distance” arises from the natural effects of every bit of matter, and that the attractive forces between any two objects can be described by a simple algebraic equation. It took the mind of Albert Einstein, the twentieth century’s most brilliant and influential, to show that we can more accurately describe gravity’s action-at-a-distance as a warp in the fabric of space-time, produced by any combination of matter and energy. Einstein demonstrated that Newton’s theory requires some modification to describe gravity accurately—in predicting, for example, the amount by which light rays will bend when they pass by a massive object. Although Einstein’s equations are fancier than Newton’s, they nicely accommodate the matter that we have come to know and love. Matter that we can see, touch, feel, and occasionally taste.

Don’t know who’s next in the genius sequence, but we’ve now been waiting well over half a century for somebody to tell us why the bulk of all the gravitational forces that we’ve measured in the universe arises from substances that we have neither seen, nor touched, nor felt, nor tasted. Or maybe the excess gravity doesn’t come from matter at all, but emanates from some other conceptual thing. In any case, we are without a clue. We find ourselves no closer to an answer today than we were when this “missing mass” problem was first identified in 1933 by astronomers who measured the velocities of galaxies whose gravity affects their close neighbors, and more fully analyzed in 1937 by the colorful Bulgarian-Swiss-American astrophysicist Fritz Zwicky, who taught at the California Institute of Technology for more than forty years, combining his far-ranging insights into the cosmos with a colorful means of expression and an impressive ability to antagonize his colleagues.

Zwicky studied the movement of galaxies within a titanic cluster of galaxies, located far beyond the local stars of the Milky Way that trace out the constellation Coma Berenices (the “hair of Berenice,” an Egyptian queen in antiquity). The Coma cluster, as it is called by those in the know, is an isolated and richly populated ensemble of galaxies about 300 million light-years from Earth. Its many thousands of galaxies orbit the cluster’s center, moving in all directions like bees circling their hive. Using the motions of a few dozen galaxies as tracers of the gravity field that binds the entire cluster, Zwicky discovered their average velocity to be shockingly high. Since larger gravitational forces induce higher velocities in the objects that they attract, Zwicky deduced an enormous mass for the Coma cluster. When we sum up all of its galaxies’ estimated masses, Coma ranks among the largest and most massive galaxy clusters in the universe. Even so, the cluster does not contain enough visible matter to account for the observed speeds of its member galaxies. Matter seems to be missing.

If you apply Newton’s law of gravity and assume that the cluster does not exist in an odd state of expansion or collapse, you can calculate what the characteristic average speed of its galaxies ought to be. All you need is the size of the cluster and an estimate of its total mass: The mass, acting over distances characterized by the cluster’s size, determines how rapidly the galaxies must move to avoid falling into the cluster’s center or escaping from the cluster entirely.

In a similar calculation, as Newton showed, you can derive the speed at which each of the planets at its particular distance from the Sun must move in its orbit. Far from being magic, these speeds satisfy the gravitational circumstance in which each planet finds itself. If the Sun suddenly acquired more mass, Earth and everything else in the solar system would need larger velocities to stay in their current orbits. With too much speed, however, the Sun’s gravity will be insufficient to maintain everybody’s orbit. If Earth’s orbital speed were more than the square root of 2 times its current speed, our planet would achieve “escape velocity” and, you guessed it, escape the solar system. We can apply the same reasoning to much larger objects, such as our own Milky Way galaxy, in which stars move in orbits that respond to the gravity from all the other stars, or in clusters of galaxies, where each of the galaxies likewise feels the gravity from all the other galaxies. As Einstein once wrote (more ringingly in German than in this English translation by one of us [DG]) to honor Isaac Newton:

Look unto the stars to teach us

How the master’s thoughts can reach us

Each one follows Newton’s math

Silently along its path.

When we examine the Coma cluster, as Zwicky did during the 1930s, we find that its member galaxies all move more rapidly than the escape velocity for the cluster, but only if we establish that velocity from the sum of all the galaxy masses taken one by one, which we estimate from the galaxies’ brightnesses. The cluster should therefore swiftly fly apart, leaving barely a trace of its beehive existence after just a few hundred million years, perhaps a billion, had passed. But the cluster is more than 10 billion years old, nearly as old as the universe itself. And so was born what remains the longest-standing mystery in astronomy.

Through the decades that followed Zwicky’s work, other galaxy clusters revealed the same problem. So Coma could not be blamed for being odd. Then whom should we blame? Newton? No, his theories had been examined for 250 years and passed all tests. Einstein? No. The formidable gravity of galaxy clusters does not rise high enough to require the full hammer of Einstein’s general theory of relativity, just two decades old when Zwicky did his research. Perhaps the “missing mass” needed to bind the Coma cluster’s galaxies does exist, but in some unknown, invisible form. For a time, astronomers renamed the missing-mass problem the “missing-light problem,” since the mass had been strongly inferred from the excess of gravity. Today, with better determinations of the masses of galaxy clusters, astronomers use the moniker “dark matter,” although “dark gravity” would be more precise.

The dark matter
problem reared its invisible head a second time. In 1976, Vera Rubin, an astrophysicist at the Carnegie Institution of Washington, discovered a similar “missing-mass” anomaly within spiral galaxies themselves. Studying the speeds at which stars orbit their galaxy centers, Rubin first found what she expected: within the visible disk of each galaxy, the stars farther from the center move at greater speeds than stars close in. The farther stars have more matter (stars and gas) between themselves and the galaxy center, requiring higher speeds to sustain their orbits. Beyond the galaxy’s luminous disk, however, we can still find some isolated gas clouds and a few bright stars. Using these objects as tracers of the gravity field “outside” the galaxy, where visible matter no longer adds to the total, Rubin discovered that their orbital speeds, which should have fallen with increasing distance out there in Nowheresville, in fact remained high.

These largely empty volumes of space—the rural regions of each galaxy—contain too little visible matter to explain the orbital speeds of the tracers. Rubin correctly reasoned that some form of dark matter must lie in these far-out regions, well beyond the visible edge of each spiral galaxy. Indeed, the dark matter forms a kind of halo around the entire galaxy.

This halo problem exists under our noses, right in our own Milky Way galaxy. From galaxy to galaxy and from cluster to cluster, the discrepancy between the mass in visible objects and the total mass of systems ranges from a factor of just 2 or 3 up to factors of many hundreds. Across the universe, the factor averages to about 6. That is, cosmic dark matter enjoys about six times the mass of all the visible matter.

Over the past twenty-five years, further research has revealed that most of the dark matter cannot consist of nonluminous ordinary matter. This conclusion rests on two lines of reasoning. First, we can eliminate with near certainty all plausible familiar candidates, like the suspects in a police lineup: Could the dark matter reside in black holes? No, we think that we would have detected this many black holes from their gravitational effects on nearby stars. Could it be dark clouds? No, they would absorb or otherwise interact with light from stars behind them, which real dark matter doesn’t do. Could it be interstellar (or intergalactic) planets, asteroids, and comets, all of which produce no light of their own? It’s hard to believe that the universe would manufacture six times as much mass in planets as in stars. That would mean six thousand Jupiters for every star in the galaxy, or worse yet, 2 million Earths. In our own solar system, for example, everything that is not the Sun adds to a paltry 0.2 percent of the Sun’s mass.

Thus, as best we can figure, the dark matter doesn’t simply consist of matter that happens to be dark. Instead, it’s something else altogether. Dark matter exerts gravity according to the same rules that ordinary matter follows, but it does little else that might allow us to detect it. Of course, we are hamstrung in this analysis by not knowing what the dark matter is. The difficulties of detecting dark matter, intimately connected with our difficulties in perceiving what it might be, raise the question: If all matter has mass, and all mass has gravity, does all gravity have matter? We don’t know. The name “dark matter” presupposes the existence of a kind of matter that has gravity and that we don’t yet understand. But maybe it’s the gravity that we don’t understand.

To study dark matter beyond deducing its existence, astrophysicists now seek to learn where the stuff collects in space. If dark matter existed only at the outer edges of galaxy clusters, for example, then the galaxies’ velocities would show no evidence of a dark matter problem, because the galaxies’ speeds and trajectories respond only to sources of gravity interior to their orbits. If the dark matter occupied only the clusters’ centers, then the run of galaxy speeds as measured from the center of the cluster out to its edge would respond to ordinary matter alone. But the speeds of galaxies in clusters reveal that the dark matter permeates the entire volume occupied by the orbiting galaxies. In fact, the locations of ordinary matter and dark matter loosely coincide with each other. Several years ago, a team led by the American astrophysicist J. Anthony Tyson, then at Bell Labs and now at UC Davis (he’s called “Cousin Tony” by one of us, though we have no family relationship) produced the first detailed map of the distribution of dark matter’s gravity in and around a titanic cluster of galaxies. Wherever we see big galaxies, we also find a higher concentration of dark matter within the cluster. The converse is also true: regions with no visible galaxies have a dearth of dark matter.

The discrepancy between
dark and ordinary matter varies significantly from one astrophysical environment to another, but it becomes most pronounced for large entities such as galaxies and galaxy clusters. For the smallest objects, such as moons and planets, no discrepancy exists. Earth’s surface gravity, for example, can be explained entirely by what’s under our feet. So if you are overweight on Earth, don’t blame dark matter. Dark matter also has no bearing on the Moon’s orbit around Earth, nor on the movements of the planets around the Sun. But we do need it to explain the motions of stars around the center of the galaxy.

Does a different kind of gravitational physics operate on the galactic scale? Probably not. More likely, dark matter consists of matter whose nature we have yet to divine, and which clusters more diffusely than ordinary matter does. Otherwise, we would find that one in every six pieces of dark matter has a chunk of ordinary matter clinging to it. So far as we can tell, that’s not the way things are.

At the risk of inducing depression, astrophysicists sometimes argue that all the matter that we have come to know and love in the universe—the stuff of stars, planets, and life—are mere buoys afloat in a vast cosmic ocean of something that looks like nothing.

But what if this conclusion were entirely wrong? When nothing else seems to work, some scientists will understandably, and quite rightly, question the fundamental laws of physics that underlies the assumptions made by others who seek to understand the universe.

During the early 1980s, the Israeli physicist Mordehai Milgrom of the Weizmann Institute of Science in Rehovot, Israel, suggested a change in Newton’s laws of gravity, a theory now known as MOND (MOdified Newtonian Dynamics). Accepting the fact that standard Newtonian dynamics operates successfully on size scales smaller than galaxies, Milgrom suggested that Newton needed some help in describing gravity’s effects at distances the sizes of galaxies and galaxy clusters, within which individual stars and star clusters are so far apart that they exert relatively little gravitational force on each other. Milgrom added an extra term to Newton’s equation, specifically tailored to come to life at astronomically large distances. Although he invented MOND as a computational tool, Milgrom didn’t rule out the possibility that his theory could refer to a new phenomenon of nature.

MOND has had only limited success. The theory can account for the movement of isolated objects in the outer reaches of many spiral galaxies, but it raises more questions than it answers. MOND fails to predict reliably the dynamics of more complex configurations, such as the movement of galaxies in binary and multiple systems. Furthermore, the detailed map of the cosmic background radiation produced by the WMAP satellite in 2003 allowed cosmologists to isolate and measure the influence of dark matter in the early universe. Because these results appear to correspond to a consistent model of the cosmos based on conventional theories of gravity, MOND has lost many adherents.

During the first half million years after the big bang, a mere moment in the 14-billion-year sweep of cosmic history, matter in the universe had already begun to coalesce into the blobs that would become clusters and superclusters of galaxies. But the cosmos was expanding all along, and would double in size during its next half million years. So the universe responds to two competing effects: gravity wants to make stuff coagulate, but the expansion wants to dilute it. If you do the math, you rapidly deduce that the gravity from ordinary matter could not win this battle by itself. It needed the help of dark matter, without which we would be living—actually not living—in a universe with no structure: no clusters, no galaxies, no stars, no planets, no people. How much gravity from dark matter did it need? Six times as much as that provided by ordinary matter itself. This analysis leaves no room for MOND’s little corrective terms in Newton’s laws. The analysis doesn’t tell us what dark matter is, only that dark matter’s effects are real—and that, try as you may, you cannot credit ordinary matter for it.