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> This article appeared in Scientific American, May 2005 > > Too Cold for Comfort > DARK ENERGY CHILLS OUR GALACTIC NEIGHBORHOOD BY GEORGE MUSSER > > 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 > dont need to go so far to find dark energy, > says Andrea Macci 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 lightyears > 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. > 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 > Maccis group indicate that dark energy > works after all. If and only if you include > dark energy, there is a very good agreement, > Macci 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 Maccis 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. > ------------------------ Yahoo! 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