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

by Richard Panek

Forty years earlier Edwin Hubble had arrived at the evidence for an expanding universe by studying the behavior of galaxies. By tracing that expansion backward, as if running a film of the outward-flying galaxies in reverse, Georges Lemaitre had arrived at the idea of a primeval atom. Peebles hadn’t believed that the universe could be that simple, but now he was becoming one of the leading interpreters of a simple universe: homogeneous and isotropic—one that looked the same no matter where you were in it and no matter which way you looked. After Penzias and Wilson, as well as Peebles and his colleagues at Princeton, had found evidence for what those initial “Primeval Fireball”* conditions might actually have been, Peebles began using that knowledge to refine his understanding of the expansion itself. It was as if he were running the film of the history of the universe again, only instead of rewinding it to the beginning he would be running it forward to today.

Peebles would be performing what scientists call an N-body simulation. Take a number—N—of points, program them to interact according to whatever properties you want, and see how the action unfolds. In this case, Peebles would be taking 300 points and treating each as if it were a galaxy in one particular part of the universe—the Coma Cluster, the closest and most-studied galaxy cluster. He would assign each galaxy a position and velocity based on rough observations of real galaxies in the cluster, and he would teach the computer the law of universal gravitation. And then he’d let the model do whatever galaxies interacting gravitationally in an expanding universe do over billions of years. He’d already been thinking about how clusters develop, and he’d done some cluster calculations during his time at Caltech. Now he took that initial research and converted it into a computer program. Then he punched the holes in the 7%-by-3%-inch computer cards himself, stacked them in the metal feeder, and ran a simulation. At the end of the simulation the points had moved a bit. He transferred the image to a frame of 35-millimeter film, and then he ran the next simulation, using the galaxies’ positions at the end of the previous simulation as a starting point. Again, over a period equivalent to millions of years, the galaxies shifted slightly. When Peebles had enough frames, he ran them together, loaded the film in a projector, and sat back. The universe swirled to life. Galaxies moved o...

Georges Lemaitre and Aleksandr Friedman had attached a theoretical interpretation to those observations: a universe expanding from a Big Bang.

“While in a limited sense gravitation is of no great importance to a physicist, this is much too naive an interpretation.” In the first paragraph of the “Cosmology” section Peebles expanded on that philosophy. For physicists, he wrote, cosmology doesn’t satisfy just “the obvious interest” in the origins of the universe; “we need cosmology as a basis of any complete theory of the galaxies, or for that matter, of the solar system.” If you wanted to understand specific problems concerning the evolution and structure of the universe—the clustering of galaxies, for instance—then you had to abandon any residual “island universe” assumptions. You had to learn to think about the universe not only as a collection of individual galaxies but as the sum of its galaxies—a single unit, a whole. You had to keep in mind that while the whole—the universe—was expanding, its parts—the galaxies—were evolving. “The moral of this section,” Peebles offered in conclusion, twelve pages later, “is the unity of the universe.”

“The radiation,” Peebles explained in one of his many papers during this period, “performs the great service of defining the epoch at which the galaxies can start to form.” That epoch occurred when the temperature of the primeval fireball fell below 4000 K. At that point the electrons and protons that had been ricocheting independently since the first instants of the universe recombined to form atoms of matter. This matter now took on a “life” of its own and decoupled from the radiation—the fossil radiation that survived today as the cosmic microwave background. And although that background seemed to be as uniform—as homogeneous—as theory had predicted, it couldn’t be entirely uniform. It had to contain the irregularities, or inhomogeneities, that identified the concentrations of matter that existed at the moment that matter and radiation decoupled and went their separate ways—the matter that through gravitational interactions would have grown into the large-scale “distributions of mass and size” that we see today: “galaxies, and clusters of galaxies, and the material within galaxies,” including us. The universe was simple. It just couldn’t be perfectly simple. Yet to the radio telescopes at the time, the microwave background was absolutely uniform; it lacked the inhomogeneities that had to be there in order for us to be here. Eventually, astronomers or physicists wanting to test the Big Bang theory would have to develop instruments sensitive enough to detect those subtle ...

Ostriker had been working on rotating celestial objects since he was a graduate student at Cambridge; he had written his thesis on rotating stars. Scientists had known since the nineteenth century that if you rotated an initially spherical liquid drop it would become oblate, increasingly so, and eventually compress into a bar shape. Ostriker had treated stars as liquid drops—as compressible objects—and found that they, too, would become oblate over time. Recently, he told Peebles, he had looked at a rendering of the Milky Way—a flat disk like the other spiral galaxies that astronomers had been collecting by the thousands. He could see at a glance that it should have become bar-shaped or broken up into two galaxies after one rotation. Yet by now the Milky Way was old enough to have completed a dozen rotations. “There’s something wrong here,” Ostriker said. Peebles agreed. He began work on creating an N-body simulation for the Milky Way. He laid the points into a spiral shape and set it rotating. Sure enough, it wobbled catastrophically during its first 200-million-year rotation. He and Ostriker would need something else to stabilize it: a surrounding mass of something to hold it together gravitationally. Something you couldn’t see with a telescope—at least not yet—but something that had to be there. Something that Peebles and Ostriker would now add to the computer program. For the first simulation of the rotation of the galaxy with this missing component, the amount of mass...

The problem of “missing mass” had been shadowing astronomy for decades, for almost as long as astronomers had known of the existence of galaxies. But the problem had always related to clusters of galaxies. In 1933 the Swiss-born astrophysicist Fritz Zwicky, working at Caltech, studied eight galaxies in the Coma Cluster, comparing the mass he derived from their velocities relative to one another with the mass he expected just judging from appearances. His conclusion was that the density of mass had to be four hundred times as large as what the luminosity alone suggested.* If astronomers couldn’t resolve this discrepancy, he wrote in a Swiss journal, “we would arrive at the astonishing conclusion” that the density of luminous matter in Coma must be minuscule compared with the density of some sort of dunkle—or dark —Materie—matter. Three years later, the astronomer Sinclair Smith published an article in the Astrophysical Journal about a similar pattern he’d noticed in the Virgo Cluster, suggesting the presence of “a great mass of internebular material within the cluster.” That same year, Edwin Hubble addressed the problem in his landmark book The Realm of the Nebulae: “The discrepancy seems to be real and is important.”

Peebles and Ostriker kept giving their galaxy a larger and larger halo. Only when they had simulated an invisible halo with roughly the same mass as the visible parts of the galaxy—the disk of stars and gas, of central bulge and spiral arms—did the system stabilize.

Yin/ysng balance

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.

“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.”

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.”

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?

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).

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.

“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.

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.

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?

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 than 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.

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. But what about over time? A new syllogism presented itself: One, the universe is expanding; two, the universe is full of matter attracting other matter through gravity; therefore, the expansion must be slowing down. The lingering challenge to aspiring cosmologists was no longer Why wasn’t the universe collapsing? It was Will it collapse?

Ever since Hubble’s discovery of evidence for an expanding universe, astronomers had known how to measure how much the expansion was slowing down, at least in principle. Hubble had used Henrietta Swan Leavitt’s period-luminosity relation for Cepheid variable stars to determine distances to nearby galaxies. And he had used the redshifts for those galaxies as equivalent to their velocities as they moved away from us. When he graphed those distances against those velocities, he concluded they were directly proportional to each other: the greater the distance, the greater the velocity. The farther the galaxy, the faster it was receding. This one-to-one relationship showed itself as a straight line on a 45-degree angle. If the universe were expanding at a constant rate, that straight line would continue as far as the telescope could see. But the universe is full of matter, and matter is tugging gravitationally on other matter, so the expansion can’t be uniform. At some far reach of the Hubble relation, the galaxies would have to deviate from the straight line. The graph of their points would begin to gently curve toward brighter luminosity. How much they deviated from the straight line would tell you how much brighter they were at that particular redshift than they would be if the universe were expanding at a constant rate. And how much brighter they were—how much nearer they were—would tell you how much the expansion was slowing.

In 1934, the same Caltech astrophysicist who had recently suggested that galaxy clusters might be full of dark matter, Fritz Zwicky, collaborated with the Mount Wilson astronomer Walter Baade on a calculation showing that, under certain conditions, the core of a star could undergo a chain of nuclear reactions and collapse. The implosion would race inward at 40,000 miles per second, creating an enormous shock wave and blowing off the outer layers of the star. Baade and Zwicky found that the surviving ultracompact core of the star would consist of Chadwick’s neutrons, weighing 6 million tons to the cubic inch and measuring no more than 60 miles in diameter.

Baade and Zwicky decided that their exploding star deserved a classification all its own: “super-nova.” Almost at once, Zwicky initiated a search for supernovae. He helped design an 18-inch telescope that became the first astronomical instrument in use on Mount Palomar, and soon newspapers and magazines across the country were keeping a running tab of how many “star suicides” his survey had discovered.

In 1988, the National Science Foundation awarded the University of California, Berkeley, six million dollars over five years to establish the Center for Particle Astrophysics. The center would take multiple approaches to the mystery of dark matter. One was to try to detect particles of dark matter in the laboratory. Another looked for signs of dark matter in the cosmic microwave background. A third approach explored dark matter through theory. And another group would try to determine how much matter was out there, dark or otherwise, by using supernovae as standard candles.

In the late 1920s the physicist who would become the lab’s namesake, Ernest Lawrence, conceived of an accelerator that shot particles not in straight lines, as linear accelerators did, but in circles. Strategically placed magnets would deflect the particles just enough to prod them to follow the closed curve, around and around, faster and faster, to higher and higher levels of energy. Lawrence’s first “proton merry-go-round”—or cyclotron—was five inches in diameter, small enough to fit inside any room bigger than a broom closet in the physics building on campus. In 1931 he’d moved his operations into an abandoned building, the former Civil Engineering Testing Laboratory—the first official site of the Berkeley Lab “Radiation Laboratory.” By 1940, a version of the cyclotron had reached a diameter of 184 inches, and the experiment had outgrown the Rad Lab.

the LBL director at the time refused to give him access to lab funds. Not because of what the experiment would be doing—particle physics was what LBL did. Rather, the problem was how HAPPE would be doing it: aboard a balloon. “We are an accelerator lab,” the lab director Edwin McMillan, a Nobel laureate in Physics, told Alvarez. “If we stop doing accelerator physics, our funding will disappear.” For Alvarez, turning your back on a particle physics experiment because it went up in the air instead of around and around betrayed a lack of imagination. Alvarez quit the leadership of his own LBL physics group in protest and got funding elsewhere, including from NASA, to see if HAPPE could fly. (It crashed.) By the mid-1970s, Alvarez had his own Nobel Prize in Physics, and the lab had a new director. One day Alvarez was reading a magazine article about an experiment being done by an acquaintance of his. Stirling Colgate, toothpaste scion by birth and thermonuclear physicist by choice, had planted a 30-lnch telescope on a surplus Nike missile turret in the New Mexico desert. The plan was to program it to automatically look at a different galaxy every three to ten seconds. The telescope would then transmit the information through a microwave link to the memory of an IBM computer on the campus of the New Mexico Institute of Mining and Technology, eighteen miles away. There, software would search the images for supernovae.

Muller’s battles with the bureaucracy were legion. Year after year, LBL leadership would tell Muller that this was the last time he’d be getting funding, because if the Department of Energy was going to cut anything from the LBL budget, it was going to be the speculative astrophysics project,

When astronomers using photographic plates hunted for supernovae by eye, they used an optical device called a comparator. By rapidly switching back and forth between two images of a galaxy taken several weeks apart, the comparator would allow an astronomer to see whether any new pinpoint of light had appeared in the Interim. The comparator blinked the two images. New computer technology, however, allowed astronomers to take all the light from the earlier image and remove it from the later one. It subtracted the first image from the second. If the computer signaled that a telltale bit of light remained, then a real live human being analyzed the data. Sometimes the source of the bit of light was “local”—a fluctuation in the output, a cosmic ray from space striking the instrument, a subtraction error. Sometimes the source of the light was “astronomical”—asteroids, comets, variable stars. But once in a while the blip of light was a star erupting in a farewell explosion that stood out even against the background of all the tens or hundreds of billions of other stars in its host galaxy combined: that is, a supernova.

1980, Luis Alvarez, along with his son Walter Alvarez, had hypothesized that the mass extinction of the dinosaurs 65 million years ago, at the cusp of the Cretaceous and Tertiary periods, had been caused by a comet or asteroid impact that had disrupted the global ecosystem. Then, in 1983, a pair of paleontologists announced that they had discovered evidence of a cycle of mass species extinctions every 26 million years. The following year, Muller and some colleagues published a paper speculating on the existence of a companion star to the Sun—Nemesis. Every 26 million years, they wrote, the highly elliptical orbit of Nemesis would bring it relatively near the Sun, and its gravitational influence would draw comets from the farthest reaches of the solar system into the orbital paths of the planets nearest the Sun, including Earth. The idea wasn’t as fanciful as it might seem; studies of Sun-like stars had shown that about 84 percent were in binary systems, meaning that the Sun, if solo, would be an anomaly. Muller assigned Perlmutter the task of looking for Nemesis (or the Death Star, as the media called it), and in 1986 Perlmutter completed his thesis, “An Astrometric Search for a Stellar Companion to the Sun.”

Thanks to BASS, Pennypacker and Perlmutter now knew they could do an automated supernova search; they had added two more supernovae in 1986 and another in 1987. But could they do an automated supernova search at cosmologically significant distances?

Supernovae remained attractive as potential standard candles for a couple of reasons. They’re bright enough to be visible from the farthest recesses of space, meaning that astronomers can use them to probe deep into the history of the universe. And they operate within human time frames, their luminosity rising and falling over the course of weeks, meaning that, unlike most astronomical phenomena (such as the formation of a solar system or the coalescing of galaxies into a cluster), supernovae offer a soap opera that astronomers can actually watch. But supernovae were also problematic for at least three reasons. As the LBL group put it, “they are rare, they are rapid, and they are random.” In our own Milky Way galaxy, supernovae pop off at an average rate of maybe once a century. So astronomers pursuing supernovae have to devise a way to look at a great number of galaxies, whether individually in quick succession or in great gulps of the sky all at once—or, ideally, both: great gulps in quick succession. Supernovae also require a fast response. Once astronomers identify a supernova, they have to move quickly to do the necessary follow-up studies—not always possible when time on telescopes is assigned months in advance. And they’re random. You never know where or when one’s going to go off, so even if you could reserve follow-up time on telescopes months in advance, you wouldn’t know whether you would have a supernova worth studying on that date.

They calculated that if they looked not at one galaxy at a time but at clusters of galaxies, they could beat the once-a-century-per-galaxy odds of finding the right kind of supernova. They selected clusters with well-established distances. And they timed their searches carefully, choosing the nights just before and after a new moon so that they were able not only to capitalize on dark skies but to compare images about twenty days apart, a period that, through happy coincidence, corresponds to the natural life (or, more aptly, death) cycle of the kind of supernova they wanted. They found one. Possibly a second, on what would be their final night of observing, though they didn’t bother to follow up that detection. They were ahead of their time, and, after two years, their time was up. Their telescope turned out to be too small for their purposes, their rate of discovery too low. Unless they could garner access to a larger telescope and a more powerful detector, they would need dozens of years to collect a suitable sample. Even at the rate of one good supernova a year, they would still need ten years, minimum, to complete their program.

Problems began even before they could start observing. The contractor constructing the camera delivered a mirror that didn’t fall within “tolerances,” as opticians call the allowable imperfections. The second cut was spoiled when cleaning fluid spilled on the mirror. Finally, the third cut of the mirror worked. Pennypacker, however, had ordered a camera without a filter, figuring that the more light he got, the better—and not understanding that if you want to compare the brightness of an object on different days, you need to observe in different filters in order to “equalize” the light level. After the crack technical crew at LBL designed an after-the-fact filter wheel, Pennypacker handed it to a graduate student, sent her to Australia, and provided her with the number to give to customs. When she got to customs, she went up to the two clerks and said, “I have this number.” They looked at each other, then back at her. “Number for what?” one of them said. (A few days later the director of the observatory in Australia managed to convince the authorities to unconfiscate the wheel.) Even when the team did perform supernova searches, the subsequent logistics were daunting. The on-site computers didn’t have sufficient bandwidth for the Berkeley project’s purposes, so team members had to take the computer data to the Sydney airport, fill out volumes of paperwork, and put the cargo on the next plane to San Francisco. There, someone else had to fill out more paperwork before claimi...

Every few months the supernova search had to justify its existence as part of the Center for Particle Astrophysics to an internal Program Advisory Committee. Every few months it also had to justify its existence to an External Advisory Board.

Robert Cahn, the new director of the Physics Division at LBL, first approached the senior researcher on the project, Gerson Goldhaber. But for Goldhaber the chance to work on supernovae had represented a freedom from the kind of responsibilities he’d held for four decades at behemoth particle accelerators. Muller had moved on. The next choice was Saul Perlmutter. Cahn consulted with Muller: Was the kid ready? Muller thought maybe so.

By ten or eleven in the evening, Perlmutter was alone and the images would begin to emerge on his screen. Each image held hundreds of galaxies; by the end of the night he would collect dozens of images. He printed out each one, just in case. Sometimes the computer told him that a blip of light had appeared that hadn’t been there the previous month, and he would bend close to the screen and try to figure out what was wrong. The view of the wide-field camera distorted the geometry, so he didn’t trust blips near the edge of the frame. Sometimes a blip would be too near the center of a galaxy, meaning that its light would be impossible to distinguish from the background light during follow-up observations. Sometimes the blip turned out to be an asteroid. One night he found a blip that he couldn’t discount, and he had to wonder what obvious error he was overlooking, but he couldn’t think of one, so he asked himself what subtle error he was overlooking, but he couldn’t think of one, and then he wondered what he was doing wrong, when suddenly he realized, “Wait—this is what we’re supposed to be looking for”: the potential supernova you can’t throw away.

They had a supernova. Still no cause for celebration. Again, the data was meaningless for cosmology unless they knew how distant the supernova was—its redshift. For that, they would need a spectroscopic analysis. Twelve times, at four observatories around the world, astronomers agreed to make the follow-up observations. Eleven times the weather didn’t cooperate. The twelfth, the instrument malfunctioned.

The old record redshift, the one set by the Danish team, had been 0.31, corresponding to roughly 3.5 billion years ago. The new record redshift was 0.458, or 4.7 billion years ago.

the first question that the LBL team had asked was: Can we find distant supernovae? It was, Kirshner thought, the wrong question to ask first. The right one was whether distant supernovae were worth finding. Could they really serve as standard candles?

Edwin Hubble spent much of the last twenty years of his life working under the assumption that galaxies might be standard candles, even though they weren’t entirely uniform. Maybe they were similar enough that he could use them to discern the universe’s shape and fate.

nova” paper, argued that Hubble had it backward: “You must understand the galaxies before you can get the geometry right.”

Through a telescope on Earth, the two types would look the same, even though one is an implosion and the other is an explosion. But a spectroscope would show the difference—hydrogen or no hydrogen, Type II or Type I.

In the 1980s, however, the clear distinction between Type I and Type II began to blur. Spectroscopic analysis of three supernovae—one each in 1983, 1984, and 1985—showed that they consisted of huge amounts of calcium and oxygen, consistent with the interiors of massive stars that end their lives as Type II supernovae, but no hydrogen, consistent with white dwarfs that end their lives as Type I supernovae. Some astronomers, including Kirshner, suggested that they were seeing a third type of supernova, essentially a hybrid of the other two. It was the product of a core collapse that had already lost its outer shell: a hydrogen-free implosion. They added this specimen to the Type I column, calling it Type Ib. The old Type I, a thermonuclear explosion with no hydrogen, was now Type Ia. In 1991, even that classification—Type Ia—began to blur. On April 13, five amateur observers in four locations around the world discovered a supernova designated 1991T.* On December 9, an amateur astronomer in Japan discovered a supernova designated 1991bg. Follow-up spectroscopic observations by professional astronomers—including Kirshner, on April 16, for 1991T—showed that they were both Type Ia supernovae. But their luminosities differed widely. Supernova 1991T was much brighter than the usual Type Ia at its particular distance, and 1991bg was much dimmer than the usual Type Ia at its particular distance. Astronomers could rule out the possibility that they were simply miscalculating distance...

the Hubble constant was around 50 to 55. Despite its name, the Hubble constant wasn’t a constant—a value unchanging over time. It told you only how fast the universe was expanding now—its current rate of expansion—and for this reason astronomers sometimes referred to it as the Hubble parameter. It told you nothing, however, about how much the expansion rate was changing over time. That value—Sandage’s second number—astronomers called the deceleration parameter because it would tell you to what extent the universe was slowing down. From the Hubble parameter you could extrapolate backward into the past and, depending on the amount of matter in the universe, derive the universe’s age. From the deceleration parameter you could extrapolate forward into the future and, depending on the amount of matter in the universe, derive the universe’s fate.

In that sense, there were only two numbers to measure in cosmology: the alpha and the omega of the universe.

Both measurements would require a standard candle, and at the time that Suntzeff received his Carnegie Fellowship in 1982, Sandage (along with Gu...

On February 23 of the following year, 1987, a supernova went off right overhead. SN 1987 A appeared in the Large Magellanic Cloud, one of the few galaxies visible to the unaided eye—and only from the Southern Hemisphere. It was the first unaided-eye supernova since 1604, and among astronomers it prompted a worldwide viewing party. It wasn’t a Type Ia, the explosive kind of supernova that Phillips and Suntzeff had studied. It was a Type II, the implosive kind.

“There are only two numbers to measure in cosmology.”

Maybe they were wrong in thinking that Type Ia supernovae were not standard candles. Maybe Leibundgut, who after all had been studying other Type Ia while they were busy with the Type II 1987 A, was right. And if he was right, then maybe they could use nearby Type Ia supernovae to measure the Hubble parameter—the current rate of the universe’s expansion. And if that program worked, they could go to farther supernovae to measure the deceleration parameter—the rate at which the expansion was slowing down.

Ideally, a supernova search would combine the widest-field camera with the latest CCD technology, but that option wasn’t available to the collaboration. Instead they had to choose between a telescope that couldn’t accommodate a CCD camera but had a wide-field view and a telescope that could accommodate a CCD but had a narrow-field view. They chose the wide-field, no-CCD view, the 24-inch Curtis Schmidt Telescope on Cerro Tololo. When hunting prey as rare and elusive as supernovae, the more galaxies you can grab at a time, the better your chances of finding even one, and in identifying supernova candidates, quantity still trumped quality. The photographic plates were big—eight inches by eight inches—and covered a patch of sky equal to one hundred full moons.

The bright one declined more gradually. The dim one declined more abruptly. Bright . . . gradually. Dim . . . abruptly.