The Quintessential Universe; January 2001; Scientific American Magazine; by Jeremiah P. Ostriker and Paul J. Steinhardt; 8 Page(s)

Is it all over but the shouting? Is the cosmos understood aside from minor details? A few years ago it certainly seemed that way. After a century of vigorous debate, scientists had reached a broad consensus about the basic history of the universe. It all began with gas and radiation of unimaginably high temperature and density. For 15 billion years, it has been expanding and cooling. Galaxies and other complex structures have grown from microscopic seeds-quantum fluctuations-that were stretched to cosmic size by a brief period of "inflation." We had also learned that only a small fraction of matter is composed of the normal chemical elements of our everyday experience. The majority consists of so-called dark matter, primarily exotic elementary particles that do not interact with light. Plenty of mysteries remained, but at least we had sorted out the big picture.

Or so we thought. It turns out that we have been missing most of the story. Over the past five years, observations have convinced cosmologists that the chemical elements and the dark matter, combined, amount to less than half the content of the universe. The bulk is a ubiquitous "dark energy" with a strange and remarkable feature: its gravity does not attract. It repels. Whereas gravity pulls the chemical elements and dark matter into stars and galaxies, it pushes the dark energy into a nearly uniform haze that permeates space. The universe is a battleground between the two tendencies, and repulsive gravity is winning. It is gradually overwhelming the attractive force of ordinary matter-causing the universe to accelerate to ever larger rates of expansion, perhaps leading to a new runaway inflationary phase and a totally different future for the universe than most cosmologists envisioned a decade ago.