VOYAGE TO THE GREAT ATTRACTOR: EXPLORING INTERGALACTIC SPACE

By Alan Dressler. Alfred A. Knopf, 1994 ($25).

If the title of this book carries a whiff of science fiction, that is understandable. "Voyage to the Great Attractor" is very much a journey into uncharted realms of outer space. Alan Dressler, a young and respected astronomer at the Carnegie Observatories (better known by its ancestral name, the Mount Wilson Observatory), takes us with him as he becomes one of astronomy's "Seven Samurai," a group of scientists searching for nothing less than the structure of our universe. The trip is all the more remarkable because every detail of it is real.

Dressler's voyage parallels the maturation of cosmology into a quantitative science. An important step in this process took place in the 1920s and 1930s, when Edwin P. Hubble and his associates discovered the redward shift of light from distant galaxies, a finding now understood as a consequence of the expansion of the universe. Astronomers have since learned to work backward, deducing the approximate distance to galaxies by measuring how much their light has been stretched, or redshifted. With that information, we can break out of the two dimensions that we see in the sky and begin to get a full, three-dimensional view of the universe.

Dressler and his colleagues set out to chart our section of the universe by studying in detail the distribution of bright elliptical galaxies. Ever since the time of Hubble, our mind-set has been to think of the universe as expanding uniformly. But as Dressler explains, his team was forced to break with that paradigm when it found groups of galaxies whose redshifts seemed unexpectedly large given their brightness--that is, they seemed to be expanding away too quickly.

This startling finding soon garnered Dressler and his six collaborators (Sandy Faber, Donald Lynden-Bell, Roberto Terlevich, Dave Burstein, Gary Wegner and Roger Davies) the Seven Samurai nickname. Then, much like D'Artagnan joining the Musketeers, Ofer Lahav of the University of Cambridge became the eighth samurai when he provided a map that revealed a vast clumping of galaxies, qualitatively different from any known cluster. The gravity from this cosmic gathering seemed to be accelerating all the surrounding galaxies in its direction, causing the anomalous redshifts. In an offhand comment during a press conference, our author referred to the galactic pileup as the "Great Attractor," a flashy name that captured attention within and well beyond the astronomical community.

Perhaps such a term is necessary these days when people also hear astronomers talk about an immense "slice of the universe" and its "Great Wall," although Dressler reflects on the negative as well as the positive points of having such a simplistic peg on which to hang a mental image. Indeed, we see how Dressler and his co-workers feel left behind by the immediate shift in public interest to the Great Wall of galaxies discovered by a Harvard-Smithsonian team. Time and again "Voyage to the Great Attractor" brings us behind the scenes to reveal the feelings of the scientists as their work progresses, sometimes one step back for every two or three forward.

Dressler introduces us, but not too deeply, to his fellow samurai. We see how they interact, as well as how they clash for personal and scientific reasons. From his position at the Carnegie Observatories, Dressler was able to contribute time on his institution's telescope in Chile, in addition to his own ideas and approach to research. He describes the evolution of the data sets that eventually brought him and his collaborators to the revolutionary idea that our universe's expansion is far less regular than astronomers once thought. Large regions of the cosmos are held back from the overall expansion (known as the Hubble flow) by the gravity of massive collections of objects like the Great Attractor. Such large-scale structures and motions surprised cosmologists, because other evidence indicates that the universe was very smooth soon after the time of the big bang.

In recounting the tortuous path toward the discovery of the Great Attractor, Dressler wends through a wide variety of today's cosmological thought. We face the uncertainties, errors and problems of interpretation that affect real scientific data. Philosophers of science may not be able to define precisely what the scientific method is, but readers of this book will understand better by the end how science is done.

Dressler often explains by analogy. In one passage he recounts spending the night at the observatory with an assistant, clicking off the redshifted spectrum of a new galaxy every five minutes: "I, the telescope, and the computer, were chugging along like some infernal contraption; charts flying, motors whirring, computer beeping, and, at the heart, two humans piloting their way through deep space at a madcap pace. I can think of no better image than Toad in his motorcar in Disney's animated version of Kenneth Grahame's "The Wind in the Willows"--eyes lit up like headlights, bouncing, exulting, merrily on his way to 'nowhere in particular.'"

Dressler paints an honest picture of cosmology, describing its problems as well as its successes. He includes the viewpoint of those who object to the idea of the Great Attractor and evaluates their reservations. He leaves the reader in good shape to understand the methodology and importance of results now flowing in from the Hubble Space Telescope, most notably the much discussed recent measurements of the expansion of the universe that seem to leave our universe younger than some of its parts. Dressler's lucid explanations benefit from the well-chosen graphs and diagrams scattered throughout.

In retrospect, the evidence for the Great Attractor has held up well, although doubters remain. Some observers have reported what might be called an "Even Greater Attractor." Above all, astronomers now widely accept the once radical idea that huge mass concentrations distort the (overall) uniform Hubble flow.

I agree with Dressler that he has earned the right, at the end of the book, to "speculate with something less than the full scientific rigor required for the body of the paper." He expounds on the nature of the scientific enterprise and on the value of science for society. He is an optimist in some general sense, remembering that "millions of people live more comfortable and healthy lives than any Pharaoh ever did."

Dressler's most interesting reflections concern the broader implications of his work. He concludes that "in all likelihood, OUR universe contains all that we will ever know; the UNIVERSE of other realms of existence that lies beyond is inaccessible, probably forever. If this is true, then it is not outrageous to believe that we can know 'all there is to know.'" (Although the book is, on the whole, carefully put together, I would have been happier with fewer italics and with a proper distinction made between "enormity" and "enormousness." The forthcoming paperback edition will incorporate some of these changes.)

After a few hundred pages of scientific and personal details, Dressler returns to earth with his thoughtful conclusion that "it is time to take full stock of the discovery that life is the most complex thing we know of in the universe, and, as such, most worthy of our admiration." We have traveled outward from our galaxy to the largest known structures in the universe. It is refreshing to be reminded that our own brains provide the ship for the voyage.--Review by Jay M. Pasachoff


JAY M. PASACHOFF is Field Memorial Professor of Astronomy at Williams College.


SCIENTIFIC AMERICAN  August 1995  Volume 273  Number 2  Page 106


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