Two astronomers have discovered that our own Milky Way galaxy and most of its neighboring galaxies, contained within a huge volume of the universe, one billion light-years in diameter, are drifting with respect to the more distant universe. This startling result may imply that the universe is "lumpier" on a much larger scale than can be readily explained by any current theory. "The new observations thus strongly challenge our understanding of how the universe evolved," says Dr. Tod Lauer of the National Optical Astronomy Observatories (NOAO).
This surprising conclusion comes from the deepest survey of galaxy distances to date, conducted by Dr. Tod R. Lauer in Tucson, Arizona, and Dr. Marc Postman of the Space Telescope Science Institute (STScI) in Baltimore, Maryland. The two astronomers used NOAO telescopes at Kitt Peak National Observatory, near Tucson, Arizona, and at Cerro Tololo Inter-American Observatory, near La Serena, Chile to study galaxy motions over the entire sky out to distances of over 500 million light years. They explored a volume of space about thirty times larger than had been surveyed previously. The results of this survey will be published in the April 20 issue of The Astrophysicd Journal. The expansion of the universe causes all the galaxies in the volume surveyed to be moving away from us. Galaxies at the edge of the volume are receding from us at 5% of the speed of light. The large flow that Postman and Lauer discovered comes from looking at the galaxy motions "left over" once the expansion of the universe had been accounted for. The flow means that the nearby universe, as well as expanding, appears to be drifting with respect to the more distant universe.
Astronomers generally assume that the diffuse glow of microwave radiation left over from the Big Bang provides the backdrop or rest frame of the universe. In the mid 70's astronomers found that temperature of this radiation is slightly hotter towards the direction of the constellation of Leo.
This effect has been interpreted to mean that the Milky Way is drifting with respect to the rest of the universe at about 380 miles per second in this direction. It has also been assumed that most of this motion is due to the gravitational attraction of more distant galaxies; however, these galaxies have never been positively identified.
In the mid-80's a group of seven astronomers surveyed the motions of galaxies out to about one-third of the distance studied by Lauer and Postman, finding the galaxies to be flowing as a group with respect to the more distant universe. This team postulated that this flow was due to the gravitational pull of a large concentration of galaxies dubbed "The Great Attractor." The Great Attractor is located deep inside the volume surveyed by Postman and Lauer, however, and would not be massive enough to cause their much larger sample of galaxies to drift.
In fact, the new result implies that the Milky Way and its neighbors are affected by much larger concentrations of galaxies at much larger distances than can be easily explained by popular theories of how the universe is organized.
Lauer and Postman started their project in 1989 to measure the drift of the Milky Way with respect to 119 clusters of galaxies located all over the sky at distances as far as 500 million light-years. If the motion of the Milky Way was caused by galaxies closer to us than the distant clusters, as was then presumed to be the case, then its motion with respect to the clusters should have been essentially identical to that with respect to the microwave background radiation.
Because the galaxy clusters are at a variety of distances from us, galaxies in the more distant clusters appear dimmer than the ones more nearby. However, once the various distances are accounted for, the brightest galaxy in each cluster is always found to give off roughly the same amount of light. Astronomers refer to such objects as "standard candles." The distances to the clusters are estimated from how fast they are moving away from us as the universe expands.
If the Milky Way Galaxy is drifting, however, its motion makes measurement of the expansion speeds dependent on which direction we are looking. If the drift is not corrected for, then the cluster galaxies will appear to vary slightly in brightness in a smooth pattern across the sky. Postman and Lauer used images of the cluster galaxies to detect this pattern and determine the motion of our own galaxy.
The motion of the Milky Way that Postman and Lauer measured from the distant clusters is in a completely different direction from that inferred from the microwave background. The most likely solution to this dilemma is that the clusters themselves are moving with an average velocity of 425 miles per second towards the constellation of Virgo. Because of the enormous size of the volume containing the clusters, however, this implies the existence of even more distant and massive concentrations of matter.
Most theories explaining the structure of the universe predict that the universe should be nearly uniform on the scale of the Lauer and Postman cluster sample. The motion of the Milky Way and its neighbors would then be due t o concentrations of mass relatively close by. If instead, the portions of the universe as big as a billion light-years in diameter are still drifting with respect to the larger universe, then the universe has structure or "lumps" of matter on much larger scales than predicted by most theories. The detection of galaxy flows across large volumes of space should improve our understanding of how the universe came to be organized the way we see it today.
A more provocative but probably less likely interpretation of the Postman and Lauer result is that the large volume of clusters really is at rest, with the temperature variation of the imicrowave background around the sky being a relic of the conditions of the Big Bang, rather then being caused by the motion of our galaxy. In this case, the microwave temperature variation would tell about the properties of the very early universe rather than about large scale motions of galaxies.
Ray Villard, STScI
Karie Meyers, NOAO
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Dr. Marc Postman
Dr. Tod R. Lauer