A Fleeting Attraction?

Does the Great Attractor exist?

By David Lindley|Tuesday, September 01, 1992
RELATED TAGS: COSMOLOGY
Our galaxy and its neighbors, most astronomers think, are hurtling through space at more than a million miles per hour. What’s more, our local group is not alone. In the 1980s astronomers discovered that hundreds of galaxies, over a region of space spanning tens of millions of light-years, are streaming in the same direction as the local group and at roughly the same speed. Where to and why the rush? The answer, it seemed, was that the galaxies were being drawn by gravity toward the center of some enormous concentration of mass--a Great Attractor--somewhere in the direction of the constellations Hydra and Centaurus.

Three years ago Alan Dressler of the Carnegie Institution and Sandra Faber of the University of California at Santa Cruz reported that they had located the center of the attractor. By surveying galaxies even farther away in the Hydra-Centaurus direction, they had found the point at which the streaming motion stopped, and beyond which galaxies began to stream the other way, back toward us and back toward the center of the attractor. With the discovery of back-side infall, the case for believing in the attractor seemed open and shut.

Three astronomers at the Mount Stromlo Observatory in Australia want to open it again--and along with it, perhaps, the whole issue of whether the Milky Way itself is rocketing through space. Donald Mathewson, Vince Ford, and Markus Buchhorn have surveyed the same part of the universe Dressler and Faber did but have looked at five times as many galaxies. They have come to the opposite conclusion: There is no back-side infall into the Great Attractor, they wrote in a recent paper, which suggests that the Great Attractor does not exist.

How can the two studies yield such contradictory results? We have much more data than they do, Mathewson says simply. Having a lot of data is important when you’re trying to decide whether a galaxy is streaming toward an attractor, because the measurement process is a minefield of potential errors. The only thing that is measured directly is the galaxy’s redshift--the degree to which its light is stretched toward longer wavelengths by its motion away from us. Most of that redshift is caused by the expansion of the universe, which is constantly creating new space between us and the distant galaxy and which has nothing to do with galactic streaming. To factor out cosmic expansion, you have to know the galaxy’s distance (the expansion velocity is proportional to distance), and there’s no foolproof way of measuring that. Having a lot of data helps compensate for inevitable measurement errors.

Even after you’ve factored out cosmic expansion, you still have to take the Milky Way’s own streaming motion into account. The evidence for that motion comes from the cosmic microwave background: the faint glow, left over from the Big Bang, that rains down on us from all directions from the edge of the visible universe.

Fifteen years ago measurements of this radiation revealed a striking pattern. The wavelength of the radiation was slightly shorter in the direction of Centaurus and slightly longer in the exact opposite direction in the sky. That’s precisely the pattern the Doppler effect would produce if the background radiation were motionless--if it defined an absolute reference frame for the universe--and if the Milky Way were moving through space toward Centaurus. Since this presumed motion affects the redshift of a distant galaxy, astronomers routinely subtract it out to determine what the galaxy’s own motion is.

Mathewson and his colleagues went through all these steps and calculated the streaming velocities of galaxies well beyond where Dressler and Faber had put the center of the attractor, which was around 150 million light-years away. Unlike Dressler and Faber, they never found a point where the streaming stopped or reversed direction. As far as 200 million light- years or so, the galaxies still seemed to be streaming in the same direction as the Milky Way and at about the same speed. Furthermore, Mathewson points out, a separate survey of galaxies on the opposite side of the sky from Centaurus has found that they too are headed in the same direction, again at roughly the same speed.

If one takes all these measurements at face value, then, there is not a stream but a river of moving galaxies, stretching hundreds of millions of light-years from one side of the sky through our own galaxy to the other side, and disappearing over the horizon toward a Great Attractor that, if Mathewson is right, still remains to be discovered. Only now, to keep all these extra galaxies in motion, it would have to be even more massive than the 10,000-trillion-sun figure Dressler and Faber had suggested.

Mathewson finds that scenario hard to swallow. It is possible, he admits, that a Really Huge Attractor lurks beyond the measurement horizon. It is also possible that he has misinterpreted his own complex data and that the original Great Attractor lives; that is what Faber believes. But Mathewson suggests a heretical alternative. If these galaxies are moving in the same direction and with the same velocity, he argues, maybe it would be easier just to say that none of them, including our own Milky Way, are moving at all. Maybe the dipole pattern in the background radiation, which has been taken as direct evidence for our galaxy’s motion, is really a huge pattern imprinted on the universe itself.

Such an explanation, however, might be difficult to square with the Big Bang theory, which assumes that the universe looks about the same in every direction--and which has just been vindicated yet again. Satellite measurements of the background radiation have revealed small fluctuations (much smaller than the dipole), which suggests that the Big Bang, while creating an essentially uniform universe, left ripples of mass that could grow into galaxies. (How galaxies formed has been even harder to explain than why they might move.) Fresh from a reassuring success, most cosmologists would rather believe in a Great Attractor and a moving Milky Way than follow Mathewson (Eppur non si muove!) into cosmos-shaking heresy.
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