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All this spurs the question of what exactly distant galaxy clusters can tell us about the origins of the universe. What makes such painstaking work worthwhile? Sometimes it can reveal things totally unexpected and of potentially profound importance. Greg Fish, a blogger for Discovery.com and BusinessWeek, tells of a strange new finding called "dark flow." In 2008, a team led by Alexander Kashlinsky of NASA's Goddard Space Flight Center found evidence that many galaxy clusters were moving toward a single small section of the vast horizon of our visible universe. Last month, new observations by the team confirmed these unexpected findings and extended them to 1,400 galaxy clusters as far as 3 billion light-years from Earth, again using the SZ effect to help identify and measure the clusters. The velocity of the clusters is as high as a thousand kilometers per second, suggesting that some immense, unknown force is acting upon the galaxies like flotsam caught in an invisible current. This is the dark flow.
Kashlinsky's team doesn't speculate on what might be causing the dark flow, but others have weighed in with their own ideas. The simplest explanation, Fish says, is some massive object or body outside of our visible universe, gravitationally tugging the galaxies toward it. But such a huge discrepancy in what, on the largest scales, otherwise appears to be a very spatially homogenous universe would require a rethinking of some of the fundamental principles of cosmology. Notably, it would require a reconsideration of the prevailing notion that our surroundings, on cosmic scales, are strictly average. If we occupy a cosmological position that is somehow "special," this uniqueness may taint our extrapolations about the conditions in other distant regions and eras of the cosmos. In other words, it would become more difficult to pin down the correct large-scale structure and behavior of the universe.
Here's the link to the whole article.