The Dark Matter Problem: A Historical Perspective

The Dark Matter Problem: A Historical Perspective, Robert H. Sanders Cambridge U. Press, New York, 2010. $60.00 (205 pp.). ISBN 978-0-521-11301-4

“The prevailing view of the Universe now is radically different than it was 40 years ago when I began my career as a professional astronomer,” says Robert Sanders in The Dark Matter Problem: A Historical Perspective. In this readable and enjoyable book, Sanders takes us through the historical development of the theory of dark matter, including attempts at its direct detection, and up to the present state of affairs in astronomy, cosmology, and particle physics. And he does so with a sympathetic nod toward an unpopular competing theory.

I found the first half historically informative. It was fascinating to learn that the instability of rotationally supported disks of spiral galaxies was one of the earliest pieces of evidence for dark matter. The author deftly discusses the post–World War II emergence of radio astronomy and weaves that story into the history of spiral galaxy rotation-curve observations. He also carefully presents the roles played by those involved in the discovery process. For example, the contributions of radio astronomers Morton Roberts, Albert Bosma, and others appear alongside those of Vera Rubin, whose name is most strongly associated with the optical observations of rotation curves in the 1970s.

Several chapters in the second half of the book contain an overview of early-universe physics, cosmology, and particle physics. Those chapters are terse and technical, providing neither explanations nor equations. They belong to a category of writing that won’t satisfy any audience. A reader already familiar with the topics they discuss will not learn much, and I doubt a nonspecialist will either. Some topics, it seems, are resistant to intermediate-level expositions. Sanders also includes a discussion of dark energy, which is generally viewed as a separate problem. I found it unnecessary and a little distracting.

In an idiosyncratic twist, the book offers a detailed treatment of dark matter versus modified Newtonian dynamics, or MOND, an alternative hypothesis to explain galaxy rotation curves. A chapter is devoted to MOND, and references to MOND are ubiquitous throughout the book. The author presents many arguments for and against both dark matter and MOND, with varying degrees of emphasis. In his thorough coverage of galaxy rotation curves, Sanders emphasizes that dark matter has yet to explain their small-scale features, the “conspiracy” of maximum disks, or the slope of the Tully–Fisher relation, while MOND, apparently, fares well in addressing those observations.

But MOND’s weak spots—explaining the mass content of clusters and the observations of the colliding clusters of the Bullet and other systems where mass and light are separated on the sky—are presented as problematic, although potentially explainable. Sanders also notes that various, sometimes unrelated observations can be explained by dark matter. To most cosmologists, that is a powerful and persuasive argument for the yet-to-be-detected particles. To the author, it is a convenient compilation of problems forced together under the leaky umbrella of dark matter.

Given that the book is sympathetic toward modifications of Newtonian gravity, I would have liked to see more details about MOND. What is the form of the function that interpolates between Newtonian and MOND regimes in the force law? Can MOND account for the observed temperature fluctuations in the cosmic microwave background? How does the cosmic network of galaxies and clusters develop in MOND? What about the extra fields introduced in the tensor-vector-scalar theory, a relativistic version of MOND?

On the whole, The Dark Matter Problem will benefit advanced undergraduates, graduate students, and some researchers. I can imagine two different ways in which the author’s nonstandard view of dark matter will play out with the reader. Nonspecialists might buy the narrative wholesale and believe that MOND is a competitive alternative—a conclusion at odds with the consensus of researchers. (Although the fraction of MOND papers has tripled since tensor-vector-scalar theory was introduced in 2004, they still make up only 3% of the papers that deal with the same “problems” addressed by dark matter.) Students and professionals working in the field may receive this book as a training tool that highlights the current deficiencies of the dark-matter paradigm. Understanding those deficiencies is an important part of the learning process.