While scanning the sky for telltale patterns is an ancient endeavour, modern astronomical tools and methods have tremendously sharpened the precision and broadened the scope. Thanks to contemporary instruments such as the Hubble Space Telescope, astronomers have been able to map out the expansive reach of the observable universe, pinning down precise values of locations and speeds of the galaxies, clusters of galaxies, and other celestial objects.
We have long known that the universe is expanding. Because of the growth of space, galaxies (except for nearest neighbors clinging together) are moving away from each other at an ever-increasing pace, like suburban sprawl gone mad. Unlike the spreading out of our own communities, there is apparently no limit to spatial growth -- cosmic real estate seems unlimited.
Much attention has been paid to measuring the exact expansion rate of the universe, gauging how it has changed over time. Such information can be used to help ascertain the origin and fate of the cosmos. That's one reason why pinning down the coordinates of galaxies, such as those of the spiral shown below, is so important (NGC 5584. Credit: NASA, ESA, A. Riess, L. Macri, et al., Hubble Heritage):
As humble dwellers on a middling planet in a peripheral region of the Milky Way galaxy, we have long thought that there is no special part of space. We have assumed that, aside from space's uniform expansion, galactic motions should be as haphazard as fish swimming in a tank--before any food pellets are dropped in, that is. Yet, remarkably, several recent studies have pointed to an improbable flow of clusters of galaxies in a particular direction--like fish swarming toward a certain point after tasty morsels had been dropped into their tank. What is the lure that draws all these clusters? Strangely enough there are no known agents powerful enough to provide such attraction.
Could there be incredibly vast collections of mass just beyond observation (or, alternatively, composed of undetected dark matter) providing the gravitational incentive for such an enormous cosmic flow? Or could it be that we live in a special part of the cosmos that happens by chance to have a greater than usual flow in a certain direction?
The latter possibility would negate the long-cherished Copernican Principle that no part of the universe is central or otherwise different from any other region. We expect Earth to lie in an average location, not an unusual locale. But could we be defying the odds?
As researcher Mike Hudson of the University of Waterloo remarked in a recent talk on the subject at the University of Pennsylvania:
"It is possible that we just happen to live in an unusual place where the velocity is that high. It is possible, but not likely."
Cosmic flow is a truly profound mystery. Its resolution could determine whether or not our part of space is truly special after all, despite the legacy of Copernicus. For the first time in centuries, Earth's normality hangs in the balance!