Inflation in a Low-Density Universe; January 1999; Scientific American Magazine; by Bucher, Spergel; 8 Page(s)

Cosmology has a reputation as a difficult science, but in many ways explaining the whole universe is easier than understanding a single-celled animal. On the largest cosmic scales, where stars, galaxies and even galaxy clusters are mere flecks, matter is spread out evenly. And it is governed by only one force, gravity. These two basic observations-large-scale uniformity and the dominance of gravity-are the basis of the big bang theory, according to which our universe has been expanding for the past 12 billion years or so. Despite its simple underpinnings, the theory is remarkably successful in explaining the velocity of galaxies away from one another, the relative amounts of light elements, the dim microwave glow in the sky and the general evolution of astronomical structures. The unfolding of the cosmos, it seems, is almost completely insensitive to the details of its contents. Unfortunately for biologists, the same principle does not apply to even the simplest organism.

Yet there are paradoxes inherent in the big bang theory. Two decades ago cosmologists resolved these troubling inconsistencies by incorporating ideas from particle physics-giving rise to the theory of "inflation." But now this elaboration is itself facing a crisis, brought on by recent observations that contradict its prediction for the average density of matter in the cosmos. Cosmologists are realizing that the universe may not be quite so simple as they had thought. Either they must posit the existence of an exotic form of matter or energy, or they must add a layer of complexity to the theory of inflation.