The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality

by Richard Panek

“I know,” said Nick. “You don’t know,” said his father. —Ernest Hemingway

wouldn’t be the first time that the vast majority of the universe turned out to be hidden to us. In 1610 Galileo announced to the world that by observing the heavens through a new instrument—what we would call a telescope—he had discovered that the universe consists of more than meets the eye.

The new universe consists of only a minuscule fraction of what we had always assumed it did—the material that makes up you and me and my laptop and all those moons and planets and stars and galaxies. The rest—the overwhelming majority of the universe—is . . . who knows?

23 percent something mysterious that they call dark matter, 73 percent something even more mysterious that they call dark energy. Which leaves only 4 percent the stuff of us.

If you know how often a variable pulsates, then you know how bright it is relative to other variables; if you know how bright it is relative to other variables, then you know how distant it is relative to other variables.

In 1912 the American Vesto Slipher began examining the nebulae with a spectrograph, an instrument that registers the wavelengths from a source of light. Much like the sound waves of a train whistle as the train approaches or departs from a station, light waves are compressed or stretched—they bunch up or elongate—depending on whether the source of the light is moving toward you or away from you. The speed of the light waves doesn’t change; it remains 186,282 miles (or 299,792 kilometers) per second. What changes is the length of the waves. And because the length of the light waves determines the colors that our eyes perceive, the color of the source of light also seems to change. If the source of light is moving toward you, the waves bunch up, and the spectrometer will show a shift toward the blue end of the spectrum. If the source of light is moving away from you, the waves relax, and the spectrometer will show a shift toward the red end of the spectrum. And as the velocity of the source of light as it moves toward you or away from you increases, so does the blueshift or redshift—the greater the velocity, the greater the shift.

He found out when he compared the velocities of eighteen of these nebulae with their distances: The two measurements seemed to be directly proportional to each other—the farther the galaxy was, the faster it appeared to be receding. In other words, the universe might seem to be expanding.

Suddenly the universe had a story to tell. Instead of a still life, it was a movie. And like any narrative, the story of the universe now had not only a middle—the present, swarming with galaxies fleeing one another—but the suggestion of a beginning.

Hubble himself, as an observer, hoarding evidence and leaving the theorizing to the theorists, preferred to remain agnostic as to whether the universe really was expanding or whether another interpretation might explain the apparent correlation.

film. The Belgian priest Georges Lemaitre, a physicist and astronomer, imagined the expansion unreeling in reverse, the size of the universe shrinking, smaller and smaller, the galaxies rushing back together, faster and faster, until the infalling matter would reach a state that he called the “primeval atom” and that other astronomers would come to call a “singularity”: an abyss of infinite density and incalculable mass and energy.

But words such as “infinite” and “incalculable” aren’t of much use to mathematicians, physicists, or other scientists. “The unrestricted repeatability of all experiments...

Rather than a big bang—the term Hoyle applied, during a BBC radio broadcast in March 1949, to the idea of a universe expanding* from, as he wrote in his paper, “causes unknown to science”—they postulated a steady state. Through “continuous creation of matter,” Hoyle wrote, “it might be possible to obtain an expanding universe in which the proper density of matter remained constant.”

Over the course of cosmic history, the creation of even infinitesimal amounts of matter could become cumulatively significant. Such a universe wouldn’t have a beginning or an end; it would just be.

with nothing to set it in motion? no initial cause/mover?

For many astronomers, however, “continuous creation” was no more appealing than a “singularity.” Both the Big Bang and Steady State theories seemed to require a leap of faith, and faith not being part of the scientific method, there they let the matter rest.

Whereas Newton imagined gravity as a force that acts across space, Einstein’s equations cast gravity as a property that belongs to space. In Newton’s physics, space was passive, a vessel for a mysterious force between masses. In Einstein’s physics, space was active, collaborating with matter to produce what we perceive as gravity’s effects. The Princeton physicist John Archibald Wheeler offered possibly the pithiest description of this co-dependence: “Matter tells space how to curve. Space tells matter how to move.” Einstein in effect reinvented physics.

According to general relativity, the background starlight should appear to “bend” by a certain amount as it skirted the great gravitational grip of the Sun. (Actually, in Einstein’s theory it’s space itself that bends, and light just goes along for the ride.)

Einstein himself downplayed the theory’s power to predict “tiny observable effects”—its influence on physics. Instead, he preferred to emphasize “the simplicity of its foundation and its consistency”—its mathematical beauty. Mathematicians tended to agree, as did physicists such as Dicke’s professor at the University of Rochester. General relativity’s known effects in the universe—an anomaly in the orbit of a planet, the deflection of starlight—were obscure in the extreme; its unknown effects on the history of the universe—cosmology—were speculative in the extreme. Even so, Einstein also acknowledged that if the theory made a prediction that observations contradicted, then, as would be the case with any theory under the standards of the scientific method, science should amend or abandon it.

His experiments over the coming years would involve placing occulting disks in front of the Sun to determine its precise shape, which affects its gravitational influence on the objects in the solar system, including Mercury; bouncing lasers off the Moon and using the round-trip time to measure its distance from Earth, which would indicate if its orbit was varying from Einstein’s math in the same way that Mercury’s orbit varied from Newton’s math; and using the chemical composition of stars to trace their age and evolution, which in turn would be important for tracing the age and evolution of the universe, which in turn would involve an attempt to detect the relic radiation from the primeval atom, cosmic fireball, Big Bang, or whatever you wanted to call it.

Dicke wondered if a theory of the universe could avoid not only a Big Bang singularity but the Steady State’s spontaneous creation of matter, and he proposed a compromise of sorts: an oscillating universe. Such a universe would bounce from expansion to contraction to expansion throughout eternity, without ever reaching absolute collapse or, between collapses, eternal diffusion. During the expansion phase of such a universe, galaxies would exhibit redshifts consistent with what astronomers were already observing. Eventually the expansion would slow down under the influence of gravity, then reverse itself. During the contraction phase, galaxies would exhibit blueshifts as they gravitated back together. Eventually the contraction would reach a state of such compression that it would explode outward again, before the laws of physics broke down. Dicke’s oscillating universe would therefore neither emerge from nor return to the dreaded singularity, though the earliest period of its current expansion would resemble a Big Bang.

The trouble, Peebles saw, started with Einstein. In 1917, two years after arriving at the theory of general relativity, Einstein published a paper exploring its “cosmological considerations.” What might general relativity say about the shape of the universe? In order to simplify the math, Einstein had made an assumption: The distribution of matter in the universe was homogeneous—that is, uniform on a large scale. It would look the same no matter where you were in it. In calculating the implications of Einstein’s theory, Georges Lemaitre and, independently, the Russian mathematician Aleksandr Friedman had adopted the same assumption and added one more, that the universe is isotropic—uniform in every direction. It would look the same no matter which way you looked. Then the Steady State theory went the Big Bang one assumption better: The universe is homogeneous and isotropic not only throughout space but over time. It would look the same in every direction no matter where you were in it and no matter when.

“Boy, this is silly,” Peebles thought. Why, he asked himself, would anyone imagine the universe to be, of all the things that a universe could be, simple? Yes, scientists preferred to follow the principle of Ockham’s razor, dating back to the fourteenth-century Franciscan friar William of Ockham: Try the simplest assumptions first and add complications only as necessary. So Einstein’s invocation of a homogeneous universe had a certain logic to it, a legacy behind it—but not enough to be the basis of a science that made predictions that led to observations.

The first hint that radio waves might offer a new way of seeing the universe dated to the 1930s—again, through an accidental detection at Bell Labs. In 1932 an engineer who had been trying to rid transatlantic radiotelephone transmissions of mystery static figured out that the noise was coming from the stars of the Milky Way.

The wavelengths to which human eyes have evolved to be sensitive range from 1/700,000th of a centimeter (red) to 1/400,000th of a centimeter (violet). To either side of that narrow window of sight, the lengths of electromagnetic waves increase and decrease by a factor of about one quadrillion, or 1,000,000,000,000,000.

Peebles began by using the present constitution of the universe to work backward toward the primordial conditions. The present universe is about three-quarters hydrogen, the lightest element; its atomic number is 1, meaning that it has one proton. In order for such an abundance of hydrogen to have survived to the present day, the initial conditions must have contained an intense background of radiation, because only an extraordinarily hot environment could have fried atomic nuclei fast enough to keep all those single protons from fusing with other subatomic particles to form helium and heavier elements. As the universe expanded—as its volume grew—its temperature fell. Extrapolate from the current percentage of hydrogen how intense the initial radiation must have been, calculate how much the volume of the universe has expanded since then, and you have the temperature to which the initial radiation would by now have cooled. A radio antenna, however, doesn’t measure temperature, at least not directly. The temperature of an object determines the motions of its electrons—the higher the temperature, the greater the motions. The motions of the electrons in turn are what produce radio noise—the greater the motions, the more intense the noise. The intensity of the noise therefore tells you how much the electrons are moving, which tells you the temperature of the object—or what engineers call the “equivalent temperature” of the radio noise. In a box with opaque walls, the only sourc...

Dicke radiometer, invented by Dicke to refine radar sensitivity during the war,

Arno Penzias had described for Burke the work he and Bob Wilson were doing on Crawford Hill. He had told Burke that they hoped to study the radio waves from the stars not in the big bulge at the center of the Milky Way, where most astronomers had been looking, but in the other direction, at the fringe of the Milky Way halo.

“We have something we don’t understand,” Penzias said. He explained that he and Wilson couldn’t get rid of an excess noise corresponding to a temperature near, but not quite, absolute zero.

The Big Bang was a creation myth, but by 1965 it was a creation myth with a difference: It came with a prediction. By the time Penzias placed his call to Dicke, Peebles had arrived at a temperature of approximately 10° Celsius above absolute zero, which is more commonly referred to as 10 Kelvin.* Penzias and Wilson had found a measurement of 3.5 K (plus or minus 1 K) in their antenna. Because Peebles’s calculations were rudimentary and Penzias and Wilson’s detection was serendipitous, the approximation of theory and observation was hardly definitive. Yet it was also too close to dismiss as coincidence.

At the very least it was worth recording for posterity. After the Crawford Hill meeting and a reciprocal meeting at Princeton, the two sets of collaborators agreed that they would each write a paper, to appear side by side in the Astrophysical Journal. The Princeton foursome would go first, discussing the possible cosmological implications of the detection. Then the Bell Labs duo would confine their discussion to the detection itself, so as not to align their measurement too closely with a wild interpretation that, as Wilson said, it “might outlive.”

On May 21, 1965, even before their papers appeared, the New York Times broke the story: “Signals ...

A subsequent search of the literature turned up other predictions and at least one previous detection. In 1948, the physicist George Gamow had written a Nature paper that predicted the existence of “the most ancient archeological document pertaining to the history of the universe.” He was wrong on the details but right on the general principle: The early universe had to be extremely hot to avoid combining all the hydrogen into heavier elements. That same year, the physicists (and sometime collaborators of Gamow’s) Ralph Alpher and Robert Herman published their calculation that “the temperature in the universe” should now be around 5 K, but astronomers at the time assured them that such a detection would be impossible with current technology. (In retrospect Wilson felt that they probably could have performed it with World War II-era technology, as long as they had properly connected the antenna to the cold load.) In a 1961 article in the Bell System Technical Journal, a Crawford Hill engineer wrote that the Echo antenna was picking up an excess of 3 K; but that reading fell within the margin of error, and the discrepancy wasn’t going to make a difference for his purposes anyway, so he ignored it. In 1964, Steady State champion Hoyle, working with fellow British astronomer Roger J. Tayler, investigated the oscillating-universe scenario and performed calculations similar to those of Alpher and Herman. Also in 1964, even as Penzias and Wilson were directing their antenna to ev...

His initial paper on the temperature of the universe—Dicke had forwarded a preprint to Penzias after the phone call about the Bell Labs detection—had repeatedly bounced back from the Physical Review referee because it was duplicating earlier calculations by Alpher, Herman, Gamow, and others. Peebles finally withdrew the paper in June 1965. He managed to rectify some of those oversights in the paper on the cosmic microwave background he wrote with Dicke, Roll, and Wilkinson. Even that paper, however, referred only to Gamow’s work on the primordial creation of elements, not to his work predicting the temperature of the cosmic background. Gamow sent an angry note to Penzias, listing citations of his early work and concluding, “Thus, you see, the world did not start with almighty Dicke.”

By December 1965, Roll and Wilkinson had mounted their antenna on the roof of Guyot Hall and gotten the same reading as Penzias and Wilson. Within months two more experiments (one by Penzias and Wilson) had found what a sound scientific prediction demands: a duplication of results—in this case, a detection of what was already being called “the 3-K radiation.”

Both the Steady State and Big Bang interpretations had relied not just on math and observation but on speculation. They were modern counterparts to Copernicus’s attempt to save the appearances; they were theories in need of evidence. And just as Galileo, with the aid of the telescope, had detected the celestial phenomena that decided between an Earth-centered and a Sun-centered cosmos, forcing us to reconceive the universe, so radio astronomers, with the aid of a new kind of telescope, were now detecting the evidence that decided between the Steady State and Big Bang cosmologies, necessitating a further reconception of the universe.

The introduction of radio astronomy could have left the Newtonian conception of the universe intact. But seeing beyond the optical did mean seeing more phenomena and having to accommodate new kinds of information. This new universe would still run like clockwork; the laws that had arisen through Galileo’s observations and Newton’s computations would still presumably apply. But now, so would Hubble’s and Einstein’s, and in their universe the motions of the heavens weren’t cyclical so much as linear; their cosmos corresponded not so much to a pocket watch, its hands and gears grinding and turning but always returning to the same positions, as to a calendar, its fanning pages preserving the past, recording the present, and promising the future.

Not that the always-cautious Peebles now embraced the Big Bang theory. But the uniformity of the microwave background that he had predicted and that Penzias and Wilson had detected would certainly correspond to a universe that looked the same on the largest scales no matter where you were in it. Einstein had posited an elephant on an incline, and that’s what the universe turned out to be: homogeneous. “Which is an amazing thing,” Peebles thought. “But there it is: The universe is simple.”

Vera Cooper was born in 1928, three years after Edwin Hubble announced that our Milky Way galaxy was hardly singular, and one year before he presented evidence that the galaxies seemed to be receding from one another—the farther apart, the faster. The only universe she’d known was full of galaxies, and those galaxies were in motion.

she tried to update the old clockwork view of the cosmos for the new expanding universe. She reasoned that since the Earth rotated on its axis, and the solar system rotated, and the galaxy rotated, then maybe the universe had an axis too. Maybe the whole universe rotated. The premise seemed reasonable. Her husband, Robert Rubin, a doctoral candidate in physics at Cornell, had shown her a brief, speculative article by George Gamow in the journal Nature, “Rotating Universe?” Then she heard that Kurt Gödel, at Princeton, was working on a theory of a rotating universe. Her approach also seemed reasonable. She gathered data on the 108 galaxies for which astronomers had managed to measure a redshift. Then she separated out the motions that were due to the expansion of the universe—what astronomers call recessional motions. Did the motions that remained—the peculiar motions—exhibit a pattern? She plotted them on a sphere and thought they did. In December 1950, at the age of twenty-two, still half a year shy of getting her master’s degree, Vera Cooper Rubin presented her thesis at an American Astronomical Society meeting in Haverford, Pennsylvania.

Gamow was nearly alone among astronomers, and Gödel among theorists, in finding the question of a rotating universe worthy of serious consideration. Gamow had admitted, in the Nature paper, that the idea of a rotational universe was “at first sight fantastic”—which, at first sight, it was. But what if you didn’t trust first sight? First sight—the evidence of the senses, unaided by technology—tells you that the Earth is stationary, that the Sun revolves around the Earth, that Jupiter is moonless and Saturn ringless and the stars motionless, and that the stars are as far as there is. The point Gamow was trying to make was that astronomers needed to go beyond first sight, because now they had a new scale of the universe to consider.

Just as our eyes didn’t need to evolve to see radio waves in order for us to survive, maybe our minds didn’t need to evolve to understand the numbers that astronomers were now trying to incorporate into their thinking. Like cultures that count “One, two, three, more,” we tend to regard the scale of the universe—to the extent that we regard it at all—as “Earth, planets, Sun, far.” Consider: How long would it take you to count to a million at the “one Mississippi” rate of one second per number? Eleven days—or, to be exact, 11 days, 13 hours, 46 minutes, and 40 seconds. How long would it take you to count to a billion at the same rate? A billion is a thousand million—that is, a million one thousand times over. So you would have to count a million Mississippis—eleven days of counting—a thousand times. That’s 31 years, 8% months. To reach a trillion, you’d have to count to a billion a thousand times—31 years a thousand times, or 31,000 years. A light-year—the distance light travels in a year—is about six trillion miles. To count to six trillion, you would need six sets of 31,000 years, or 186,000 years. Earlier generations of astronomers had to learn to adjust their thinking to accommodate successive discoveries about new scales of the universe: that the Sun is 93 million miles distant; that the nearest star after the Sun is 4.3 light-years, or 25 trillion miles, away (that’s 186,000 years of Mississippis 4.3 times, or about 800,000 years); that our “island universe” consists o...

Was the distribution of galaxies throughout the universe random and uniform, as most astronomers assumed? Hubble himself had thought so. “On a large scale the distribution is approximately uniform,” he had written in his highly influential 1936 book, The Realm of the Nebulae. “Everywhere and in all directions, the observable region is much the same.” In a sense, he was simply reiterating the two assumptions of modern cosmology, homogeneity and isotropy, in layman’s terms. But the way he was framing the issue was also reminiscent of the premodern island-universe thinking—emphasis on “island.” In Hubble’s view, and therefore the view of a generation of astronomers, the galaxy clusters that astronomers had observed would be accidents of nature, or perhaps a sort of cosmic optical illusion arising from multiple galaxies falling along our line of sight. But Gamow was thinking on a different scale. Maybe the peculiar motions of galaxies—the motions that were separate from a straightforward outward expansion—weren’t random, as most astronomers assumed. Maybe the gravitational interactions among galaxies, even across previously unthinkable distances, were sometimes strong enough to counteract the expansion on a local level. Maybe no—or at least not every—galaxy is an island.

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?

“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.

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.

“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.

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.

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 ...

“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.”

even though there 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.

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.