Gravity's Shadow: The Hunt for Omega

Evidence for a dark, unknown matter permeating the universe is rooted in Newton's basic law of gravitation. In what may be the most influential science document ever written, the Principia Mathematica, the illustrious British scientist Isaac Newton in 1687 showed mathematically that there exists a force of attraction between any two bodies. Although Newton was working from observations made by the Danish astronomer Tycho Brahe and calculations earlier completed by Brahe's German assistant, Johannes Kepler, he extrapolated this work on celestial bodies to any two objects with mass, such as the apocryphal apple falling on his head. The attractive force between any two bodies, Newton demonstrated, increased as the product of their masses and decreased as the square of the distance between them. This revelation led to a systematic way of calculating the velocities and masses of objects hurtling through space.

Indeed, it was by applying Newton's formulae to certain observational data in the 1930s that Fritz Zwicky, a Swiss-born astronomer working at the California Institute of Technology, obtained astronomy's first inkling that dark matter was present in the universe. Concern about dark matter, noted Geller, "is not a new issue." By analyzing the velocities of the galaxies in the famous Coma cluster, Zwicky noticed that they were moving around within the cluster at a fairly rapid pace. Adding up all the light being emitted by the Coma galaxies, he realized that there was not enough visible, or luminous, matter to gravitationally bind the speeding galaxies to one another. He had to assume that some kind of dark, unseen matter pervaded the cluster to provide the additional gravitational glue. "Zwicky's dynamical estimates—based on Newton's laws which we think work everywhere in the universe—indicated that there was at least 10 times as much mass in the cluster of galaxies as he could account for from the amount of light he saw," said Geller. "The problem he posed we haven't yet solved, but we have made it grander by giving it a new name: Now we call it the dark matter problem.'' Over the last 15 years, astronomer Vera Rubin with the Carnegie Institution of Washington has brought the dark matter problem closer to home through her telescopic study of the rotations of dozens of spiral galaxies. The fast spins she is measuring, indicating rates higher than anyone expected, suggest that individual galaxies are enshrouded in dark haloes of matter as well; otherwise each galaxy would fly apart.

One of the reasons astrophysicists refer to the dark matter as a problem is related to the potential gravitational effect of all that dark matter on the fate of the cosmos, a question that arises when the

equations of Einstein's theory of general relativity are applied to the universe at large. General relativity taught cosmologists that there is an intimate relationship between matter, gravity, and the curvature of space-time. Matter, said Einstein, causes space to warp and bend. Thus if there is not enough matter in the cosmos to exert the gravitational muscle needed to halt the expansion of space-time, then our universe will remain "open," destined to expand for all eternity. A mass-poor space curves out like a saddle whose edges go off to infinity, fated never to meet. The Big Bang thus would become the Big Chill. On the other hand, a higher density would provide enough gravity to lasso the speeding galaxies—slowing them down at first, then drawing them inward until space-time curls back up in a fiery Big Crunch. Here space-time is "closed," encompassing a finite volume and yet having no boundaries. With a very special amount of matter in the universe, what astrophysicists call a critical density, the universe would stand poised between open and closed. It would be a geometrically flat universe.

Scientists refer to this Scylla and Charybdis dilemma by the Greek letter Ω. Omega is the ratio of the universe's true density, the amount of mass per volume in the universe, to the critical density, the amount needed to achieve a flat universe and to just overcome the expansion. An Ω of 1 indicates a flat universe, greater than 1 a closed universe, and less than 1 an open universe. By counting up all the luminous matter visible in the heavens, astronomers arrive at a value for Ω of far less than 1; it ranges between 0.005 and 0.01. When astronomers take into account the dark matter measured dynamically, though, the amounts needed to keep galaxies and clusters stable, Ω increases to about 0.1 to 0.2. But theoretically, Ω may actually be much higher. In 1980 physicist Alan Guth, now with the Massachusetts Institute of Technology, introduced the idea that the universe, about 10-36 second after its birth, may have experienced a fleeting burst of hyperexpansion—an inflation—that pushed the universe's curvature to flatness, to the brink between open and closed. And with the geometry of the universe so intimately linked with its density, this suggests that there could be 100 times more matter in the cosmos than that currently viewed through telescopes.