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

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Prologue

The time had come to look inside the box. On November 5, 2009, scientists at sixteen institutions around the world took their seats before their computer screens and waited for the show to begin: two software programs being run by two graduate students — one at the University of Minnesota, the other at the California Institute of Technology — simultaneously. For fifteen minutes the two scripts would sort through data that had been collecting far underground in a long-abandoned iron mine in northern Minnesota. Over the past year, thirty ultrasensitive detectors — deep-freeze cavities the size of refrigerators, shielded from stray cosmic rays by half a mile of bedrock and snug blankets of lead, their interiors cooled almost to absolute zero, each interior harboring a heart of germanium atoms — had been looking for a particular piece of the universe. The data from that search had sped from the detectors to offsite computers, where, following the protocol of a blind analysis, it remained in a “box,” out of sight. Just after 9 a.m. Central Time, the “unblinding party” began.
 Jodi Cooley watched on the screen in her office at Southern Methodist University. As the coordinator of data analysis for the experiment, she had made sure that researchers wrote the two scripts separately using two independent approaches, so as to further ensure against bias. She had also arranged for all the collaborators on the project — physicists at Stanford, Berkeley, Brown; in Florida, Texas, Ohio, Switzerland — to be sitting at their computers at the same time. Together they would watch the evidence as it popped up on their screens, one plot per detector, two versions of each plot.
 After a few moments, plots began appearing. Nothing. Nothing. Nothing.
 Then, three or four minutes into the run, a detection appeared — on the same plots in both programs. A dot on a graph. A dot within a narrow, desirable band. A band where all the other dots weren’t falling.
 A few minutes later another pair of dots on another pair of plots appeared within the same narrow band.
 And a few minutes later the programs had run their course. That was it, then. Two detections.
 “Wow,” Cooley thought.
 Wow, as in: They had actually seen something, when they had expected to get the same result as the previous peek inside a “box” of different data nearly two years earlier — nothing.
 Wow, as in: If you’re going to get detections, two is a frustrating number — statistically tantalizing but not sufficient to claim a discovery.
 But mostly Wow, as in: They might have gotten the first glimpse of dark matter — a piece of our universe that until recently we hadn’t even known to look for, because until recently we hadn’t realized that our universe was almost entirely missing.

It wouldn’t be the first time that the vast majority of the universe turned out to be hidden to us. In 1610 Galileo announced to the world that by observing the heavens through a new instrument — what we would call a telescope — he had discovered that the universe consists of more than meets the eye. The five hundred copies of the pamphlet announcing his results sold out immediately; when a package containing a copy arrived in Florence, a crowd quickly gathered around the recipient and demanded to hear every word. For as long as members of our species had been lying on our backs, looking up at the night sky, we had assumed that what we saw was all there was. But then Galileo found mountains on the Moon, satellites of Jupiter, hundreds of stars. Suddenly we had a new universe to explore, one to which astronomers would add, over the next four centuries, new moons around other planets, new planets around our Sun, hundreds of planets around other stars, a hundred billion stars in our galaxy, hundreds of billions of galaxies beyond our own.
 By the first decade of the twenty-first century, however, astronomers had concluded that even this extravagant census of the universe might be as out-of- date as the five-planet cosmos that Galileo inherited from the ancients. The new universe consists of only a minuscule fraction of what we had always assumed it did — the material that makes up you and me and my laptop and all those moons and planets and stars and galaxies. The rest — the overwhelming majority of the universe — is . . . who knows?
 “Dark,” cosmologists call it, in what could go down in history as the ultimate semantic surrender. This is not “dark” as in distant or invisible. This is not “dark” as in black holes or deep space. This is “dark” as in unknown for now, and possibly forever: 23 percent something mysterious that they call dark matter, 73 percent something even more mysterious that they call dark energy. Which leaves only 4 percent the stuff of us. As one theorist likes to say at public lectures, “We’re just a bit of pollution.” Get rid of us and of everything else we’ve ever thought of as the universe, and very little would change. “We’re completely irrelevant,” he adds, cheerfully.
 All well and good. Astronomy is full of homo sapiens–humbling insights. But these lessons in insignificance had always been at least somewhat ameliorated by a deeper understanding of the universe. The more we could observe, the more we would know. But what about the less we could observe? What happens to our understanding of the universe then? What currently unimaginable repercussions would this limitation, and our ability to overcome it or not, have for our laws of physics and our philosophy — our twin frames of reference for our relationship to the universe?
 Astronomers are finding out. The “ultimate Copernican revolution,” as they often call it, is taking place right now. It’s happening in underground mines, where ultrasensitive detectors wait for the ping of a hypothetical particle that might already have arrived or might never come, and it’s happening in ivory towers, where coffee-break conversations conjure multiverses out of espresso steam. It’s happening at the South Pole, where telescopes monitor the relic radiation from the Big Bang; in Stockholm, where Nobelists have already begun to receive recognition for their encounters with the dark side; on the laptops of postdocs around the world, as they observe the real-time self-annihilations of stars, billions of light-years distant, from the comfort of a living room couch. It’s happening in healthy collaborations and, the universe being the intrinsically Darwinian place it is, in career-threatening competitions.
 The astronomers who have found themselves leading this revolution didn’t set out to do so. Like Galileo, they had no reason to expect that they would discover new phenomena. They weren’t looking for dark matter. They weren’t looking for dark energy. And when they found the evidence for dark matter and dark energy, they didn’t believe it. But as more and better evidence accumulated, they and their peers reached a consensus that the universe we thought we knew, for as long as civilization had been looking at the night sky, is only a shadow of what’s out there. That we have been blind to the actual universe because it consists of less than meets the eye. And that that universe is our universe — one we are only beginning to explore.
 It’s 1610 all over again.