Getting the drift of galaxies.

The Cosmological Principle is taking a beating these days. The principle states that the universe is homogeneous and isotropic, and it once seemed so obvious. Lately, maps of the locations of galaxies have shown that galaxies tend to lie along sheets (or the surfaces of "bubbles" by one description) with voids between, a distribution that violates homogeneity and maybe isotropy (SN: 1/18/8, p. 38). Now there is evidence for large-scale motions of galaxies, galactic drifts one moght say, that reinforce the notion of large-scale inhomogeneities in the arrangement of the universe.

The motion study, performed by seven astronomers, is first an analysis of the motions of 400 elliptical galaxies in a volume surrounding our position in the universe. To this the group added reanalyses of two previous surveys of spiral galaxies. From all of this they derive a general drift of galaxies at something like 600 or 700 kilometers per second toward a point approximately at galactic longitude 290[deg.] and galactic latitude zero.

What is new in this survey, besides the use of elliptical galaxies as a reference group, is a simple transformation of coordinates, but one that nobody had done before, according to Sandra M. Faber of Lick Observatory, headquartered in Santa Cruz, Calif., who spoke to Science News for the group. (The others are David Burstein, Roger L. Davies, Alan Dressler, Donald Lynden-Bell, Roberto Terlevich and Gary Wegner, from institutions ranging from California and Chile to England.) Previous surveys had compared the motions of the local group of galaxies, including the one in which we live, to farily large reference groups of spiral galaxies. The results seemed to show that the local group was moving in radically different directions with respect to different samples of galaxies.

What these seven astronomers did was to refer the motions of the local group and the refeence galaxies to the point of view of the microwave background, the three-degree blackbody radiation that pervades the universe. This is a frame of reference that goes back to the earliest moments of the universe and is rooted in the Big Bang in such a way that cosmologists generally take it as a standard of absolute rest for the universe. Comparing all the motions with absolute rest shows this general drift in all three surveys.

There are discrepancies, up to 40[deg.] in direction, and the astronomers do not wish to gloss them over, but the agreement on the drift is much better with reference to the microwave background than it is with other frames of reference. The remaining discrepancies probably come from the other surveys having less thorough coverage in some directions than in others, a result of those observers not being able to get enough telescope time at a geographically widespread variety of observatories. Another contribution to the discrepancies comes from the tendency of spirals to inhabit different parts of space than do ellipticals.

The results indicate that, over and above the expansion of the universe, there are large-scale motions of galaxies to the order of 2,000 km per second. The most obvious cause of such motions is gravitational attraction. The attractors have to be large concentrations of matter, two or three times as big as the largest that can be accounted for by the simplest theory of the distribution of matter in the universe, the "cold dark matter" theory. So there is work for theorists here.

These conclusions are based on te widespread assumption that the microwave background is the absolute standard of rest for the universe. The notion is one of the bases of modern cosmology; if for some reason it should ever go by the boards, the shole game will need new rules. The conclusions are also based on assumptions about how to estimate the intrinsic brightnesses of galaxies, which cannot be directly measured, from physical characteristics that can be measured, such as the rotation speeds within the galaxy. One needs an estimate of the intrinsic brightness of a galaxy to get an idea of its distance.