The 4-Percent Universe

by Richard Panek

  The premise seemed respectable to Rubin, and not just because the visionary George Gamow was suggesting it. Shortly after that first phone call from Gamow, she had received a letter from Gérard de Vaucouleurs—a French astronomer then working in Australia—and then she heard from him again, and then again. She found the correspondence relentless; she always seemed to owe him a letter. But she couldn't complain. As was the case with Gamow, de Vaucouleurs wanted to discuss her master's thesis. He wrote to her that he had noticed a pattern among the galaxies similar to the one she had possibly detected, and in February 1953, midway through her doctoral work, her patience with the persistent de Vaucouleurs paid off. He began an article in the Astronomical Journal with a citation from her work: "From an analysis of the radial velocities of about a hundred galaxies within 4 megaparsecs Mrs. V. Cooper Rubin recently found evidence for a differential rotation of the inner metagalaxy." To de Vaucouleurs, however, her evidence seemed to suggest not the rotation of the universe but the motion of a cluster of galaxy clusters—a supercluster. Even so, his argument was a variation on the theme Gamow was now asking Rubin to consider: Did galaxies cluster, and if so, why?

  Once again she marshaled the data that was already out there, available to anyone, this time galaxy counts from Harvard. And once again she applied a conceptually straightforward analysis, this time plotting the galaxies in three dimensions by comparing locations in the sky with distances suggested by their redshifts.

  She learned to balance becoming an astronomer and being a mother, sometimes literally: a thick German textbook in one hand, the handle of a baby carriage in the other. Two or three evenings a week she attended classes in the observatory at Georgetown. She worked on her thesis at night, after the children were in bed. She finished her studies in two years, and her thesis, "Fluctuations in the Space Distribution of the Galaxies," appeared in the July 15, 1954, issue of Proceedings of the National Academy of Sciences. Her conclusion: Galaxies don't just bump and clump arbitrarily; they gather for a reason, and that reason is gravity.

  This time she didn't receive a drubbing, as she had at Haverford. The reaction was worse: silence.

  During an AAS meeting in Tucson in 1963, Vera Rubin went on a tour of the Kitt Peak National Observatory, in the desert mountains fifty-five miles southwest of the city. Rubin was by then the mother of four and an assistant professor in astronomy at Georgetown, but she was still not a practicing astronomer. "Galaxies may be pretty remarkable," she liked to explain, "but to watch a child from zero to two is just incredible." Her youngest, however, was now three.

  She secured time at Kitt Peak later that year. With her students at Georgetown she had been studying the motions of 888 relatively nearby stars, following the same methodology she'd used in her master's and doctoral theses—by consulting catalogues.* Now she would be continuing that work, only she would be using a telescope and collecting the evidence herself.

  While most astronomers at the time were studying the motions of stars in the interior of the Milky Way galaxy, she went in the other direction—the galactic anticenter, astronomers call it: the stars that lie at greater distance from the galaxy's central bulge than our own star, the Sun. The following year she received an invitation to become the first woman to observe at Mount Palomar, in the mountains northeast of San Diego.† She decided that the time had come to do what she couldn't do as an assistant professor using the increasingly limited resources at Georgetown: become a full-time astronomer.

  By now she was living in a quiet, leafy neighborhood near the Department of Terrestrial Magnetism, where she used to meet Gamow in the library. Occasionally she would make the fifteen-minute walk there to visit her friend Bernard Burke and discuss the radio-astronomy analysis he was performing on the rotation of the Milky Way. In December 1964, however, she visited for a different purpose. Even though DTM, founded in 1904, had never had a woman staff member, she walked into Burke's office and asked for a job.

  "He couldn't have looked more surprised if I'd asked him to marry me," she told her husband that night.

  Once Burke had recovered his composure, he took her to the communal lunchroom and introduced her to his colleagues; she was impressed that one of them, W. Kent Ford, had recently returned from Mount Wilson. Someone encouraged her to go to the blackboard—it was that kind of lunchroom—and talk about her latest work. Before Rubin left DTM that afternoon, Merle Tuve, DTM director and a longtime (since the 1920s) staff scientist, gave her a two-inch-by-two-inch photographic plate and asked her to perform a spectroscopic analysis. After she returned the plate along with her analysis, Tuve phoned to set up an appointment.

  She said she could be there in ten minutes.

  He said the following week was fine.

  She said she could be there in ten minutes.

  As was the case at all the Washington labs of the Carnegie Institution, the responsibilities of a staff scientist at DTM included no teaching, no tenure, and infrequent, if any, grant-writing. All it required was the ability to maintain a collegial atmosphere with colleagues and produce meaningful science. In Rubin's case, she had the choice of sharing an office with her friend Bernie Burke* or with W. Kent Ford. Burke was an on-staff radio astronomer, as were Tuve and Kenneth Turner; she noticed that the radio astronomers had commandeered a room on the first floor, thoroughly blanketing a large table with geological layers of charts and other paperwork. Rubin didn't want to immerse herself in their world. She wanted a world of her own, and she figured she was more likely to find it in Ford's office.

  Ford was an instrumentalist. He had recently built an image-tube spectrograph—a variation on the standard instrument that records the electromagnetic spectrum from a source of light. His version, however, didn't photograph the light from a distant object. Instead, it converted those faint photons into a fountain of electrons, which in turn sprayed onto a phosphorescent screen, which in turn gave off a vivid glow—and that was what his instrument photographed with all the clarity of a "normal" camera. The intensity of the image compensated for the dimness of the distant source of light. As a result, the instrument reduced the exposure time to one-tenth that of an unaided photographic plate. In Ford's new spectrograph—officially the Carnegie Image Tube Spectrograph—Rubin saw the chance to join the hunt for what was then astronomy's hottest prey.

  Quasars—short for quasi-stellar radio sources—were extraordinarily powerful pointlike signals, possibly from the farthest depths of space. Their discovery in 1963 provided breathtaking evidence for astronomers that the universe visible in radio waves is not the universe we see with our eyes. And the quasar work that Rubin and Ford did with the new image-tube spectrograph was not unrewarding. Only months after they'd published one of their findings, Jim Peebles was using their data to advance a theoretical exploration of the early universe. Rubin was thrilled. Her research, she marveled, was contributing to a subject she had never even thought to investigate.

  On the whole, though, the two years she spent chasing quasars were burdensome. The field was too crowded, competition for time on the big telescopes favored more established astronomers from more mainstream institutions, and the pressure to provide data to her non-image-tube-blessed colleagues was crushing. Constantly they insisted on answers even though she wasn't yet sure her answers were right.

  This wasn't the way she wanted to do astronomy. She already had enough personal pressures in her life; she didn't need professional pressures, too. So she quit quasars. She was beginning to realize that in her earlier work she hadn't known what the mainstream was because she herself wasn't working at mainstream institutions. Cornell was no Harvard or Caltech when it came to astronomy; Gamow and de Vaucouleurs weren't the masters of Mount Wilson or Mount Palomar. In those days, though, her outsider status had been inadvertent. Not this time. At least now, she told herself, she knew what the mainstream was; she knew what she was leaving behind. She would shape her observing program accordingly.

  She needed to find a subject she could
explore with small telescopes, the kind that generally would be more available to someone of her relatively junior status. She wanted a research program that nobody would care about while she was doing it. But she also wanted it to be work that the community would eventually be glad someone had done.

  She found it next door, cosmically speaking: Andromeda, the nearest galaxy that resembles our own.

  "Within a galaxy, everything moves," Rubin would write. "In the universe, all galaxies are in motion." Every two minutes "the earth has moved 2500 miles as it orbits the sun; the sun has moved 20,000 miles as it orbits the distant center of our galaxy. In a 70-year lifespan, the sun moves 300,000,000,000 miles. Yet, this vast path is only a tiny arc of a single orbit: it takes 200,000,000 years for the sun to orbit once about the galaxy."

  Yet such is the scale of the universe that astronomers don't see galaxies actually rotating. If observers in Andromeda were studying our galaxy—a scenario that Rubin enjoyed imagining—they would see an apparently motionless spiral. So do we when we look at Andromeda. The spectrograph, however, would tell a different story: how much the light from Andromeda had shifted toward the blue or the red end of the electromagnetic spectrum—how fast it was advancing toward or receding from Vera Rubin.

  In effect, she had inverted her earlier approach to the image-tube spectrograph. She would still be looking at fainter and fainter objects. But rather than pushing deeper and deeper into space, she would be looking at subtler and subtler details close to home. And she would be doing it in record time.

  When the American astronomer Francis G. Pease studied that same galaxy in 1916, he needed eighty-four hours of exposure time over a three-month period to record a spectrum along one axis of the galaxy; the following year he needed seventy-nine hours over a three-month period to record a spectrum along the other axis. Instruments had improved since then, but even by the mid-1960s obtaining a single spectrum of a galaxy still took tens of hours over several nights (assuming that you could even guide the telescope precisely enough and keep the spectrograph stable enough for such a long period, always iffy propositions). Ford's new instrument, however, could reduce the exposure time by 90 percent. Obtaining four to six spectra in one night was routine. In Ford's instrument Rubin saw the potential to measure the rotation motions of Andromeda farther from its central bulge than any astronomer had ever measured on any galaxy before.

  Again and again Ford and Rubin made the trip to the two main observatories in Arizona. On occasion their families joined them— Ford had three children, and the two families socialized in Washington—but mostly they went alone. In the dark of the dome, Rubin and Ford would sometimes bump heads—literally—as they each tried to be the one to guide the instrument. In general, though, their competitiveness was restricted to who would spot the first saguaro on the drive south from the Lowell Observatory in Flagstaff. For part of the way on the three-hundred-mile drive—through Phoenix and Tucson to Kitt Peak, with Ford's image tube safely tucked in back— they talked about their children. But mostly, during those days and nights out west, they talked about science.

  At the December 1968 AAS meeting Rubin announced that she and Ford had achieved their goal. They had gone farther from the center in their observations of Andromeda than any other astronomers had gone in observing a galaxy. After Rubin's talk, Rudolph Minkowski, one of the most eminent astronomers of the era, asked her when she and Ford were publishing their paper.

  "There are hundreds more regions that we could observe," she said, referring to Andromeda alone. She could gather this sort of data forever. It was beautiful. It was clean. It was unobjectionable. It was what it was.

  Sternly, emphatically, Minkowski addressed her. "I think you should publish the paper now."

  So she and Ford did. But they knew that before they could submit a formal paper on their research, they would have to address a problem that had bothered them from almost the first night of observing Andromeda.

  Going into the darkroom, Rubin had expected to detect the pattern that holds for the planets in our solar system: the farther the planet from the Sun, the slower the orbit—just as Newton's universal law of gravitation predicted. A planet four times as far from the Sun as another planet would be moving at half the velocity. A planet nine times as distant would be moving at one-third the velocity. Pluto is one hundred times as far from the Sun as Mercury, so it should be moving—and does move—at one-tenth the velocity of Mercury. If you plotted this relationship between distance and velocity on a graph—the farther the distance, the slower the velocity—you would get a gradual falling-off, a downward curve.

  That's what Rubin and Ford had assumed they would see in plotting the relationship between distance and velocity in the different parts of a galaxy: The farther the stars were from the center of the galaxy, the slower their velocity would be. That's what astronomers had always done—assumed they would get a downward curve, as if the great mass of stars making up the central bulge in the galaxy affected the wispiest tendrils in the same way that the great mass of the Sun in our solar system affected the wimpiest planet. But those astronomers hadn't actually made those observations because, without the benefit of Ford's spectrograph, they couldn't have. Instead, they drew their assumption as a dotted line. Rubin and Ford, however, had pushed the observations farther than ever, as far as the image-tube spectrograph would allow them, to the farthest edges of the spiral. But they couldn't help noticing that the outermost stars and gas seemed to be whipping around the center of the galaxy at the same rate as the innermost stars and gas. It was as if Pluto were moving at the same speed as Mercury. Plot the rotation curve of Andromeda, and it wasn't a "curve" at all.

  Maybe the gas was interacting with the stars in some way Rubin couldn't imagine. Maybe Andromeda was just an oddball galaxy. Maybe a theorist could supply a logical explanation. They submitted their paper to the Astrophysical Journal in the summer of 1969, and in it Ford and Rubin declared that "extrapolation beyond that distance is clearly a matter of taste." Her taste, Rubin would say in private, was that plotting data that didn't exist was "offensive." So she and Ford agreed that they would plot only what they got. It was what it was.

  And what it was, was a flat line.

  Shortly after Rubin finished her work on Andromeda, her good friend Morton Roberts, at the National Radio Astronomy Observatory in Charlottesville, Virginia, called to say he was driving over. He had something he wanted to show her.

  They met in a basement conference room at DTM, along with a group of three or four other DTM astronomers. Roberts, too, had been studying the rotation curve of Andromeda, except his observations were at radio wavelengths. He placed a copy of the Hubble Atlas of Galaxies on the table and opened it to a photograph of Andromeda. Then he laid the plot of his radio observations on the photograph. He had pushed far past the familiar cyclone of stars and gas, far past the point that Ford and Rubin had managed to reach with their optical probes, into a ring of hydrogen gas clouds. But a graduate student from Harvard who was spending some time at DTM, Sandra Faber, seemed unimpressed.

  "There's nothing new in this," she said. "It's all part of the same problem. Velocity has never made sense."

  She was right. As Rubin herself had shown, velocities of galaxies varied all over the map of the heavens. But for Faber the problem was a given. Unlike Rubin, she'd come of age in a universe that was in motion in more ways than anyone had ever imagined.

  "Don't you understand?" Roberts said. "The galaxy has ended, but the velocities are flat." He gestured at the points he'd plotted. "What is the mass out there? What is the matter? There's got to be matter there."

  They all stared at the photograph. Here was this beautiful swirl of billions of stars—the kind of majestic image that had captivated astronomers for more than half a century—though that's not where they were looking. They were looking beyond it. Beyond the bulge, beyond the stars, beyond the gas of the spiral arms—beyond all of the light, whether optical or radio. And even though the
re was nothing to see there, the small group of astronomers understood that they were nonetheless looking at the Andromeda galaxy.

  It was what it wasn't.

  3. Choosing Halos

  IN THE SUMMER OF 1969, Jim Peebles decided to find out just how simple the universe was.

  He had spent the previous academic year at Caltech, and now he and his wife, Alison, were driving back across the country to their home in Princeton. Along the way they stopped at Los Alamos Scientific Laboratory. The lab had invited Peebles to spend a month there as part of a program to bring outside perspectives into what would otherwise be an insular scientific community in the middle of the New Mexico desert. Los Alamos was where the first atomic bombs were designed: one for the Trinity test, on July 16, 1945, two hundred miles south of Los Alamos, in the arid flatlands outside Alamogordo; then Little Boy, twenty days later, over Hiroshima; then Fat Man, another three days later, over Nagasaki. In 1969, Los Alamos was one of two government facilities (along with Lawrence Livermore National Laboratory, in California) designing nuclear weapons. When Peebles looked around at the supercomputers at the facility, he realized, with characteristic restlessness, that as long as he was there he might as well get some work done.