Tuesday, January 15, 2013

Up Against The Wall

Reading a truly excellent science popularization, Stephen S. Hall’s Mapping the Next Millennium, made me aware of developments in astronomy of which the first, the discovery of The Great Wall, had first alerted me in the early 1990s that major changes were afoot.

Back in the 1950s, theories about the universe as a whole had a pleasingly simple duality. The universe had but one ultimate fate: it would end in total heat death after continuous expansion until all energy had been exhausted—or the universe would begin contracting at some point, the galaxies converging again, until it all ended in the Big Crunch. In this view what mattered was the velocity of the expansion. If the velocity of the expanding universe was less than “escape velocity”—meaning that gravity would prevail over the outward impulse—expansion would halt and then reverse. Observations then (of the universe’s mass, of the velocity) were not precise enough to determine which was more likely—but the ratios of mass to velocity were close enough so that it could go either way. The out-in, out-in sequence appealed to me then. You know: Vishnu breathing.

Both models, to be sure, crucially depended on observations, credited to Edwin Hubble, that the universe, now, was definitely expanding—and had done so ever since the Bang. Indeed the Big Bang theory was the consequence of Hubble’s observations. If the universe is expanding now, that expansion had to have a start, and reading the observations backward—after all galaxies were moving away from every other galaxy in a uniform pattern—then in the beginning there must have been a great explosion from a mere point.

News of the first problem with that theory were published in 1989 by astronomers Margaret Geller and John Huchra, she a theorist, he an observer. A survey (or map) of the Nordic sky produced the first image of The Great Wall, usually and more humbly called Cf2A. That stands for the (Harvard-Smithsonian) Center for Astrophysics; the 2 stands for “second survey.” The great wall is a very massive, thick clustering of galaxies, thus a great structure. It challenges the theory of a uniform distribution of matter in the universe. The formation of such a wall also takes a huge amount of time—far more time than would seem to have elapsed since the Big Bang, thus approximately 14 billion years ago. Then, in succession others, in essence replicating the work of Geller and Huchra, discovered a number of other walls in turn, the largest of all being the Sloan Great Wall, named after the Alfred P. Sloan Foundation. Half a dozen such walls have now been mapped.

At around that same time (1986), but based on work conducted in the 1970s, seven astronomers, known as the Seven Samurai†, discovered that the Milky Way itself, along with all 39 galaxies of our Local Cluster (prominent in that list ourselves, Andromeda, and Triangulum), were themselves in uniform motion toward a distant spot. Other surveys later followed showing other galactic clusters also heading towards an enormously dense region (difficult to see because it is shadowed by our own galaxy’s clouds). That region is itself part of one of these walls. It was named The Great Attractor by one of the Samurai, Alan Dressler. After that arose the prediction that if one attractor has been found, others may exist as well. And the work goes on.

What are we to conclude? One conclusion may be that galactic expansion has already stopped—and what we see out there is in a much more distant past. The cosmos may already be in process of gathering its errant sheep—and that that gathering is very, very ancient. Those wall are very old, and yet still in formation. Pondering such discoveries not only enlarges my understanding of the cosmos but also of the nature of science. Young tendrils of it are exposing new knowledge—while the orthodoxy grimly clings to exciting news a hundred years old.
†David Burstein, Roger Davies, Alan Dressler, Sandra Faber, Donald Lynden-Bell, Roberto J. Terlevich, and Gary Wegner.

The first image, from Wikipedia (link), shows some of the walls, including the largest, the Sloan Great Wall. In astronomical terminology, these are called filaments. And it turns out that the universe is quite thick with them—as shown in the second image (link), taken from a YouTube film produced by the Sloan Digital Sky Survey in New Mexico.

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