Part of the rush to consensus in the community was sociological. Corroboration for any scientific result is always necessary, and if only one team had reached a surprising result, the response of the community would have been intense skepticism. That two teams had independently arrived at the same conclusion was notable. So, too, that the two teams had used mostly independent sets of data (very few of the same supernovae), had relied on several independent methods of analysis (including the corrections for dust), and had arrived at a conclusion that was the opposite of what they expected. But two teams that met all those criteria and had been infamously competitive? "Their highest aspiration," Turner said, "was to get a different answer from the other group."
And part of the rush to consensus was aesthetic. Just as inflation in 1980 solved the flatness and horizon problems, a positive lambda in 1998 made the universe understandable again. As Turner said, "It made eve ry thing fit!"
Those measurements of the Hubble constant on the "wrong" side of 60 that displeased Allan Sandage because they suggested a universe younger than its oldest stars? Problem solved. Those large-scale structures of supercluster filaments that seemed too mature for such a young universe? Problem solved. The universe was "too" young only if you assumed that the expansion rate had been decelerating throughout the history of the universe, or at least holding steady. A car that had been accelerating from 50 to 60 miles per hour and was only now reaching 65 would have needed more time to cover the same stretch of road than a car that had already been cruising at 65 miles per hour or slowing down from 70. If the expansion were decelerating, hitting the brakes, it would have been going faster in the "recent" past, and therefore taking less time to reach the present, than if it had just been constant. But an expansion that was accelerating today, hitting the gas, going faster and faster, would have been going less fast in the recent past, taking more time to reach the present. Thanks to acceleration, the age of the universe seemed to be, roughly, in the range of fifteen billion years, safely in the older-than-its-firstborn, old-enough-to-have-mature-filaments range.
But what made the supernova results perhaps most aesthetically pleasing wasn't just the presence of a positive value for lambda but the value itself.
If you were Bob Dicke or Jim Peebles in the late 1970s and you wanted the observation of a uniform cosmic microwave background to make sense, you wanted a theoretical explanation for homogeneity and isotropy. And then you got one: inflation. If you were Mike Turner or Rocky Kolb in the 1980s and you wanted the theory of inflation to work, you wanted an observation that revealed a flat universe. And then you got one—or half of one, anyway. COBE indicated that inflation was correct, meaning that the universe had to have an omega of 1. But numerous other observations indicated that the amount of mass in the universe was less than critical density, meaning that omega had to be less than 1—significantly so.
But now lambda explained away that contradiction. The amount of matter in the universe wasn't enough to halt the expansion, but the amount of matter and energy in the universe was. According to Einstein, matter and energy are equivalent, so while the mass, whether in the form of dark matter or regular matter, might well fall short of the critical density, the energy causing the acceleration—lambda—could make up the difference. A mass density of 40 percent or so plus an energy density of 60 percent or so added up to 100 percent of the critical density, or an omega of 1.
The universe did have a low matter density.
The universe was flat.
"Admit it," Jim Peebles once teased Brian Schmidt, "you didn't know what you had. You'd never heard of inflation."
"Inflation!" Schmidt answered. He informed Peebles that at Harvard he'd shared an office with the theorist Sean Carroll while Carroll was writing "The Cosmological Constant," one of the influential pre-1998 papers that explained how lambda could save inflation. "Alan Guth used to drop by once a week!" he added.
A positive lambda solved so many problems that when Turner approached Peebles in Aspen in 1998, he already knew what he wanted to debate: "Is Cosmology Solved?"
It was a debate Turner had already been framing. That March, for a dark-matter workshop in Gainesville, Florida, he had titled his talk "Cosmology Solved? Maybe." The following month, for a conference in Kyoto, Japan, he had dropped the qualifier from the title and went with a more straightforward "Cosmology Solved?" The published versions of both papers included the same sentence in the abstract: "These are exciting times in cosmology!" For the Smithsonian debate he took the exclamation point out of the body of the talk and promoted it to the title: "Cosmology Solved? Quite Possibly!"
Peebles would have to handle "quite possibly not," which was fine by him. It wasn't in his nature to argue passionately for a specific side of an unresolved issue, if only because having convictions about unresolved issues was unscientific. If he felt passionately about anything, it was that, in the absence of facts, you shouldn't feel too passionately. Fourteen years after he himself had used inflation's prediction of a flat universe as the basis for a lambda argument, Peebles still thought the community's embrace of inflation was premature—"distasteful," even. When he thought about physics, he divided its practitioners into classicists and romantics. The classicists were inventive but followed the rules; the romantics were respectful of the rules but followed their intuition. The romantics waved their hands and came up with a homogeneous, isotropic universe, and then, if they were lucky, an observation came along that could test their assumptions and predictions. A classicist looked to that observation—the one that suggested an expanding universe—then made a prediction of a temperature for the background radiation that observations would test. Then it was the romantics' turn again, waving their hands and invoking inflation and dark matter and, now, "missing energy"—the explanation that some classicist was going to have to invent for whatever physical presence in the universe corresponded to a positive lambda and caused the expansion to accelerate. Jim Peebles liked to think of himself as a classicist.
Mike Turner liked to think of Jim Peebles as "half enthusiast, half curmudgeon."
The debate took place on a wet Sunday afternoon in October at the National Museum of Natural History on the National Mall in Washington. The setting was Baird Auditorium, the same hall where the astronomers Heber Curtis and Harlow Shapley had "debated" in 1920 whether the Milky Way was the universe in its entirety or whether other "island universes" existed outside of it. Back then, Vesto Slipher's spectroscopy showing redshifts of nebulae was less than a decade old. Einstein's cosmology was only three years old and still applied to a static universe, thanks to his insertion of lambda. Hubble's discoveries that the nebulae were separate island universes and that, when their distances were graphed against their velocities, they seemed to be receding, lay in the decade to come—and with them, the apparent obsolescence of lambda. But now, some seventy years later, lambda was back. On their way into the auditorium, audience members received buttons bearing "A." If they were bewildered by the symbol, they weren't for long.
Like the earlier debate, the 1998 version wasn't going to solve anything; its purpose was to educate and entertain. And in terms of showmanship, as Peebles would have known in advance from having attended numerous talks by Turner, the debate was over before it began. All Turner had to do to win over the audience was to display one of his usual colorful viewgraphs, complete with Keith Haring-like dancing silhouettes:
COSMOLOGY is EXCITING!... for at least the next 20 years
STRONG FOUNDATION: Hot Big-bang Model
BOLD IDEAS DEEPLY ROOTED IN FUND. PHYSICS: Inflation + CDM
FLOOD OF DATA
(And all Turner had to do to make Peebles wince was say the words "precision cosmology.")
When the audience left four hours later, the drizzle might have felt like exclamation points dancing over their heads. But the question "Cosmology Solved?" was, by Turner's own admission, "ridiculous." As he acknowledged at the end of the debate, he was being "purposefully provocativ
e." Debates might need a yes-or-no question, but Turner couldn't answer "Yes" and Peebles couldn't answer "No" without seeming foolish.
In a way, their roles on stage, while suiting their personalities, were almost perversely reversed. Turner argued, "I believe we will ultimately refer to 1998 as a turning point in cosmology as important as 1964"—the year that Wilson and Penzias inadvertently detected the cosmic microwave background at a temperature that Peebles himself had predicted. He cited the progress in establishing the most fundamental numbers in cosmology—the two that Sandage had always cited, plus the third that inflation had introduced. Astronomers were converging on a Hubble constant in the mid-60s. They were agreeing to a matter density of 0.4, give or take. And despite that seeming shortfall, they had discovered observational evidence that bumped the ratio between the overall density and the critical density—between the matter/energy density and the density necessary to keep the universe from collapsing—up to 1.
Yet Turner himself acknowledged that there was a problem that a positive value of lambda didn't solve. Cosmology had a new syllogism: One, the expanding universe was full of matter attracting other matter through gravity; two, the expansion was speeding up; therefore, something other than matter, dark or otherwise, had to be overwhelming the influence of gravity. So: What was it?
Cosmology solved? Hardly!
To astronomers, lambda was just a fudge factor, a symbol in an equation. It might equal zero. It might not. But if you had confidence in the usefulness of Type Ia supernovae for cosmology, and if you satisfied yourself that you'd checked your results, then you accepted its value. Brian Schmidt had been aware of the implications of a positive lambda for the theory of inflation, but Adam Riess, for instance, had not. In the days after Riess's computer code told him that the universe had negative mass unless he balanced it with a positive lambda, he'd had to educate himself—happily—about all the problems that a cosmological constant would settle.
For particle physicists, however, a positive lambda didn't solve a problem. It created one.
From a particle physics perspective, lambda wasn't just a number. It was a property of space. And space, in particle physics, wasn't empty. It was a quantum circus, a phantasmagoria of virtual particles popping into and out of existence. Not only did those particles exist, as experiments had shown, but they possessed energy. And energy, in general relativity, interacts with gravity. The result of quantum particles possessing energy that interacts with gravity was what physicists called the Casimir effect, after the Dutch physicist Hendrik Casimir. Put two parallel plate conductors closer and closer together, Casimir proposed in 1948, and you could measure the increase in the vacuum energy. Numerous experiments since then had found agreement with his predictions. As the mathematician Stephen Fulling noted, "No worker in the field of overlap of quantum theory and general relativity can fail to point this fact out in tones of awe and reverence."
So positive energy itself wasn't a surprise. And theorists even had two forms of vacuum energy in mind—or two names for them, anyway. One form of vacuum energy would be constant over space and time, and they would call it the cosmological constant. Another would vary over space and time, and they would call it quintessence (the fifth element in ancient Greek physics). In order to discourage astronomers from assuming that the terms "lambda" and "cosmological constant"—which they'd been using nearly interchangeably—were identical, Turner started testing other terms. "Funny energy" he auditioned at the Fermilab conference in May 1998, but that didn't stick. His next try—"dark energy," with its deliberate echo of "dark matter"—did.
The problem with the supernova result of a positive energy density in the universe, however, was that quantum mechanics predicted a value larger than the 0.6 or 0.7 that astronomers measured. A lot larger. Ten-to-the-power-of-120 larger. That's of 1,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000 times larger. As the joke went, even for cosmology that's a big discrepancy. The stretching of space under the influence of such a ridiculously large energy density would be so extreme that, as Turner said, "you wouldn't be able to see the end of your nose." Not that the universe would be here for your nose to have an end on: A density that high would have cooled the cosmic microwave background below 3 K in the first 1/100,000,000,000,000,000, 000,000,000,000,000,000,000,000th of a second after the Big Bang. So given the choice between an energy density with a value of 10120 and one with a value of 0.7, most particle physicists would have been perfectly content to assume that somehow someone someday would manipulate the math, or figure out how particles were annihilating one another in just the right proportion, in order to make the result be what everyone had always been perfectly content to assume it was: A = 0.
Skeptics liked to quote a saying: "You get to invoke the tooth fairy only once"—meaning dark matter—"but now we have to invoke the tooth fairy twice"—meaning dark energy. An epithet became inescapable, or at least a commonplace on the conference circuit: epicycles. Were astronomers and their inflation-theorist enablers simply saving the appearances, like the ancients and their desperate measures to make the math correspond to the motions in the heavens?
One could cite modern precedents. Scientists in the nineteenth century figured that the phenomenon of waves of light propagating across space like waves across water didn't make sense unless they inferred the presence of a cosmic pond. The Scottish physicist William Thomson, eventually Lord Kelvin, spent the entirety of his career trying to find equations to describe this "ether." In 1896, on the occasion of the golden jubilee of his service to the University of Glasgow, he wrote to a friend, "I have not had a moment's peace or happiness in respect to electromagnetic theory since Nov. 28, 1846." He died in 1907, two years after Einstein established the theory of special relativity by eliminating the need for absolute space, thereby making the ether "superfluous."
Would future generations look on the whole of modern cosmology as a similar lesson in the limitations of inferences from indirect evidence? The motions of galaxies didn't make sense unless we inferred the existence of dark matter. The luminosities of supernovae didn't make sense unless we inferred the existence of dark energy. Inference can be a powerful tool: An apple falls to the ground, and we infer gravity. But it can also be an incomplete tool: Gravity is...?
Dark matter is...?
Dark energy is...?
Astronomers might not have been able to identify dark energy, but some theorists knew what it was: an inference too far. Just because a positive lambda would solve many problems didn't mean it existed.
"You observational astronomers," a theorist told Alex Filippenko in 1998, "are wasting a lot of valuable Keck and Hubble time, because your result must be wrong. We have no theory that could be compatible with a tiny non-zero vacuum energy"—tiny in the sense that lambda would be equal to 0.6 or 0.7 of critical density, rather than 10120—"and there's no theory that could possibly be compatible with this."
"Look," Filippenko said, "this is an observational result. I only know what end of the telescope to look through. You're a lot smarter than I am. But with additional observations, we will either confirm this, or we will find that we were wrong—hopefully for some subtle reason, and not '2 plus 2 equals 5' in some computer program."
In other words: Just because a positive lambda created a problem didn't mean it didn't exist.
In the end, sociology—the fact that two intense rivals had independently reached the same surprising result—wasn't going to be enough to convert the skeptics or, for that matter, to convince appropriately cautious astronomers that they weren't fooling themselves. Neither would aesthetics—whether the result solved problems or created problems. Not even the honor of being Science's "Breakthrough of the Year" for 1998. Filippenko's point was that only science, only further observations, could test a positive value for lambda.
And so astronomers did what scientists do in such circumstances:
They set out to prove that the effect didn't exist. What problems might they have overlooked that could cause distant supernovae to appear dimmer than they should? Two possibilities immediately presented themselves.
One was an exotic kind of dust. Astronomers knew that regular dust within galaxies makes the light redder, and they knew how to correct for that dust—thanks in large part to Riess. His paper on the MLCS—multicolor light-curve shapes—correction method for dust won the 1999 Trumpler Award, an honor that recognizes a recent PhD thesis of unusual importance to astronomy. But now astronomers were mentioning the possibility of gray dust, and positing its presence between galaxies.
"Nobody has ever seen gray dust between galaxies," Riess thought. "But," he reminded himself, "nobody has ever seen a cosmological constant either."
Or what if the unusually faint appearance of supernovae at great distances was the result of supernovae being different back then, when the universe was younger and less complicated? What if the nature of Type Ia supernovae had changed over the life of the universe, and the recipe for a relatively nearby supernova was different from the recipe for a distant supernova? Maybe more distant supernovae had a simpler cocktail of elements, making them intrinsically fainter and giving the illusion that they were more distant.
There was one way to find out. If the interpretation of the supernova evidence was correct, then we were living at a time when dark energy was dominant over matter; the anti-gravitational force of dark energy was winning a tug of war with the gravitational force of matter. In that case, the expansion of the universe would be accelerating, and, as the two teams found, distant supernovae would appear dimmer than we would expect.
In earlier eras, however, the universe would have been smaller and therefore denser. The earlier the era, the smaller and denser the universe; the denser the universe, the greater the cumulative gravitational influence of matter. If astronomers could see far enough across the universe—far enough back in time—they would reach an era when dark matter was dominant. At that point, the gravitational influence of dark matter would have been winning the tug of war with the anti-gravitational force of dark energy. The expansion would have been decelerating, and supernovae from that era would therefore appear brighter than we would expect.
The 4 Percent Universe Page 20
