When you first meet dark energy, it seems so charming. An alluring stranger, outsider to the Standard Model of particle physics, it entered astronomers' lives a decade ago and won their hearts by fixing all kinds of problems, such as discrepancies in the age of the universe and the cosmic census of matter. Cosmic expansion has got its groove back: once thought to be winding down, it is actually speeding up. But astronomers have come to realize that dark energy has a dark side. The cold grip of its repulsive gravity is strangling the formation of large cosmic structures. And now observers see it prowling the neighborhood of our own Milky Way. "You don't need to go so far to find dark energy," says Andrea Macciò of the University of Zurich. "Dark energy is also around us."
Up until recently, those seeking the exotica of the universe--dark matter as well as dark energy--focused on the very largest scales (galaxy clusters and up) and on comparatively small ones (a single galaxy). But in between is a poorly studied cosmic mesoscale. The Milky Way is part of the Local Group of galaxies, which in turn is part of the Local Volume, about 30 million light-years in radius. We and the rest of our gaggle are flocking en masse at 600 kilometers per second, lured by the Virgo Cluster of galaxies and other outside masses. Tracking relative motions within the volume, though, is tough; it requires distance and velocity measurements of high precision. Early efforts by Allan R. Sandage of the Carnegie Observatories in Pasadena, Calif., and others in the 1970s, confirmed in recent years, hinted that stuff is moving abnormally slowly--on average, somewhere around 75 kilometers per second. Simulations predict that galaxies, pulled together by gravity, should buzz around at closer to 500 kilometers per second. By analogy with a gas of slow-moving molecules, the Local Volume is "cold."
Another way to think of the problem is in terms of cosmic expansion. Theory predicts that you'd have to go out hundreds of millions of light-years, where matter is spread randomly rather than finely structured, before the overall expansion should outgun localized motions. Yet in the Local Volume, you have to go out only about five million light-years.
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One explanation, championed by Igor Karachentsev of the Russian Academy of Sciences, is that galaxies and their individual cocoons of dark matter swim in a sea of dark matter. The sea would mute the density contrasts and hence the gravitational forces that drive galactic motions. The only trouble is that matter, whether dark or visible, should not spread out into a sea. It should clod. So others have looked to dark energy. Its gravitational repulsion would offset galaxies' gravitational attraction, thereby deadening their motion. In and near the Milky Way, attraction wins, but beyond a certain distance, repulsion does. As Arthur Chernin of Moscow University and his colleagues calculated in 2000, this distance is five million light-years--exactly where galactic motions deviate from standard predictions.
The initial calculations actually only halved the galactic velocities, which is not enough. But the new full-up simulations by Macciò's group indicate that dark energy works after all. "If and only if you include dark energy, there is a very good agreement," Macciò says. "This is why we state that we have found the signature of dark energy." Not everyone agrees. In 1999 Rien van de Weygaert of the University of Groningen in the Netherlands and Yehuda Hoffman of Hebrew University in Jerusalem argued that the Local Volume is caught in a cosmic tug-of-war between surrounding galaxy clusters. This, too, would pull galaxies apart, offsetting their own gravity.
To decide whether this mechanism or dark energy is more important, astronomers have to compare the Local Volume with similar regions. If those not caught in a tug-of-war behave similarly, the dark energy must be to blame. Unfortunately, the teams disagree on what "similar" means, so the debate goes on. If Macciò's model proves to be right, then dark energy, once considered the most "out there" idea in science, an ethereal abstraction of little relevance, will bump a little closer down to earth.
