Trespassing on Einstein's Lawn by Amanda Gefter

opposite sides of a hot or cold spot could be used to form two equal sides of a long, thin triangle, their length given by the light’s travel time since the photons were freed simultaneously from the plasma. The length of the triangle’s third side was determined by the distance a sound wave can travel in 380,000 years—that is, the stretch of space that the accordion compressed or expanded to form the hot or cold spot in the first place.
    Knowing the lengths of all three sides, physicists used some basic trigonometry to calculate the angles at the triangle’s base: 89.5 degrees apiece, summing to 179. Now they just needed the third angle—the one at the near tip. If the photons had traveled in straight lines to get there, the angle would be 1 degree, bringing the total to a flat 180. If their paths had bent outward as they journeyed through a positively curved universe, the angle would be larger, and if their paths had bent inward due to negative curvature, it would be smaller. According to WMAP, the third angle was precisely 1 degree. Way to call it, Euclid.
    There was just one problem. The universe’s geometry is determined by the amount of mass—or, using E = mc 2 , energy—it contains. As Wheeler would say, mass tells space how to curve. A flat universe requires a critical density of mass to flatten it, the equivalent of an average of six hydrogen atoms per cubic meter. It didn’t sound like much. You’d think there’d be more than enough stuff out there, considering all the galaxies swirling around. But there’s not. Not even close.
    Ordinary matter—particles such as protons, electrons, and quarks—accounts for a pathetic 4 percent of the total you’d need. Our planet, the stars, ourselves, everything we see and know, is virtually negligible in the cosmic scheme of things, the sad, shining tip of a larger, darker iceberg.
    So what else was out there? The physicists had a few ideas.
    For one, they already knew that there’s more to matter than meets the eye, thanks to the simple fact that galaxies aren’t bursting at the seams, sending billions of rogue unshackled stars flying off in all directions. Somehow gravity is holding them together in tight spiral and elliptical formations, despite the fact that the total mass of all the stars in a given galaxy doesn’t provide nearly enough gravity to do the trick. Something else had to be lurking there, hidden in the dark spaces between stars or encircling each galaxy like an invisible fence, preventing stars from wandering off. In order to provide the necessary gravity but also to have remained unseen all this time, it had to be something sturdy and solid, like matter, but indiscernible to electromagnetism. Dark.
    Astronomers calculated how much of this dark matter was skulking out there, but when you add it to the ordinary luminous stuff,you’re still only at 27 percent of the total mass and energy needed to flatten the universe. A disturbing 73 percent is still missing.
    Enter dark energy. In the late 1990s, two teams of astrophysicists—one led by Saul Perlmutter, the other by Brian Schmidt and Adam Riess—went supernova hunting, hoping to measure the expansion rate of the universe. They knew it had begun in a burst of inflation, but figured it had been slowing down ever since, reined in by the grip of gravity.
    Perlmutter, Schmidt, and Riess realized that the expansion history of the universe is encoded in light from exploding stars. Certain kinds of supernovae—so-called standard candles—always burn with the same intrinsic brightness, even though they appear dimmer when they’re farther away. Just how dim a standard candle appears reveals exactly how far away it is. As its light travels through an expanding space, it gets stretched out, its wavelengths shifted toward the red end of the electromagnetic spectrum. This redshift measures how much the universe expanded during the