Re: What has become of The Great Attractor ?

First, it is indeed still the case that the current evidence points to not only an expanding Universe, but one in which the rate of expansion is accelerating. I won't repeat the evidence here, as it has been covered in previous MadSci articles and in various other astronomy fora.

What is the impact of that on the Great Attractor? To be accurate, the current evidence suggests that there isn't a single "Great Attractor," but a series of "Great Attractors." The original discovery of the Great Attractor, in the mid-80s, came at a time when astronomers were just beginning to survey the local Universe and determine distances to large numbers of galaxies. The expectation had been that the surveys would reveal a rather uniform distribution of galaxies. Instead, astronomers found rapid motions, in the same general direction, of many galaxies in our neighborhood. The implication was that there must be an extremely large concentration of galaxies in that direction, the combined gravitational effect from which could induce the observed motions.

There were two issues or concerns with this conclusion, however. The first was the simple fact that these initial surveys were still relatively limited so there were numerous uncertainties. There were also some theoretical concerns about how such a large concentration of galaxies could come together in the age of the Universe. The second concern was that the apparent direction of the Great Attractor ended up being behind the Milky Way Galaxy. As this image shows, there are vast clouds of dust and gas within the disk of the Milky Way Galaxy. (The dark areas in the linked image are not areas devoid of stars in the Milky Way Galaxy, rather they are regions where the gas and dust density is so high that the light from background stars cannot penetrate. Similarly, galaxies behind the Milky Way Galaxy also cannot be seen. This so-called "Zone of Avoidance" has complicated efforts to study the Great Attractor(s).

Over the past 20 years, however, there has been a concentrated, multi-wavelength effort to find galaxies in this direction. Astronomers have used radio, infrared, optical, and X-ray telescopes to conduct surveys for galaxies. The current evidence is that there is not a single cluster of galaxies in the direction of the Great Attractor, but several. Potential clusters (or superclusters) in this direction include the Norma, Pavo II, CIZA J1324.7−5736, and Abell S0639 clusters. (The Norma and Pavo clusters are named after the constellations in which they are found, the CIZA J1324.7-5736 cluster is named after the coordinates on the sky of a powerful radio and X-ray source, and the Abell S0639 cluster is named after a catalog of clusters compiled by G. Abell.) These combined mass of these clusters alone may be 10,000 times the mass of the Milky Way Galaxy, and there may be more clusters in this direction.

Further, our theoretical understanding has improved. (We think!) Astronomers can now conduct large simulations. The results show that galaxies seem to form a "cosmic web". The linked image shows the resulting distribution of matter in one such simulation. Galaxies and hot gas concentrate in "filaments," with large clusters forming at the intersections ("nodes") of filaments. If one scans this image, the eye can easily pick out long "chains" of clusters. That is likely to be the explanation for the Great Attractor, but we have the misfortune that much of it lies behind the Milky Way, making it difficult to observe. I'll also note that other such long chains or "walls" of galaxies are seen in other galaxy surveys.

So if that is what the Great Attractor is, what does the future hold? Today, it is clear that we are falling toward the Great Attractor. Unfortunately, knowing whether the Milky Way Galaxy will continue to fall toward the Great Attractor depends upon the future of the Universe's expansion, which we don't understand. What's causing the expansion of the Universe to accelerate is something we've named "dark energy," but that doesn't mean that we understand it.

The most simple explanation is that the rate at which the expansion of the Universe is accelerating will remain constant. If so, at some point in the future, the dark energy expansion will overwhelm the gravitational attraction between us and the Great Attractor(s), and we'll start expanding away from it. However, there are also explanations for dark energy in which it becomes less important in the future. Then what happens depends on when dark energy becomes less important and how much weaker it becomes. We could see the Milky Way Galaxy's fall toward the Great Attractor slow down for a while before speeding back up or we could see that the Milky Way will just continue to fall into the Great Attractor because dark energy weakens before it can break the gravitational attraction between us and the Great Attractor(s).

I suspect that that is less than a fulfilling answer, but that's also why astronomy is interesting. There are still questions!

(For a more technical, if somewhat aging, summary of the Great Attractor, see Kraan-Korteweg & Lahav, "The Universe behind the Milky Way" (Astronomy and Astrophysics Review, Volume 10, Issue 3, pp. 211-261, 2000).