Department of Dark Energy
By Michael Riordan
Published: April 13, 2003
THE EXTRAVAGANT UNIVERSE
Exploding Stars, Dark Energy,
and the Accelerating Cosmos.
By Robert P. Kirshner.
Illustrated. 282 pp. Princeton, N.J.:
Princeton University Press. $29.95.
IN 1929, the astronomer Edwin Hubble discovered that the universe is not at rest. By measuring the speed and distance of various galaxies, he established that all but a few are receding from us and that their ''recessional velocities,'' the speeds at which they are moving away, increase steadily with distance. His surprising conclusion, accepted by scientists ever since, is that the universe is expanding.
Until Hubble published his findings, most astronomers had thought the universe was static, unchanging. Not many years earlier, in fact, Albert Einstein noted that his equation for general relativity would lead to a contracting universe and inserted an artificial term, called the ''cosmological constant,'' into his formula to counteract the force of gravity and yield a static universe. After 1929, Einstein abandoned his fabrication, calling it his worst blunder.
This term has been widely regarded as ''theoretical poison ivy'' by mainstream cosmologists until recently, Robert P. Kirshner writes in ''The Extravagant Universe.'' But in the past few years, using ever more sophisticated telescopes and other instruments, astronomers have begun to perceive that the universe is not just expanding; its expansion is speeding up, seemingly unrestrained by gravitational attractions among the galaxies. Some kind of antigravity seems to be at work in the universe. Suddenly one hears cosmologists -- scientists who seek to explain the whole universe -- talking again about Einstein's cosmological constant as one way to account for the apparent acceleration.
Kirshner gives a detailed, opinionated account of how these extremely difficult measurements -- made by observations of exploding stars, or supernovas -- became possible in the last 25 years. He has been observing these rare celestial convulsions ever since he was a Caltech graduate student in the early 1970's. Now, as head of the optical and infrared division at the Harvard-Smithsonian Center for Astrophysics, he leads an international team of scientists chasing supernovas.
Supernovas have the great advantage of being extremely bright celestial objects, often emitting as much light during their first several weeks of existence as an entire galaxy does normally. Thus they can be examined even when they occur at tremendous distances. And one category of them, called Type Ia supernovas, which are the nuclear explosions of white-dwarf stars, seem to have nearly the same intrinsic brightness no matter where or when they occur. Thus astronomers can use these as ''standard candles'' -- a kind of cosmic yardstick -- to measure the expansion of the universe at both large and small distances. If you know the intrinsic brightness of such a supernova, then its apparent brightness as seen in a telescope tells you its distance from Earth fairly precisely.
Accurately gauging the distances to faraway galaxies has always been the most problematical part of making such measurements. In determining the apparent brightnesses of Type Ia supernovas over a wide range of distances, Kirshner's team, and a competing group from the Lawrence Berkeley National Laboratory led by Saul Perlmutter, noticed during the late 90's that the really distant ones, located about a third of the way across the visible universe, seemed substantially dimmer than anticipated. On the surface, these observations indicate that the galaxies in which they lie are farther away than predicted by the uniform ''Hubble expansion'' and thus have been receding faster. They suggest that a cosmic acceleration is occurring. If cosmic expansion had been slowing down because of gravity, as was widely expected, these galaxies would have been nearer and their supernovas would appear brighter.
But these measurements are fraught with uncertainties; subtle errors creep in and must be weeded out. Kirshner explains in exhaustive detail how these ambiguities have been resolved so that meaningful results can be extracted from messy data. That is one of the principal strengths of his book. He takes readers inside the control rooms of large telescopes around the Pacific Basin and reveals the harried life of a leading astronomer. We even glimpse the politics of jockeying to use the Hubble Space Telescope.
Kirshner reminds us that despite the all-encompassing ''theories of everything'' being promoted today, cosmology is ultimately based on observations and experiments. Unlike many other recent books on the topic, ''The Extravagant Universe'' is the gripping story of how two competing groups of scientists came to make, and finally believe, the surprising measurements on which a radical new view of the universe depends. And it is not the simple, elegant picture that certain theorists would have us think. Instead, to help accommodate the evidence that has recently been accumulating, we require ordinary, garden-variety matter; at least three kinds of dark, invisible matter; and two types of a mysterious entity inhabiting empty space called ''dark energy,'' which powered the birth of the universe in the Big Bang and now drives cosmic acceleration. ''You would be unwise to believe such a baroque mixture, which seems to violate common sense,'' Kirshner cautions. ''The universe is wilder than we ordinarily dare to imagine.''
I ONLY wish he had written his book more carefully for a broader audience of general readers, who need this kind of exposure to the messy but rewarding process of scientific research. Instead, Kirshner seems to be addressing his colleagues and other scientists already familiar with such concepts as inflationary theories, cold dark matter and vacuum energy. He can occasionally write in a lively, accessible style, using revealing anecdotes and analogies, but even some of these miss their mark. And he repeatedly falls into astrophysical jargon and scientific notation. His book could have benefited from a strong editor.
Despite these difficulties, ''The Extravagant Universe'' is an excellent book for readers with some literacy in science. It is an evocative reminder that cosmology, too, is now a true observational, experimental science, securely grounded in the messy practical realities of making measurements.