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The Search for the Age of the Universe

Helge Kragh

HOW OLD IS THE UNIVERSE? David A. Weintraub. x + 370 pp. Princeton University Press, 2011. $29.95.

Astronomers and physicists are today convinced that the universe is 13.7 billion years old. How have they arrived at this conclusion? How is it possible to determine the age of something as strange and all-encompassing as the universe? This is what astronomer David Weintraub explains in his new book, How Old Is the Universe?, and he does it systematically and in considerable detail. At the same time he gives a popular introduction to fundamental concepts and techniques in astronomy and astrophysics. In fact, the book does not focus narrowly on the problem of the age of the universe; rather, it explores the subject by discussing a large number of age determinations of celestial objects.

The general structure of the book is based on the undeniable truth that the universe must be at least as old as the oldest objects in it, which follows from the very meaning of the term universe. Adopting a semihistorical framework, Weintraub starts with objects found in the solar system, such as meteorites and the Earth, and proceeds to larger and more distant objects, including stars, globular clusters and galaxies. Only after having explained how the ages of these objects have been determined does he turn to cosmology proper, meaning the class of big-bang models based on Einstein’s cosmological field equations. In this way readers are introduced to classical problems such as the stellar parallax method and the generation of solar energy, as well as to ideas at the cutting edge of modern astrophysics. They learn about supernovae, the cosmic microwave background, dark energy, gravitational lensing and much more. Many of the issues dealt with are only indirectly related to the age of the universe, but they are nonetheless relevant and are described carefully and pedagogically.

Although this is a popular book, it is quite ambitious and in places demands the full attention of the reader. I found Weintraub’s explications of the Hertzsprung-Russell diagram, the distance method based on Cepheid variable stars, the accelerating universe, and the dark matter problem to be particularly enlightening. The same is true of the final chapter, a careful and innovative account of how the analysis of patterns in the cosmic microwave background obtained by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite has led to the conclusion that the age of the cosmos is close to 13.7 billion years.

Weintraub has a gift for presenting complicated matters in a lucid and understandable way by employing clever analogies. For example, he invites the reader to “think of the universe as a flute” in order to understand the role of sound waves in the early universe.

For relativistic big bang models, the age of the universe is closely related to the Hubble constant, a measure of the rate of expansion of the universe. As Weintraub correctly points out, although Edwin Hubble was indeed the father of the famous constant, he held back from interpreting it in terms of an expanding universe. This was first done by the Belgian pioneer cosmologist Georges Lemaître, who was also the first to introduce the notion of a finite-age universe of the big-bang type. Unfortunately, Weintraub’s account of this crucial episode in the history of cosmology is not as clear as it might have been, for he appears to conflate Lemaître’s 1927 paper presenting the idea that the universe is expanding and his 1931 paper proposing that the universe expanded from an initial point—the “Primeval Atom.”

Given the book’s topic and its length (there are 363 pages of text), I had hoped to find in it some discussion of the concept of the universe and the meaning of its age. But Weintraub ignores philosophical questions, focusing solely on the science. He seems to take it for granted that astronomers know what the universe is, and for him, its age is no more problematic than, say, the age of a person or a used car. Astronomers know, he says, that the universe has a finite age—that it is not eternal. However, being able to say with confidence how long ago the big bang occurred does not mean that we know how much time has passed since the universe began. Medieval philosophers suggested that the universe might well be eternal in the past, and this idea has been taken up again by some modern advocates of alternative cosmological models that have been inspired by string theory.

Although the number of years that have passed since the big bang is finite, we do not know for certain whether the universe is finite or infinite in its spatial dimension. Yet Weintraub claims confidently that the universe cannot be infinite. One of the reasons he gives is that an infinite universe would violate Einstein’s theory of relativity. His arguments for a finite universe are puzzling, and particularly so because he acknowledges that we live in a flat, or Euclidean, universe. According to many cosmologists this implies, or at least strongly suggests, that we live in an infinite universe. Strangely, if I understand him correctly, Weintraub argues that in an infinitely large universe the net gravitational force on every galaxy would be zero. This is precisely what Richard Bentley, referring to stars instead of galaxies, suggested in a famous correspondence with Isaac Newton in 1692, but the argument is incorrect. As Newton realized, and as later astronomers confirmed, the so-called gravitational paradox appears even in an infinite universe uniformly populated with stars or galaxies.

How Old Is the Universe? is a welcome contribution to the popular literature on astronomy and cosmology. Unlike many other books in this very extensive genre, it offers a scientifically competent and impressively informative account of how astronomers and physicists have unveiled the secrets of the universe, in this case leading to the astounding insight that our universe can be traced back in time to an explosive event that happened 13.7 billion years ago. Although the book has much to recommend it, I find it somewhat disappointing when it comes to those conceptual and philosophical issues that are an integral part of cosmology in the wider sense. The concept of the age of the universe is not a purely scientific one, for it necessarily involves considerations of a philosophical nature.

Helge Kragh is professor of the history of science, Aarhus University, Denmark, where he does research in the history of the physical sciences. Among his publications are Cosmology and Controversy (Princeton University Press, 1996), Matter and Spirit in the Universe (Imperial College Press, 2004) and Conceptions of Cosmos (Oxford University Press, 2007).

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