48 SCIENTIFIC AMERICAN April 2009 DON DIXON I n science, the grandest revolutions are often triggered by the smallest discrepancies. In the 16th century, based on what struck many of his contemporaries as the esoteric minutiae of ce- lestial motions, Copernicus suggested that Earth was not, in fact, at the center of the universe. In our own era, another revolution began to unfold 11 years ago with the discovery of the accelerat- ing universe. A tiny deviation in the brightness of exploding stars led astronomers to conclude that they had no idea what 70 percent of the cosmos consists of. All they could tell was that space is illed with a substance unlike any other—one that pushes along the expansion of the universe rather than holding it back. This substance be- came known as dark energy. It is now over a decade later, and the existence of dark energy is still so puzzling that some cos- mologists are revisiting the fundamental postu- lates that led them to deduce its existence in the irst place. One of these is the product of that earlier revolution: the Copernican principle, that Earth is not in a central or otherwise special po- sition in the universe. If we discard this basic principle, a surprisingly different picture of what could account for the observations emerges. Most of us are very familiar with the idea that our planet is nothing more than a tiny speck or- biting a typical star, somewhere near the edge of an otherwise unnoteworthy galaxy. In the midst of a universe populated by billions of galaxies that stretch out to our cosmic horizon, we are led to believe that there is nothing special or unique about our location. But what is the evidence for this cosmic humility? And how would we be able to tell if we were in a special place? Astronomers typically gloss over these questions, assuming our own typicality suficiently obvious to war- rant no further discussion. To entertain the no- tion that we may, in fact, have a special location in the universe is, for many, unthinkable. Never- theless, that is exactly what some small groups of physicists around the world have recently been considering. Ironically, assuming ourselves to be insignii- cant has granted cosmologists great explanatory power. It has allowed us to extrapolate from what we see in our own cosmic neighborhood to the universe at large. Huge efforts have been made in constructing state-of-the-art models of the universe based on the cosmological princi- ple—a generalization of the Copernican principle that states that at any moment in time all points and directions in space look the same. Combined with our modern understanding of space, time and matter, the cosmological principle implies that space is expanding, that the universe is get- ting cooler and that it is populated by relics from its hot beginning—predictions that are all borne out by observations. KEY CONCEPTS The universe appears to be ■ expanding at an accelerat- ing rate, implying the exis- tence of a strange new form of energy—dark ener- gy. The problem: no one is sure what dark energy is. Cosmologists may not ac- ■ tually need to invoke exotic forms of energy. If we live in an emptier-than-average region of space, then the cosmic expansion rate varies with position, which could be mistaken for a variation in time, or acceleration. A giant void strikes most ■ cosmologists as highly un- likely but so for that matter does dark energy. Observa- tions over the coming years will differentiate between the two possibilities. —The Editors COSMOLOGY Does Really Exist? Maybe not. The observations that led astronomers to deduce its existence could have another explanation: that our galaxy lies at the center of a giant cosmic void By Timothy Clifton and Pedro G. Ferreira © 2009 SCIENTIFIC AMERICAN, INC.