Mysterious Gap

Astronomers detect a vast void

Fixing their gazes on the heavens, astronomers from Ptolemy to Hubble have studied moons, planets and stars. Recently four American astronomers tried a different approach. They concentrated not on the stars but on the space that surrounds them.

Using telescopes on Kitt Peak and Mount Hopkins in Arizona and Mount Palomar in California, they photographed patches of the night sky and got two-dimensional pictures showing the distribution of matter in a sector of space. To add the perspective of depth they used an astronomical yardstick called the red shift, a measure of how far an object has traveled based on how sharply its light is displaced toward the red end of the spectrum.

They knew that if they looked at a wide enough stretch of sky, the haphazard distribution of heavenly bodies would even out into a vast and more or less regular pattern. "Our idea of a small distance is 1 million light-years," explains Astronomer Paul Schechter of Kitt Peak. "At that distance it looks like a very lumpy universe. But as you step back, the universe begins to look very smooth."

The project was routine, Schechter admits, but the results were not. Their scan revealed that rather than becoming uniform, space contains a huge, mysterious "gap." Taking representative samples, they expected to find 25 galaxies the size of the Milky Way. They found only one. Moreover, the same low density seemed to extend over an area 300 million light-years across--about 1 % of the visible universe. The finding of so little matter in so much space had cosmologists all over the world scratching their heads.

Scientists may disagree about how the universe began. But virtually all of them take it as a cosmological first principle that any place in the universe will look more or less like any other. In the primeval explosion that set it all in motion, matter and energy were sent hurtling uniformly in all directions. Although matter ultimately cooled and condensed in random patterns, the overall distribution of galaxies should be roughly homogeneous. What could so large a gap, so notable an eccentricity, be doing there?

So far scientists can offer no easy answer. Skeptics have failed to solve the mystery with statistics, and more detailed photographs do not exist. The gap may yet prove more illusory than real--an optical trick, perhaps, played by dwarf galaxies too faint to be detected. The group is going back for a closer look, but even if they find a small army of dwarfs, someone will still have to explain how it got there.

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