The Extravagant Universe: Exploring Stars, Dark Energy and the Accelerating Cosmos - book review

The Extravagant Universe: Exploring Stars, Dark Energy, and the Accelerating Cosmos by Robert P. Kirshner Princeton University Press, 2002; $29.95

In February 1998 new observations of exploding stars in distant galaxies stood the world of cosmology on its ear. The expansion of the universe, far from slowing down, as earlier theories had implied it should, turned out to be speeding up. Objects in the universe are moving apart from one another at progressively greater, speeds. The new findings foretell a future in which the cosmos becomes an unimaginably vast, cold, dead, and barren expanse of near-nothingness.

How did astronomers reach such a startling conclusion?

In 1916 Einstein, shortly after completing the formulation of his theory of general relativity, discovered that the solutions to a key equation within the theory implied that the universe must always be either expanding or contracting. Einstein's pencil-and-paper discovery took him by surprise, because astronomers of the era had no evidence to suggest that the universe either expands or contracts. To fix what he then took to be an error, he restated his key equation with an additional, constant term--which quickly became known as the cosmological constant. If the constant had precisely the right value, Einstein wrote in 1917, the universe could exist in a state of perfect, static balance.

But the Russian mathematician Alexander Friedmann soon demonstrated that such a static universe must be balanced, as it were, on a knife edge: the slightest tremor would topple it over in one direction or the other, into a state of either expansion or contraction. Another, even more serious objection to Einstein's solution appeared in 1929, when Edwin Hubble discovered that the cosmos is indeed expanding. On distance scales as large as the ones between clusters of galaxies, all objects are moving away from all other objects at speeds that increase in proportion to the distances between them. (Cosmologists imagine the expanding universe most simply as the three-dimensional analogue of the skin of a balloon. As the balloon expands, every point on the skin of the balloon moves away from all others, yet no one point is motionless,) Einstein soon pronounced the cosmological constant a dead letter, calling it his "greatest blunder."

The results announced in 1998 effectively resurrected Einstein's "blunder." Those observations included two kinds of measurements: first, the distances to certain kinds of supernovas, or exploding stars, that astronomers discovered in distant galaxies; and second, the speeds with which those galaxies are receding from us. But when astronomers tried to describe the relation between those distances and speeds, they found they had to restore Einstein's full equation from 1917, including a nonzero cosmological constant.

The value that the 1998 observations imply for the cosmological constant is not equal to the value Einstein adopted to keep the universe static--after all, the two kinds of universe could hardly be more different. But the fact that the recent observations require a nonzero value for the constant carries a tremendous implication: Every cubic centimeter of what seems to be empty space instead teems with hidden energy, which astronomers now call dark energy. As the universe expands from its origins in the big bang, more space continuously comes into being, and so the total amount of dark energy also increases proportionately. The ever-growing amount of dark energy progressively accelerates the universal expansion. Although gravity acts in the opposite sense, tending to slow the expansion because all matter in the universe attracts all other matter, the expansionist tendency of the dark energy has now become dominant. The cosmos has entered a phase of accelerating expansion.

Such a striking result should be accepted with caution. Astronomers have spent years determining the distances and velocities of remote galaxies. Although the galaxies' velocities can be found relatively easily by measuring the shift in the colors of their light, finding their distances has proved much more difficult. In fact, astronomers could make only fairly crude estimates of the distance to any faraway galaxy--until they identified a marvelous type of exploding star called a type Ia supernova (or SN Ia for short).