The 4-Percent Universe

by Richard Panek

  "Time to get serious." The PowerPoint slide, teal letters popping off a black background, stared back at a roomful of cosmologists at yet one more conference. Sean Carroll had taken it upon himself to give his fellow theorists their marching orders. The "heyday for talking out all sorts of crazy ideas," as Carroll explained, was over—that heady, post-1998 period when Michael Turner might stand up at a conference and call for "irrational exuberance." Now had come the metaphorical morning after.

  The observers had done their job. They had used supernovae, weak lensing, BAO, galaxy clusters, and the cosmic microwave background to find more and more evidence for acceleration until the community agreed: The effect was genuine. Then the observers continued doing their job, trying to figure out if dark energy is quintessence or the cosmological constant. And they continued doing their job. And continued. "Dark Energy is the Pied Piper's pipe," White wrote, "luring astronomers away from their home territory to follow high-energy physicists down the path to professional extinction."

  For more than twenty years, particle physics had been pursuing one prey: the Higgs boson, a hypothetical particle that would explain the presence of mass in the universe. The Tevatron at Fermilab had been trying to create it; soon the Large Hadron Collider in Geneva would try to manufacture it. By the time the discovery of cosmic acceleration was ten years old, astronomers were beginning to wonder if they, too, were practicing a science in search of one result: w.

  Schmidt recognized that he had done his part to usher astronomy into the arena of Big Science. Together the two supernova search teams had relied on the efforts of more than fifty collaborators. But the changes that astronomy was experiencing would have been happening anyway. Not only were the areas of study becoming more specialized—supernovae, CMB, gravitational lensing, and on and on—but so were the means of studying them, the narrow bands of the electromagnetic spectrum. The South Pole Telescope, for instance: the Sunyaev-Zel'dovich effect required astronomers to use a specific sub-millimeter wavelength in order to detect the "holes" left by photons that had moved out of that frequency. In one generation astronomy had gone from the lone observer on a mountaintop taking photographs in visible light to dozens of collaborators around the globe pursuing a variety of specializations by looking at increasingly narrow bands along the electromagnetic spectrum. Even if the supply of funding globally stayed the same, the demand for it wouldn't. Increasing specialization of areas of research and increasing diversification of research methods were creating an effective shortage of resources. And nothing was demanding those resources like dark-energy research. ADEPT—the space telescope that Chuck Bennett and Adam Riess had conceived in response to Perlmutter's SNAP—had eventually adopted baryon acoustic oscillations as its primary observing strategy, though it would also incorporate Type Ia supernovae. SNAP had expanded its mission from Type Ia supernovae alone to weak lensing. The cost for either satellite would be at least $1 billion—except NASA budgeted only $600 million. "It's not worth it!" one astronomer pleaded with a NASA representative at a dark-energy conference. "Either you do it right, or you don't do it. And if you're not going to do it right, then give us the money back and we'll do other things."

  "Hear, hear," piped up Mike Turner from a seat near the front of the auditorium.

  Schmidt was married to an economist, and he took an unapologetically bottom-line approach to the idea of a space telescope dedicated to dark energy: Was the mission worth the expense? What were the trade-offs? How much good science wasn't going to be done because of the community's concentration on dark energy? The answers would have been easier if the evidence so far were indicating that dark energy was not only quintessence but unmistakably quintessence—"standing out like dog's balls," as Schmidt would say, adopting the Australian vernacular. Studying something that creates a lot of changes in the cosmos, even subtly, would be more challenging, more satisfying, and probably more revealing of the universe's secrets than studying something that stays the same. If the magic number for omega had been 1, then the un-magic number for the equation of state was -1 because, as Jim Peebles said, "then it's a number, and we have nothing to do."

  But as the first decade of dark energy drew to a close, the evidence seemed to be pointing to the cosmological constant. The more that observers continued to do their job, the closer they got to -1. The question then became, as the title of a session at one of the dark-energy conferences put it, "How Well Do We Need to Do?"—as in, "How close do we need to get to w equals -1 before we agree that w does equal -1?" Lawrence Krauss, a theorist and panelist for the session, laid out his argument on a PowerPoint slide:

  The most reasonable theoretical prediction is w = -1.

  Observations suggest w = -1.

  Measuring w approximately = -1 therefore tells us nothing.

  So observers shouldn't pursue w at all, right? Wrong. "How well do we need to do?" Krauss said, repeating the title of the session. "Better than we'll be able to do! We need to do better than anything you're ever going to be able to do in the lifetime of the people in this room, I expect, experimentally. And that's just life. In spite of the fact that you're likely to spend the rest of your lives measuring stuff that won't tell us what we want to know, you should keep doing it. But you should be prepared to have a standard model for twenty or thirty years that you don't understand." Speaking on behalf of the theorists, he was basically telling observers, Keep on doing your job; we'll catch up.

  Until they did (assuming they did!), science was stuck with—to invoke a term that Carroll helped popularize through articles, lectures, papers, and a blog—a "preposterous universe." It was a universe that had the benefit of a seemingly perfect match between observation and theory—the workings of the heavens and the equations on paper. Take the observations of supernovae and the cosmic microwave background, apply the theory of general relativity, and you had a universe that did indeed add up to the magic omega number of 1. But it was also a universe that didn't add up. Take the observations of supernovae and the cosmic microwave background, apply the other cornerstone of twentieth-century physics, quantum theory, and you got gibberish—an answer that was 120 orders of magnitude off.

  Which didn't mean that dark matter and dark energy were the modern equivalents of epicycles or the ether. But it did mean that theorists had to confront the same problem that had consumed Einstein for the final three decades of his life: how to reconcile the physics of the very big—general relativity—with the physics of the very small—quantum mechanics.

  Theorists could use the two theories simultaneously—for instance, Hawking radiation, Stephen Hawking's idea that while quantum mechanics dictated the existence of pairs of virtual particles at a black hole's horizon, general relativity dictated that sometimes one of those pairs would slip over the edge into the black hole and the other would rebound back into "our" universe. But theorists hadn't yet figured out a way to make the two theories work together—to make an observation of 0.7 consistent with a prediction of 10120. What made the two theories incompatible—where the physics broke down—was the foundation of the past four centuries' worth of physics: gravity itself.

  In physics, gravity is the ur-inference. Even Newton admitted that he was making it up as he went along. In one of his letters to Richard Bentley about the stability of a universe operating under the influence of gravity, Newton wrote that the notion of a force of attraction existing between two distant objects is "so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it." Nearly two centuries later, the German philosopher-scientist Ernst Mach wrote, "The Newtonian theory of gravitation, on its appearance, disturbed almost all investigators of nature because it was founded on an uncommon unintelligibility." Now, he went on, "it has become common unintelligibility." Einstein endowed gravity with intelligibility by defining it not as some mysterious force between two objects but as a property of space itself, and he refined Newton's equations so that the presence of matter a
nd the geometry of space are interdependent. Most of the interpretations of dark matter and dark energy arose from the right-hand side of the equation for general relativity, the side where Einstein put matter and energy. But there are two sides to every equation, and in this case what was on the other side was gravity.

  Most astronomers had dismissed Modified Newtonian Gravity, or MOND, as an explanation for dark matter back in the early 1980s; after the 2006 release of the Bullet Cluster data—the photograph that "showed" dark matter separating out from regular matter in the collision of two galaxy clusters—even its defenders began to distance themselves. The Bullet data could accommodate MOND, but you would still need some kind of dark matter to explain the rest. And at that point MOND began to lose the attraction of simplicity.

  But just because MOND might not be valid didn't mean that general relativity was. Back in the 1950s Bob Dicke had organized the Gravity Group at Princeton in part to put Einstein to the test, and his efforts helped inaugurate a generation of experiments. The discovery of evidence for such general-relativistic phenomena as black holes, pulsars, and gravitational lensing had only accelerated those efforts. Dark matter and dark energy, however, endowed those efforts with a sense of urgency.

  Physicists were testing gravity on the scale of the very large. In the Sacramento Mountains of New Mexico, the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) aimed a pulse at the Moon twenty times every second. If a cloud happened to pass, then a green dot appeared, Bat Signal-like, in the purpling twilight over Alamogordo. Otherwise the beam traced a clear path to a target 238,000 miles away: one of three suitcase-size mirrors that Apollo astronauts had planted on the lunar surface four decades earlier specifically to facilitate this kind of experiment. With every laser burst, a few of the photons from the beam would bounce off the reflecting surface and complete their return journey to New Mexico. Total round-trip travel time: 2.5 seconds, more or less.

  That "more or less" made all the difference. By timing the speed-of-light journey, researchers were measuring the Earth-Moon distance moment to moment and mapping the Moon's orbit with exquisite precision. As in the apocryphal story of Galileo dropping balls from the Leaning Tower in Pisa to test the universality of free fall, APOLLO treated the Earth and the Moon as two balls dropping in the gravitational field of the Sun. If the orbit of the Moon exhibited even the slightest deviation from Einstein's predictions, scientists might have to rethink those equations.

  Physicists were also testing gravity on the scale of the very small. Until the turn of the twenty-first century researchers didn't have the technology to measure gravity at ranges of less than a millimeter, in part because testing gravity isn't simply a matter of putting two objects close to each other and quantifying the attraction between them. All sorts of other things may be exerting a gravitational influence. In a series of jewel-box experiments at the University of Washington, researchers had to take into account the metal of nearby instruments, the soil on the other side of the concrete wall that encircled the laboratory, the changing water level in the soil after a rainfall (and since the experiments were taking place in Seattle, that was a lot of changing), a nearby lake, the rotation of the Earth, the position of the Sun, the dark matter at the heart of our galaxy. Nonetheless, they narrowed their measurements of gravitational attraction down to a distance of 56 microns, or 1/500th of an inch.

  So far, Einstein was holding, both across the universe and across the tabletop. And with every narrowing of the range where Einstein might have been wrong, another hypothesis died, and Brian Schmidt's PowerPoint slide lost one more esoteric name. Even if Einstein didn't hold, researchers would first have to eliminate other possibilities, such as an error in the measure of the mass of the Moon or the Sun, or in the level of groundwater after a spring shower, before conceding that general relativity required a corrective.

  Even so, astronomers knew that they took gravity for granted at their own peril. Why was it so weak? Why—in the example that scientists commonly cited—could the gravitational pull of the entire Earth on a paper clip be counteracted with a dime-store magnet?

  Because, some theorists proposed, gravity was a relic from a parallel universe. Theorists commonly called these universes "branes," as in membranes. If two branes were close enough to each other, or even occupying the same space, then they might interact through gravity. Gravity would be on the scale of the other three forces if we had access to those other universes. What was a powerful force in a parallel universe might be the source of the effects of dark matter or dark energy in ours. The problem with these theories, at least from an astronomer's point of view, was how to test them. A theory needs testable predictions or it's not a truly scientific theory; its validity must come down to observations. But how can you observe a universe beyond your own?

  Scientists are always wincingly aware that they are prisoners of their perceptions. It was true, for example, that if dark energy was the cosmological constant, then a hundred billion years from now all that cosmologists would see would be a handful of galaxies. But it was also true that we didn't need to wait a hundred billion years to confront a similar perceptual obstacle. Inflation already ensured that certain traces of the universe's initial conditions would be forever out of reach.

  Those conditions would put even branes to shame, in terms of defying perceptions. If inflation can pop one quantum universe into existence, why not many? In fact, according to quantum theory, it should. It would, if inflation actually happened. In that case, our inflationary bubble would be one of an ensemble of 10500 inflationary bubbles, each its own universe. That's 100,000,000,000,000,000,000, 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, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000 universes. Our universe would just happen to be the one with a value of lambda suitable for the existence of creatures that can contemplate their hyper-Copernican existence.

  This scenario was, in a way, a logical extension of the argument that Vera Rubin had tried to make with her master's thesis. The Earth is rotating, the solar system is rotating, the galaxy is rotating. Why not the universe? Similarly, the Earth turned out to be just one more planet orbiting the Sun, the Sun one more star in an island universe, the island universe one more galaxy in the universe. And the universe?

  By 1973, scientists had named this idea the anthropic principle, and by and large they hated it. Called it "the 'A' word." Wouldn't even discuss it. If any scientific speculation, no matter how wild, ultimately must come down to a prediction, then what prediction could the anthropic principle make? How could you falsify the claim that an unfathomable number of universes exist outside our own? If you can't, critics said, then you had to file the idea under metaphysics—overlooking, perhaps, that it was the metaphysics of midcentury cosmology that had gotten them to a lambda-CDM-plus-inflation universe.

  The ongoing resistance to the anthropic principle following the discovery of lambda was similar to an earlier era's discomfort with a homogeneous and isotropic universe following the discovery of the CMB. But then homogeneity and isotropy got an explanation: inflation. A universe that underwent a brief period of extraordinary growth would indeed appear the same wherever you were and wherever you looked.

  The anthropic principle was similarly ad hoc—and so what? "Have you heard a better idea?" So wrote no less an authority on initially resisting a homogeneous and isotropic universe than Jim Peebles, in 2003. "I hear complaints that this anthropic principle has been introduced ad hoc, to save the phenomenon. But the same is true of A. The cosmological constant is now seen to save quite a few phenomena." As for another explanation for a low value of lambda: "Something may turn up." But it didn't have to; inflation itself had elevated the homogeneity and isotropy of the u
niverse out of the ad hoc and into the inevitable. Maybe inflation did the same for the existence of 10500 other universes.

  Which didn't mean what critics of the anthropic principle often accused it of meaning: the end of physics. While inflation might predict a menagerie of universes, it didn't explain the mechanism that would allow lambda to vary, universe to universe. Theorists would still have to try to work out the physics for that understanding of existence.

  And that would be their legacy—the legacy of Brian Schmidt, Saul Perlmutter, Adam Riess, and the dozens of other discoverers of evidence for the acceleration of the universe. It wouldn't be personal acrimony, and it wouldn't be changes to their profession's sociology. It would be the revolution in thought that dark energy mandated. Almost certainly this revolution would require the long-awaited union of general relativity and quantum theory. It might involve modifying Einstein's equations. It could feature parallel, intersecting, or a virtually infinite ensemble of universes.