The 4-Percent Universe

by Richard Panek

  Just brilliant.

  Vera Rubin read the opening sentence of that second paper and recognized the kind of breadth of vision and distillation of ideas that could redefine a field. And she wasn't alone. The two papers created a sensation, though not the kind that Peebles and Ostriker might have hoped for. People were angry. And while Peebles hardly noticed (observers, upset with theorists?—so be it!), Ostriker felt such intense hostility that he had to wonder: Were most astronomers even reading what was on the page? Or were they just having a visceral reaction to the possibility that what they had been studying this whole time was only 10 percent of what was actually out there? Either way, most astronomers still weren't in the habit of thinking about the relationship between gravity and galaxies.

  Vera Rubin was. After she and Kent Ford completed their paper on Andromeda in 1969, she had turned her attention to the question that had motivated her master's thesis: Did the universe rotate? Or, in more mature terms, did the distribution and velocity of galaxies suggest a lack of uniformity beyond the local universe—on the kind of scale that could make the universe a little less simple?

  Twenty years had passed since that rancorous AAS meeting in Haverford. Rubin was no longer an unknown neophyte relying on the research of real astronomers. She had made a name for herself (and it wasn't Vera Hubin); her early work had been vindicated. Gérard de Vaucouleurs, Rubin's constant correspondent during the 1950s, had published several papers over the years showing results similar to her own. By the 1970s the pattern of non-uniform distribution of galaxies on a relatively local scale was, as she wrote in a paper during this period, "well discussed"; her fellow astronomers had adjusted themselves to the evidence that some galaxies were clustering even as the universe as a whole was expanding. The general assumption, however, was that at greater distances the universe would be the same in every direction—that any departures from homogeneity and isotropy were local, and that on larger scales the galaxies would adopt a more uniform distribution. The question Ford and Rubin (and her daughter, Judith, a student at Radcliffe) would address was: Did galaxies really behave this way?

  "The results," they wrote in 1973, while still collecting their data, "are so striking that we wish to present a preliminary account."

  Once again Rubin found that galaxies exhibited not just the recessional motion of the expansion but peculiar motions. In this case, a group of local galaxies seemed to be racing together toward one part of the sky. And once again much of the community rejected the conclusion. The Rubin-Ford effect, as it became known, was the subject of virulent arguments at conferences. Prominent astronomers begged Rubin to drop the line of research before she ruined her career. But she and Ford pushed their observing mission to the end and, in 1976, published the complete set of data in two papers that they felt established the Rubin-Ford effect as real.

  As usual, Rubin didn't like the controversy. She didn't like everyone challenging her on every number. She didn't want to have to defend her data. She didn't want to have to defend the universe. She would say that she wasn't "smart enough" to know why the universe was the way it was: "I could design a woman's plumbing. But the universe, I couldn't do it." The universe was what it was. And she was who she was. Shortly after publishing the papers on the Rubin-Ford effect, she attended a Yale conference on galaxies; above the entrance hung a giant banner: astronomers. She walked under it. "Okay," she thought wryly. "Now I'm an astronomer."

  Besides, she and Ford had something else to pursue. They had seen a continuation of the phenomenon that they had noted in their 1970 paper on Andromeda and that Mort Roberts had shown them in his radio observations of the same galaxy. In their observations of galaxies that led to the Rubin-Ford effect, they looked at galaxies far more distant than Andromeda, and therefore far smaller from the point of view of an observer on Earth. They could see the galaxy in one gulp. In the end they studied sixty galaxies, and even though Rubin was using the spectroscope to measure the motions of entire galaxies, the rotation curves showed up anyway, a shadowy residual effect. These rotation curves looked flat too, just like Andromeda's, at least at a glance. Would they still look flat under more rigorous, more focused examination? Rubin decided to do what an observer does: more observing.

  For their 1970 paper she and Ford had pushed as far out to the periphery of Andromeda as 1960s technology allowed. In 1974 a new 4-meter telescope opened at Kitt Peak, twice the diameter and therefore four times the surface area of the one they had used in observing Andromeda. The combination of Ford's spectrograph and a significantly larger telescope would allow them to take their study of galaxies both deeper into the universe and farther along the arms of the spirals. In 1978 Ford and Rubin published the rotation curves for eight more galaxies: all flat.

  Once again radio astronomers were getting the same results. Mort Roberts kept pushing along a ring of hydrogen gas clouds that lay beyond the visible swirl of stars and gas. In 1975 Roberts and a collaborator found that even there, half the length of Andromeda beyond what previous generations had unthinkingly assumed was the galaxy in its entirety, the rotation wasn't tapering off. It was essentially flat, as if even at this great distance the galaxy was still spinning at a seemingly suicidal rate. A 1978 survey using the same method found the same shape for the curves in twenty-two of twenty-five other galaxies: flat.

  Rubin had gotten her wish. The data spoke with one voice, and it spoke clearly: Galaxies were living fast but not dying young. Observers and theorists could question the evidence and double-check the methodologies, as they should and did. Some suggested that radio observations were of necessity indistinct; they covered too much of the sky to provide reliable data. Some suggested that optical data like Rubin's suffered from a bias; she was looking only at high-luminosity galaxies because they were the easiest to find, and maybe their masses were anomalous. Some suggested that elliptical galaxies wouldn't show the same flat rotation curves as spiral ones. But even the most ardent critics were finding it difficult to quarrel with the uniformity of the data. Plot after plot from astronomer after astronomer in journal after journal—all a skeptic had to do was look at the rotation curves. You could see where the sources of light were. You could see where the motions of the galaxy said the mass should be. And you could see that the two didn't match.

  In 1979, in an article in the Annual Review of Astronomy and Astrophysics, two astronomers—including Sandra Faber, who had been unimpressed by Mort Roberts's flat rotation curve when she was a graduate student visiting DTM—looked at all the evidence they could gather. "Is there more to a galaxy than meets the eye (or can be seen on a photograph)?" they wrote in the opening sentence. Their conclusion, forty-seven pages of exhaustive analysis later: "After reviewing all the evidence, it is our opinion that the case for invisible mass in the Universe is very strong and getting stronger."

  A couple of years earlier, Rubin had come away from the Yale conference on galaxies with the impression that, as she wrote, "many astronomers hoped that dark matter might be avoided." Now, the publication of Faber and Jay Gallagher's comprehensive argument left most astronomers agreeing that their field had a problem with "missing mass"—though this term increasingly seemed like a misnomer. After all, the problem wasn't that astronomers didn't know where the mass was. They did. It was in the halo—or at least in a "massive envelope," the term that Faber and Gallagher adopted in an effort to be "neutral" as to the shape. The problem for astronomers was that they couldn't see it. Not with their eyes, not with a traditional optical telescope, not with a telescope that could see in any wavelength of light. In which case, the mass wasn't "missing" at all. It was just—to borrow the term that Zwicky had used in 1933— dunkle: dark.

  "Nobody ever told us that all matter radiated," Vera Rubin liked to say. "We just assumed that it did." Her tone, like the reaction in Dicke's office on the day he got the phone call about the detection at Bell Labs, was not one of disappointment. Instead, she felt that by "recognizing that they study only the 5 or 10 perce
nt of the universe which is luminous," astronomers "can approach their tasks with some amusement."

  The joke was on us. In 1609 Galileo had discovered that looking farther into space than what he could see with the naked eye led to seeing more of the universe. Since the middle of the twentieth century, astronomers had discovered that looking farther along the electromagnetic spectrum than what they could see with an optical telescope led to seeing even more of the universe—including the echo of its origins. And now, if you were Vera Rubin, you could look up from your desk and gaze at the giant photograph of Andromeda that you'd hung on the ceiling, and you could ask, with greater sophistication than a ten-year-old leaning on a bedroom windowsill but with the same insatiable wonder: How could you possibly see farther than the electromagnetic spectrum—farther than seeing itself?

  PART II

  Lo and Behold

  4. Getting in the Game

  THE WEIGHT OF the universe. The shape of the universe. The fate of the universe.

  They talked about it in those terms. They used this giddy language in proposals to solicit funding. They used it in a brochure to recruit graduate students. They used it with the other members of their collaboration as they all told themselves that they were the ones who were finally going to solve some of the most profound mysteries of cosmology—of civilization itself. They also used this language when they needed to reassure themselves that they weren't rebuilding Babel or emulating Icarus, that their experiment was an exercise not in hubris but in science.

  Okay, maybe a little hubris. Saul Perlmutter wasn't a born astronomer. He hadn't collected telescope parts as a child, hadn't sketched the motions of the night sky, hadn't dreamed of solitary vigils on mountaintops, just him and the heavens. Carl Pennypacker wasn't a born astronomer either, though at least his PhD in physics was on a related topic, infrared astronomy. And the other members of their team weren't astronomers. None of them had come to Lawrence Berkeley National Laboratory to do astronomy; astronomy wasn't what LBL usually did. Still, they had reason to think they were in the right place at the right time.

  The right place, because LBL and the University of California, Berkeley, had just won a government competition to establish a major new research center. The name on the proposal was the Center for Particle Astrophysics, though because the titular "particle" was dark matter, they could have called it the Dark Matter Center—and, as the first director of the center once said, they probably would have if they'd thought of it.

  And the right time, because by the 1980s scientists could proceed under the assumption that they were in possession of the middle and the beginning of the cosmic narrative. They knew that their protagonist—the universe—was expanding. They had a reasonable explanation for how it had gotten to this point in the story—the Big Bang. Now they could ask themselves: What will become of Our Hero?

  Did the universe contain enough matter to slow the expansion so much that one day it would stretch as far as it could, stop, and reverse itself, like the trajectory of a tossed ball returning to Earth? In such a universe, space would be finite, curving back on itself, like a globe.

  Or did the universe contain so little matter that the expansion would never stop but go on and on, like a rocket leaving Earth's atmosphere? In this kind of universe, space would be infinite, curving away from itself, like a saddle.

  Or did the universe contain just enough matter to slow the expansion so that it would eventually come to a virtual halt? In this universe, space would be infinite and flat.

  Borrowing from the Big Bang example, astronomers gave the first options the cheerfully inadequate names Big Crunch (too much matter) and Big Chill (too little matter); the third option was the Goldilocks universe (just right). From only one measurement, astronomers could determine the weight, the shape, and the fate of the universe.

  Before the 1980s, astronomers had certainly known that the amount of matter in the universe would have an effect on the universe's rate of expansion. What they hadn't known was that they had been missing 90 percent or more of the matter. The possible cosmological implications of this realization had been evident from the start. "Not until we learn the characteristics and the spatial distribution of the dark matter," Vera Rubin had written in Science shortly after the idea gained widespread acceptance, "can we predict whether the universe is of high density, so that the expansion will ultimately be halted and the universe will start to contract, or of low density, and so that the expansion will go on forever."

  Now Perlmutter and Pennypacker set out to make that measurement. They recognized that writing the closing chapter in the story of the universe would be challenging in the extreme, and they figured they would be done in, oh, three years.

  The question of how the universe will end was as old as civilization, but the difference now was that scientists might be able to go out and make the crucial measurement. Because the discovery of the 3 K temperature had matched a prediction from the Big Bang theory, it had taught astronomers what to think about cosmology: It just might be a science after all. But the 3 K discovery also taught them how to think about cosmology: If you want to understand the history and structure of the universe—if you want to do cosmology—you have to do what Bob Dicke and Jim Peebles had been urging even before the discovery of the cosmic microwave background: think about gravity on the scale of the universe.

  Not that astronomers had been altogether ignoring the relationship between gravity and the universe. Much of modern physics and all of modern astronomy had arisen from Newton's epic struggles to derive a law of gravity that was universal. In his Principia, published in 1687, Newton met Plato's challenge to find the calculations on paper that matched the motions in the heavens. The telescope had given astronomers the physical tool to chronicle more and more of those motions. But it was Newton's math that had given them the intellectual tool to make sense of them. The law of universal gravitation was what made cosmology-as-science possible.

  Yet it also made cosmology-as-science problematic. A syllogism (of sorts): One, the universe is full of matter; two, matter attracts other matter through gravity; therefore, the universe must be collapsing. So why wasn't it?

  This was the question that the cleric Richard Bentley posed to Newton in 1692 while preparing a series of lectures on faith, reason, and the just-published Principia. Newton acknowledged that his argument required "that all the particles in an infinite space should be so accurately poised one among another as to stand still in a perfect equilibrium. For I reckon this as hard as to make not one needle only but an infinite number of them (so many as there are particles in an infinite space) stand accurately poised upon their points." How was such an equilibrium possible? In a later edition of the Principia, Newton appended a General Scholium in which he postulated an answer—the foresight of God: "And so that the systems of the fixed stars will not fall upon one another as a result of their gravity, he has placed them at immense distances from one another."

  What made cosmology scientifically suspect for investigators of nature wasn't just this invocation of a supernatural cause—a cause that was, literally, beyond nature. The problem was the effect. Or, more accurately, it was the absence of an effect. Newton's physics was all cause-and-effect, matter-and-motion. Yet what he was proposing in this one instance was a lack of gravitational interaction among the bodies of the cosmos. Having conceived of gravity as action at a distance, Newton was now suggesting the need for inaction at a distance.

  Over the following decades and centuries, the more that astronomers discovered about the system of "fixed stars"—that the stars aren't fixed at all but are in motion relative to one another, and that the entire system of unfixed stars, our galaxy, rotates around a common center—the less satisfying was the explanation of inaction at great distances.

  Einstein made subtle adjustments to Newton's theory of gravity. And in his 1916 theory of general relativity, he presented calculations on paper that matched the motions in the heavens slightly more accurately tha
n Newton's. Yet he, too, had to account for a universe that, as was evident in "the small velocities of the stars," wasn't collapsing of its own weight. In his 1917 paper "Cosmological Considerations on the General Theory of Relativity," he inserted a fudge factor in his equation—the Greek symbol lambda, "at present unknown"—to represent whatever it was that was keeping the universe from collapsing. Like Newton, he feigned no hypotheses as to what that something might be. It was just ... lambda. But then, little more than a decade later, came Hubble's universe, and with it an elegant and unforeseen solution to the lack-of-collapse conundrum: The reason the universe wasn't collapsing of its own weight was that it was expanding.

  Newton hadn't needed God, and Einstein hadn't needed lambda. In 1931 Einstein traveled from Germany to the Mount Wilson Observatory in the mountains northeast of Pasadena and visited Hubble. After reviewing the expansion data for himself, Einstein discarded his fudge factor. In retrospect, physicists of a philosophical bent came to realize, the problem with cosmology hadn't been a supernatural cause (God). And it hadn't been an illogical effect (inaction at a distance). It had been the unthinking assumption behind the syllogism, the premise of the whole cosmology-as-science debate: that the universe was static.

  If you took the universe at face value, as even Einstein did, you would have unthinkingly assumed it was, on the whole, unchanging over time. But the universe (yet again) wasn't what it appeared to be. It wasn't static. It was expanding, and that expansion was outracing the effects of gravity—for now, anyway.