review of Kirshner's book

Can Two Luminaries Share One Standard Candle to Reveal Cosmic History?

Reviewed by David Branch

Physics Today readers are likely aware of the startling discovery in the late 1990s that the expansion of the universe appears to be accelerating. The direct evidence is the observed relation between the brightnesses and redshifts of very distant (high-redshift or high-z) type Ia supernovae. The observed brightness of a "standard candle" such as a type Ia supernova tells its distance and therefore the time elapsed since it exploded. Furthermore, the supernova's redshift gives the factor by which the universe has expanded during that elapsed time. Thus is revealed the history of the cosmic expansion.

Normal type Ia supernovae are our best standard candles for determining distances on cosmological scales because they are luminous enough to be seen from afar and alike enough so their observed brightnesses can be used to infer distance. Such supernovae are nearly alike because, we think, they are produced by carbon-oxygen white dwarfs in nearly identical ways. The white dwarfs are thought to accrete matter from close binary companions until they approach the same limiting mass--the Chandrasekhar mass--at which point they undergo thermonuclear disruption.

The cause of the cosmic acceleration indicated by observations of type Ia supernovae may prove to be Albert Einstein's cosmological constant--nowadays equated with the energy density of the vacuum--or it may be some other kind of "dark energy." The nature of dark energy is the deepest mystery in contemporary cosmology and astroparticle physics.

Two international teams found the cosmic acceleration. The Supernova Cosmology Project (SCP), headquartered at Lawrence Berkeley National Laboratory and led by Saul Perlmutter, began to search for distant standard candles in the late 1980s to determine the cosmic expansion rate at ever earlier times. They found their first high-z type Ia supernova in 1992. The High-Z Supernova Search (HZSS) team, organized in late 1994, is led by Brian Schmidt of the Australian National University and includes astronomers at such other institutions as Harvard University; the University of California, Berkeley; and the Cerro Tololo Inter-American Observatory.

The Extravagant Universe recounts the discovery of cosmic acceleration from the viewpoint of Harvard professor Robert Kirshner, a senior member of the HZSS. Kirshner's avowed aim is to help the reader share in the adventure. In the process of conveying his personal adventure, Kirshner endows The Extravagant Universe with a strong autobiographical component: He relates many ups and downs of his professional odyssey, from winning an essay prize as a Harvard undergraduate to suffering aching gums as a middle-aged astronomical observer on a Chilean mountaintop.

The first eight chapters provide an introduction to cosmology at the level of Astronomy 101, with some emphasis on the history of the subject. That tale is often told, but seldom so engagingly. The analogies are apt, the anecdotes are amusing, and the writing is brisk and witty--in places, downright funny. Kirshner's story of his encounters with Caltech astronomer Fritz Zwicky beginning in 1970, entirely reminiscent of my own experiences with that singular character, had me laughing out loud.

The final three chapters get to the search for high-z type Ia supernovae and the discovery of the acceleration. The writing continues to sparkle and Kirshner succeeds in conveying the difficulty and excitement of the hunt for remote supernovae--made all the more stimulating for him, one senses, by his rocky relationships with senior members of the competing team. But caution: You are entering a spin zone. When it comes to acknowledging the SCP for its pioneering development of observation techniques and analysis approaches that were to be used by both teams, Kirshner turns out to be a stinter. The SCP personnel are treated with an air of condescension and inappropriately portrayed as a bit scientifically naive. Even credit for developing the breakthrough observational strategy that enabled both teams to guarantee the discovery of high-z type Ia supernovae in batches at predetermined times, so that photometric and spectroscopic follow-up with large telescopes could be scheduled in advance, is directed elsewhere rather than to the SCP, to whom it belongs.

The Extravagant Universe should be popular with science aficionados in the general public, and some Physics Today readers will want to read it too. Enjoy it with a dram of salt. There is no other firsthand book-length account of the discovery of the cosmic acceleration, but if you have not already done so, be sure also to read Perlmutter's article in Physics Today (April 2003, page 53).

David Branch George Lynn Cross Professor in the department of physics and astronomy of the University of Oklahoma in Norman, is interested in analyzing supernova spectra and using supernovae as distance indicators for cosmological applications.

2003 American Institute of Physics