The Extravagant Universe: Exploding Stars, Dark Energy and the Accelerating Cosmos

  • Robert P. Kirshner
Princeton University Press: 2002. 320 pp. $29.95, £19.95
Credit: DAVID NEWTON

The 1990s will go down in history as the decade in which we successfully inventoried the constituents of the Universe. A series of ground-breaking observations have given us a picture in which ordinary matter — atoms, stars, gas and dust — accounts for only about 5% of the total energy of the cosmos, with 25% consisting of 'dark matter', which is different from any of the known elementary particles. Most surprising of all, fully 70% of the Universe is the utterly mysterious 'dark energy', characterized by the fact that it is distributed nearly uniformly through space and evolves very slowly (if at all) with time.

The leading candidate for dark energy is Einstein's cosmological constant, which is equivalent to a non-zero minimum amount of energy density at every point in space. Even though we have measured the amounts of dark matter and dark energy, and can describe some of their simple properties, we know next to nothing about their origin and nature, and are worse than clueless about why they are as abundant as they are. So there is still important work to be done.

Robert Kirshner played a major role in the discovery that the Universe is accelerating, thereby providing the first direct evidence for the existence of dark energy. In a Universe dominated by ordinary matter, the expansion rate would slow down with time; in contrast, dark energy has a negative pressure, the gravitational effect of which is to make the recession velocity of distant galaxies appear to speed up.

In The Extravagant Universe, Kirshner tells the story of how astronomers figured out how to use type Ia supernovae as “standardizable candles” to accurately measure huge distances, and then performed large-scale searches for such objects to determine the behaviour of the Universe's expansion as a function of time. The Extravagant Universe is a personal book, rather than an objective account, intermingling the history of cosmology with an explanation of supernovae through the story of Kirshner's own research on these unique celestial events. For the general reader interested in the excitement of how science is done, this strategy makes for a fascinating account. Although several books by leading physicists have recently mixed autobiography with scientific exposition, most have been by theorists. It is refreshing to get the perspective of someone who has to organize his calendar around the phases of the Moon (in order to leave free those nights when the skies are darkest and best for observation).

Of course, writing a book from a personal viewpoint frees an author from the responsibility of devoting equal amounts of time to the work of all competing groups, although one must still be fair. After a pioneering but premature effort in the 1980s by a group of Danish astronomers, the epochal discovery of universal acceleration in 1998 was the work of two competing teams: the High-Z Supernova Search Team led by Brian Schmidt (of which Kirshner, who was Schmidt's thesis adviser, is a member), and the Supernova Cosmology Project led by Saul Perlmutter. In this mostly friendly rivalry, there is inevitably some jockeying for position in the apportionment of credit; it seems clear, however, that posterity will give both groups full credit for discovering dark energy.

Kirshner is a talented writer, and both experts and general readers will find his book a consistently enjoyable read. He takes the time to tell delightful and surprising stories about the many personalities who have contributed to modern cosmology. You will learn, for example, that the first man-made satellite to escape Earth's orbit was designed by Caltech astronomer Fritz Zwicky, and launched in 1957. (I won't give away the details.) For my taste, even more enjoyable than the stories is the language itself, which is thick with illuminating metaphors and amusing allusions of all sorts.

For a popular-level book, The Extravagant Universe is scientifically ambitious. Kirshner is not afraid to write about stellar spectra in some depth, to use plots of real data, and even to introduce an occasional simple equation. This is heartening, but I think that he might have spent more time trying to explain some of the basic physics at work. There are a number of places where a figure would have made things clearer; for example, although spectral lines are discussed frequently, there is no drawing of a canonical Bohr atom with an electron moving from one level to another. More surprisingly, in a book about the accelerating Universe, there is no plot of the size of the Universe against time, which would demonstrate the difference between accelerating and decelerating.

But these are quibbles, and most readers will learn a great deal from the book, and have fun in the process. The story being told is irresistible in its own right, and is related with verve and good humour. Nobody (or at least, very few people) expected that the Universe would be accelerating, and nobody (literally nobody) knows what dark energy is, or why it has the magnitude it does. Books like this one will help to inspire the next generation of physicists to ultimately answer these questions.