Not so the supernovae that we would see through gray dust, or that had a simpler cocktail of elements in the early universe. Those supernovae would just keep appearing fainter and fainter, the farther and farther we looked.
To distinguish between the two scenarios—dark energy versus either gray dust or changing cocktail—you would need to observe a supernova distant enough that it had exploded during that far earlier, far more distant era. You would need a supernova that had exploded before the expansion of the universe "turned over"—before the universe had made the transition from deceleration to acceleration, back when matter, not energy, was winning the tug of war. You would expect that supernova to be brighter than it "should" be. Plot it on the Hubble diagram—way out there, far beyond the nearby supernovae from Calán/Tololo, beyond the high-redshift supernovae that the two teams had discovered—and the slight upward deviation from the 45-degree straight line that High-z and the SCP had graphed would "turn over," too, just like the universe. It would dip down.
And if it didn't, you'd have to rethink dark energy.
Ground-based telescopes, however, couldn't see that far across the universe. The Hubble Space Telescope could, and it could even discover supernovae at that distance. From December 23 to 27, 1997, Ron Gilliland and Mark Phillips had used HST to try to prove that you could do just that—detect supernovae from the earliest epochs of the universe. For their search, they chose a familiar, even famous, speck of sky: the Hubble Deep Field. Two years earlier, in 1995, HST had made the most distant image of the universe. For ten days the telescope had drilled a hole in the sky the size of a grain of sand at arm's length, just soaking up photons, seeing deeper and deeper across space and therefore farther and farther back in time. In the end the Hubble Deep Field contained about three thousand galaxies, some faint blue and among the first in the universe. Gilliland and Phillips wanted to make a repeat visit and do what supernova hunters had been doing since the 1930s—compare the earlier images with a current image and see what had changed. Did any of the galaxies in 1997 contain a speck of light—a supernova—that hadn't been there two years earlier?
Two did. Those specks got the designations SN 1997ff and SN 1997fg. Without follow-up observations, Gilliland and Phillips couldn't do the photometry that would allow them to construct light curves. But they'd made their point. You could use HST to discover supernovae at distances inaccessible from telescopes on earth.
What you couldn't do, however, was what astronomers needed to do in order to test dark energy: make multiple reference images, then return to the same field in weeks to come in the hope of discovering the most distant supernova yet, for which you would have already reserved time for follow-up observations in the weeks and months ahead. You couldn't guarantee that you wouldn't be wasting HST time.
Still, those two supernovae—SN 1997ff and SN 1997fg—bugged Adam Riess. He couldn't stop thinking about them. By 2001, while remaining a member of the High-z collaboration, he was a staff scientist at the Space Telescope Science Institute, so HST results and possibilities were always on his mind. But SN 1997ff and SN 1997fg were a particularly poignant reminder of a lost opportunity. They were at a distance sufficient to test the deceleration-before-acceleration period of the dark-energy cosmological model—when the expansion was still slowing down under the dominant influence of dark matter, rather than speeding up under the dominant influence of dark energy. If only Gilliland and Phillips had been able to do follow-up work on SN 1997ff or SN 1997fg, Riess thought, astronomy would already have been able to put dark energy to a particularly compelling test.
In early 2001, Riess realized he could reframe the question.
What if one of those supernovae had been followed up? Not deliberately, by Gilliland and Phillips, but serendipitously, by HST during some other observation?
So he called up the HST search page on his office computer. He typed in the coordinates. Right ascension: 12h36m44s.11. Declination: + 62012'44".8. He requested dates that would correspond to the period during which SN 1997ff and SN 1997fg would have brightened and dimmed: December 27, 1997, to April 1, 1998.
Riess understood that the possibility of his finding what he wanted was extremely remote. HST looked at a lot of space; what were the chances that it had been staring at a particular dot of deep space during a particular period of time?
"Nobody's that lucky," he told himself.
Adam Riess was that lucky.
Earlier in 1997, Space Shuttle astronauts had added a couple of instruments to HST, including the Near Infrared Camera and Multi-Object Spectrometer, or NICMOS. By seeing in the infrared, NICMOS was particularly sensitive to distant objects whose light was so redshifted that, by the time it reached our patch of the universe, it had left the visible part of the electromagnetic spectrum. The NICMOS team had decided to test their instrument on a particularly distant patch of space with features they could easily identify: the Hubble Deep Field.
The observation program didn't begin until January 19, but the camera took some test images in the interim. And there it was: 1997ff. It was in the HST archives of the NICMOS test run, on December 26, January 2, January 6. Once NICMOS started taking data for real, 1997ff appeared in frame after frame, right at the edge. Sometimes it fell off the edge. But usually it was there. Riess spent the early part of 2001 examining SN 1977ff, establishing from the redshift that it had exploded about 10.2 billion years ago—far earlier than the period when the expansion of the universe would have gone from slowing down to speeding up. If indeed the universe had gone from deceleration to acceleration. If indeed dark energy existed.
Every spring the Space Telescope Science Institute hosted a symposium. One previous symposium topic had been the Hubble Deep Field; another had been stellar evolution. The topic in 2001 happened to be "The Dark Universe: Matter, Energy and Gravity." It was a chance for more than a hundred astronomers from around the world to reflect on their seemingly oxymoronic mission—what the symposium organizer and astrophysicist Mario Livio called "astronomy of the invisible."
STScI occupied a low, modern—and somewhat modest, considering its NASA provenance—building on a winding road in a far corner of the Johns Hopkins University campus in Baltimore.* It looked as if it were ducking its head so it could fit under the trees. At the rear of the building, outside the door to the auditorium on the first floor, was a wall of glass overlooking a creek. No Fermilab-style stampedes-around-the-accelerator-track-followed-by-a-barbecue-for-hundreds here; wine and cheese or the occasional overcaffeination was as wild as an STScI meeting would get.
By now, even the most ardent dark-energy skeptics had learned to accommodate the findings from a series of balloon experiments that had been launched from the outskirts of Antarctica and the Atacama Desert in Chile. The balloons had floated to an altitude of 100,000 feet and scraped the underbelly of outer space, at which point the on-board detectors had surveyed the cosmic microwave background. The goal was to refine COBE's measurements of the differences in temperature between points on the sky. If the differences in temperature were greatest between points separated by less than 1 degree, then the universe was open; by more than 1 degree, then the universe was closed; by 1 degree, then the universe was flat. So far, the verdict was all flat.
But saying that the universe sure looked flat wasn't quite the same as saying that the expansion of the universe was accelerating. You couldn't rely on an argument by subtraction—an omega of 1 minus a mass density of 0.3 equals a lambda of 0.7. That math showed only the same seeming paradox that had existed pre-1998: an apparently flat universe via COBE, an apparently open universe via other observations. The balloon experiments made COBE's flat universe much more compelling, to the point that a flat universe was quickly becoming cosmological orthodoxy. But acceleration? Especially if you were a particle physicist, that result still didn't make sense—still left you pining for alternatives.
Among the observers in attendance was Vera Rubin, opening the conference with a historical overview
of dark matter—or, actually, a historical overview of the idea of dark matter, since, as she pointed out, until you know what dark matter is, you can't really know its history. She recalled predicting in 1980 the discovery of dark matter within ten years, and she said she was amused to see the British astronomer Martin Rees recently making the same prediction. She said she knew what Fritz Zwicky would have said about the current state of cosmology: "Epicycles!"
Among the theorists in attendance was Michael Turner, exhorting the congregation to indulge in "irrational exuberance" and embrace the era of "precision cosmology." To a fellow theorist complaining about the 10 problem, Turner responded with exasperation: "Can't we be exuberant for a while?"
Saul Perlmutter was there too, talking up the possibilities of a space telescope dedicated to supernovae, and a couple of dozen other presenters were there to promote their own prospective research projects and report on their latest observations and postulate extravagant possibilities about the identity of dark energy. But mostly everybody was there to try to answer the question that Mario Livio had written on a transparency for his talk summarizing the symposium: "Accelerating Universe—Do We Believe It?"
Which is why it was Adam Riess who stole the show.
He choreographed his presentation, on the third day of the four-day conference, as a striptease. He had to do something to spice it up, since everybody in the auditorium knew what he would be showing. Two days earlier, on the first day of the symposium, Riess had attended a NASA press conference in Washington to announce his discovery. And one day earlier, that announcement had made the front page of the New York Times as well as other newspapers around the world. Still, now was his chance to let his fellow cosmologists examine this new evidence for themselves.
He would be using an overhead transparency. He kept most of it covered at first, while he explained what he would be showing. It was a Hubble diagram—redshift against brightness—of the supernovae from both the SCP and High-z teams. The points in this case represented not individual supernovae but averages of supernovae at similar redshifts.
Riess revealed the first three dots on the transparency: here, and here, and here, the averages of the nearby supernovae from the Calán/ Tololo survey.
Then, moving to the right, the next three dots: here, and here, and here, the averages of the distant supernovae from the SCP and High-z searches.
The dots were beginning to describe the now-familiar gentle departure from the straight line, the upward turn toward the dimmer. In six dots Riess had taken his audience from a few hundred million light-years across the universe, to a billion, then two billion, three, four. Now, he said, he had the point that represented SN 1997ff. He had determined its redshift to be about 1.7, the farthest supernova to date by a long shot, a distance of about eleven billion light-years.
They knew what they were going to see, but the hundred or so astronomers in the auditorium couldn't help themselves. They shifted in their seats. Leaned forward. Held back. Crossed arms.
There: SN 1997ff.
A gasp.
The gentle upward curve was gone. In its place was a sharp downward pivot. The supernova was twice as bright as you would naively expect it to be at that distance. The universe had turned over, all right.
While Riess went on to explain that the result ruled out the hypothetical effects of exotic gray dust or a change in the nature of supernovae at a confidence level greater than 99.99 percent, the evidence continued to loom on the screen behind him. His audience couldn't take their eyes off it. For the astronomers of the invisible, it was something to see.
PART IV
Less Than Meets the Eye
10. The Curse of the Bambino
"I'M JUST GOING to watch this for a little while."
"Because of the funny noise?"
"Because it stepped backward."
"It stepped backward?"
"It stepped backward."
"It stepped backward." A pause. "That's impossible."
The two graduate students were staring at a pinkie-sized shaft—a device that was turning gears that were turning gears inside a copper cylinder that extended some thirteen feet underground. The shaft was rotating, or "stepping," clockwise in tiny tick, tick, ticks. A counterclockwise step might have been impossible, but the first student had seen it with his own eyes. Now he needed to see it again.
He jammed his hands in his pockets. Then he took his hands out of his pockets and crossed his arms. Next he leaned one hand against a concrete pole. Then he grabbed a swivel chair and rested a knee on the cushion. He didn't take his eyes off the shaft. Another intern wandered past, asked what they were doing, and joined the staring contest.
The shaft was the first to blink. After ten minutes it stepped backward again.
The three students marched over to the indoor shack where the other members of the team were huddling in the air conditioning. Their presence pushed the shack to its capacity: eight. The first graduate student announced his finding to Les Rosenberg, one of the leaders of the project. Rosenberg, bushy-bearded and balding, smiled, but not really.
"That's impossible," he said.
"Oh, it's just the software," said yet another member of the team, not even glancing up from a desktop computer.
Still, Rosenberg had to see for himself. Soon four physicists, hands in pockets, were staring at the shaft. Tick. Tick. Tick. Tick. Tick. Tick. Tick.
And so went the search for dark matter one summer afternoon in 2007 in a tin-roofed hangar in the California desert forty miles east of the Bay Area—officially, Building 436 of Lawrence Livermore National Laboratory, but more commonly, "the shed." The experiment was state-of-the-art, though at the moment it was more state-of-the-workbench. The interns were working from blueprints they'd spread on the concrete floor, and they were variously wielding wire cutters and wrenches, drill bits and hammers and a hacksaw. Drips, dents, flakes, scrapes, and spills decorated the tables and metal shelves. The "To Do" list on the whiteboard hanging next to the shed entrance was numbered 1 to 8, though the 8 was on its side: infinity. After lunch, the software guy fixed the software glitch: infinity minus one. The experiment was nearly twenty years in the making—this incarnation of the instrument would be the second—and it had another decade or so to go. But in the end, after the experiment had run its course, the world would know whether one of the two leading candidates for dark matter actually existed.
Even as Vera Rubin and her galaxy-motion-measuring colleagues in the 1970s were converging on the evidence for "missing mass" and prompting cosmologists to ask the inevitable question What is it?, parallel developments in particle physics were coincidentally coming up with a possible answer: Not the stuff of us. Not the stuff of atoms—the protons and neutrons, collectively called baryons, that have been forming and re-forming familiar matter from the first instant of the universe. Other stuff instead, also left over from the first instants of the universe, but not forming and re-forming—not interacting with itself or any other matter. Stuff that was weighing down the universe just by being there in abundance, but not doing much else. In the 1970s theorists were coming up with these hypothetical particles by the bushel in an effort to solve some problems with the standard model of particle physics. But when they looked at the properties such particles would have, they noticed that two in particular would exist in just the right proportion to make up the amount of matter in the universe that was "missing."
One was the axion, the particle that the physicists in "the shed" were hoping to detect. If it existed, then it did so by the trillions per cubic centimeter, and several hundred trillion would be threading their way through your body right now. Physicists are used to the idea of particles passing through seemingly solid objects; a neutrino could pass through a light-year of lead without coming into contact with another particle. But as with the search for the other leading dark-matter candidate, called the neutralino, the trick with the axion was to catch it.
Karl van Bibber started chasing th
e axion in 1989, when he was still on the prodigy side of forty. Three years later he recruited Rosenberg, a former student of his at Stanford whom he considered "an absolute genius" and "a world-class experimentalist," to join him in the Axion Dark Matter Experiment (ADMX). Van Bibber grew up in Connecticut, a fan of the Boston Red Sox, the team that infamously hadn't won a World Series since 1918. He spent his childhood hearing about how the Red Sox sale of Babe Ruth to the Yankees in the 1919–20 off-season had cursed the ball club. When the Red Sox went to the World Series in 2004, van Bibber's enthusiasm proved contagious; Rosenberg, who already felt some fondness for the team from his years on the faculty at MIT, joined his colleague in rooting for the Red Sox. Van Bibber's screen saver on his desktop computer at Livermore was a floating compendium of newspaper headlines from the Red Sox World Series victory in 2004: "Ghost Busters!" "BELIEVE IT!" "SEE YOU IN 2090!" He and Rosenberg long ago agreed that being a Red Sox fan was good training for being an axion hunter.
To have any hope of catching an axion they had to build a radio receiver that could track a signal with a "strength" in the vicinity of a trillionth of a trillionth—or 1/1,000,000,000,000,000,000,000, 000th—of a watt. That's three orders of magnitude fainter than the final transmission from the Pioneer 10 spacecraft in 2002, when it was seven billion miles from Earth and well on its way out of the solar system. But with Pioneer 10, scientists at least knew the signal's frequency; they knew where to turn to on the radio dial.
The 4 Percent Universe Page 21
