The Answer in the Voids

By Jeffrey Winters|Friday, March 01, 1996
RELATED TAGS: COSMOLOGY
If Barbara Ryden is right, in a few years she’ll know the answer to one of nature’s greatest riddles: How will the universe end?

What lies ahead for the universe? Will it expand forever, all its matter thinning into cold, dark wisps? Or will the cosmos eventually collapse into a Big Crunch? The answer hangs on whether the universe contains enough mass--and thus gravity--to slow and reverse the expansion. Unfortunately, astronomers can’t directly measure the total mass of the universe--most of it appears to be in the form of dark matter. Yet this obstacle doesn’t deter Barbara Ryden, an astronomer at Ohio State University. By the decade’s end, when an upcoming sky survey will have given her the data she needs, she should be able to foretell the future of the universe.

Her secret? Ryden believes a long-overlooked idea from cosmologists Bohdan Paczy´nski of Princeton and Charles Alcock of Lawrence Livermore National Laboratory in California may help solve this ultimate riddle. Paczy´nski and Alcock realized that in principle one could measure how quickly the universe is decelerating by carefully looking at the shapes of objects in space. The expanding universe carries other galaxies away from our own Milky Way. The more distant the galaxy, the faster it moves away from us. Light waves emitted from a receding galaxy get stretched-- shifted toward the redder part of the spectrum.

If, as many cosmologists believe, the universe expanded much more quickly in its youth than it is expanding today, then the differences in redshifts between the front and back of distant objects should be greater than the differences in nearby objects. As you look farther out in space, you also look farther back in time, Ryden explains. In theory, then, objects very far from us should appear to be more distorted than those that are closer.

But those objects would have to be very big--much bigger than an individual galaxy. Galaxies are gravitationally bound objects that do not themselves expand as the universe expands, says Ryden. They don’t show the distortion at all. Even clusters of galaxies might be too small; what might look like cosmic stretching might be caused by localized movements of the galaxies themselves.

In the late 1980s astronomers discovered that the universe is like a tub of bubbles, with vast sheets of galaxies separated by huge, roughly spherical voids--the sheets limn the surfaces of the voids, much as soap film surrounds a bubble. The voids are as much as hundreds of millions of light-years in diameter--large enough, says Ryden, to show any deformation created by a decelerating cosmos. All the voids are thought to be roughly spherical in ‘real space,’ she explains. That is, if you took a really, really long yardstick and measured their size, they would be spherical. But measured from Earth with the only yardstick astronomers have available to them--redshift--the voids may look distorted.

Assuming the expansion is slowing, a void’s surface farthest from us should be receding faster than its near surface, which would appear to stretch the sphere into an ellipsoid along the line of sight. By measuring the redshift of the galaxies that define a void’s surfaces, says Ryden, it should be possible to detect that distortion. Astronomers could then estimate--from the observed distortions of voids at increasingly greater distances--the rate at which the universe is slowing down, a number they refer to as the deceleration parameter.

Last year Ryden reported that she’d analyzed hundreds of voids for signs of distortion--and had come up empty-handed. She wasn’t completely surprised by her failure. The maps of voids that she used show only the nearest billion light-years of space; the stretched voids should become more obvious three or four times farther out. That is one reason that, over the next five years, an international project known as the Sloan Digital Sky Survey will map 1 million galaxies in roughly the nearest 3 billion light-years of space. With those data, Ryden thinks she’ll be able to measure the amount of stretching and the deceleration parameter.

If she finds a large deceleration parameter (a value above .5, it turns out), the universe will sooner or later cave in. A smaller value means the universe has reached escape velocity, with matter becoming ever more dispersed. With a deceleration parameter right at .5, the universe avoids both fates. It expands slower and slower, until at some point in the infinite future, it simply stops.
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