The Dark Side of the Universe

by Jan Smit and Renske Smit

To put us, as book reviewers, in context, like many scientists, we study our surroundings in an effort to understand where we came from. Jan, the historical geologist, likes to begin with the birth of our solar system 4.56 billion years ago. Before that, in his mind, there was nothing. Renske, the astrophysicist, works on much larger time scales, beginning with the birth of the earliest galaxies 13.2 billion years ago. For her, our solar system is just a little speck in recent history. It is therefore rare to come across a publication that piques the interest of both of us. Such was the case with particle physicist Lisa Randall’s new book,/Dark Matter and the Dinosaurs./

The first thought that crossed both our minds when reading the title of this book was: “Oh, no, not again, another outlandish proposal for the extinction of dinosaurs….” However, we were relieved to find that, right from the start, Randall dismisses almost all connections between dark matter and the mass-extinction event that wiped out the dinosaurs. Instead, the book takes the reader on a journey through the cosmos, describing what we know about dark matter and what more we are poised to learn as new and better equipment becomes available.

Randall starts with an outline of “The Big Questions”: Why is there something rather than nothing? What happened during the Big Bang? What came before the Big Bang? She explains that there has to be dark matter because the behavior of merged galaxy clusters like the Bullet Cluster cannot be explained otherwise. She skims through the Big Bang, cosmic inflation, and how the galaxies formed when normal matter hitchhiked along with dark matter to form the seeds of the first stars. Without dark matter, we learn, our Milky Way and Earth as we know it would not exist.

Having covered the creation of stars, the book turns to our solar system. Here, Randall vividly describes the comets and asteroids that have hit or will hit Earth. She recounts how meteorites may have brought essential amino acids and perhaps some water to Earth and describes the Chicxulub asteroid/comet and its role in the mass extinction at the end of the Cretaceous. The story of the Chicxulub impact and its role in the extinction of the dinosaurs is highly entertaining and largely correct.

In the last chapters of the book, Randall outlines an important new way of thinking that applies to the search for viable dark matter models. The only way to find out whether something is allowed or even preferred is to evaluate the consequences of new assumptions and determine their experimental implications. Although observations indicate that dark matter consists of a mostly noninteracting substance, no experiment can currently rule out that dark matter may have a weak interaction with its own particles or, alternatively, that a small fraction of particles of dark matter have a moderately large interaction with one another. If true, these new models would be an extension of current theories, some of which can be tested in new experiments over the next few years.

One model explored in this book shows how a narrow disk of dark matter in the galactic plane could potentially explain the periodicity in the crater records on Earth and could have contributed to the impact 66 million years ago that generated the mass extinction of the dinosaurs. Randall is quick to admit that the data that favor such a model are still tenuous and that the theory requires further testing. So does that mean that dark matter and dinosaurs are connected? Likely not, although there is a chance that they could be. In the end, drawing a definitive connection between the two is not really the aim of the book.

Randall has a manner of writing that is pleasant and compelling: Cliff-hangers at the end of each chapter reel you into the next. Her method of using everyday situations as metaphors for explaining complicated concepts in physics is also very effective. For example, she describes how scientists know that dark matter is present in much the same way that you may be able to infer that George Clooney is in busy downtown New York: You may not see, smell, or hear him, but you can observe that all faces on the street are directed toward him.

Despite the provocative title, the scientific reasoning set out in this book is sound and interesting. Randall succeeds in guiding the reader through the history of the cosmos and the Earth from the Big Bang to the emergence of life as we know it in a fun and captivating way. The rich metaphors and personal anecdotes peppered throughout the book make this a very enjoyable read for both lay readers and scientists of various backgrounds.

URL: http://www.sciencemag.org/search?author1=Jan+Smit&sortspec=date&submit=Submit

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