By William J. Cromie

Gazette Staff

Looking 7 billion years into the past, astronomers have gotten a glimpse into the future of the universe.

They discovered the universe has less matter in it than they thought, and it will keep on expanding forever.

Roughly, 15 billion years ago, space and time began with a stupendous explosion known simply as the Big Bang. From then until now, stars and galaxies have been rushing away from each other in all directions. The key question is how fast. If this expansion slows down enough, gravity will pull everything back together, a situation that scientists, if they survived, might call the Big Crunch.

Astronomers from Harvard and other institutions have made the most accurate measurements to date of such slowing down, and found it is so small that gravity will never be able to pull things back together again.

"We find, to a 95 percent level of confidence, that the density of matter is insufficient to halt the expansion of the universe," says Peter Garnavich of the Harvard-Smithsonian Center for Astrophysics (CfA).

Garnavich and his colleagues looked at exploding stars, or supernovae, 5 billion to 7.7 billion years old, and measured how fast they were moving away from Earth. They compared this with the movement of nearby and much younger supernovae. Because of gravity, every bit of matter attracts every other bit, so the expansion must eventually slow down. The sky-watchers found this to be so, but the deceleration is smaller than expected.

Einstein's Folly

The observations raise another intriguing question. Is the slow expansion of the universe due only to the fact that it contains less matter than generally believed, or is there a mysterious anti-gravitational force that pushes matter apart and slows down the slowing down?

The great thinker Albert Einstein never believed in an expanding universe. He was happiest with a steady state; stars exploding into oblivion at the same rate as gravity put together new ones from the fragments. To make his equations work out that way, he added what amounts to a cosmological fudge factor, a force that works like gravity in reverse.

But in 1923, astronomer Edwin Hubble upset this idea by showing that every galaxy and gas cloud in the universe is streaking away from each other as if space is an expanding balloon. The farther away an object is from Earth, the faster it is moving.

Einstein admitted his mistake, but now some scientists are thinking that he might have been right after all. Although not actually detected, the counter-gravity might come from a weak but omnipresent energy force present in the vacuum of space. Rather than being empty of both matter and energy, the vacuum may contain enough energy to take over in the absence of strong gravity and push the universe apart.

Slow deceleration, then, can be accounted for either by less material alone, or by a combination of sparse matter and vacuum energy.

"Our data on expansion rates are not accurate enough to tease apart the two," Garnavich told a meeting of the American Astronomical Society in Washington, D.C., last week.

But he added in an interview that he is "not a big fan of the cosmological constant," the name given to Einstein's fudge factor. "I think that what we see is what we get. The universe simply doesn't contain enough material to halt its expansion. I can't see any reason for bringing in an additional parameter."

Age of the Universe

If the universe is lighter than scientists thought, and has been expanding more or less steadily, than it would be older than one filled with more matter. A heavier universe would take more time to expand to the same size because of the braking action of gravity.

"Using our best estimates of the expansion speed gives an age of about 15 billion years," Garnavich says. Using a heavier universe, other astronomers arrived at an age of 8 billion to 12 billion.

Such a young universe causes a serious problem. The oldest stars in sight are thought to be 14 billion to 15 billion years old. A 12-billion-year-old universe would make those stars older than the space they occupy, but a 15-billion-year-old universe accommodates them nicely.

Missing Mass

A "lite" universe sheds light on another thorny problem. Gravitational attraction moves huge clusters of galaxies around in ways that can be measured. But when these measurements are made, something is missing. Sky-watchers cannot see enough galaxies and other visible material to account for the movement they measure.

The movement obviously takes place, so there must be more to celestial matter than meets the eye. The universe must contain a great deal of invisible, or dark, matter. By some calculations, 80 to 90 percent of space should consist of stuff that can't be seen -- missing mass.

But with a lite universe that's going to expand forever, less matter becomes missing. "We still need some of the dark matter to account for the movement of galactic clusters that we see," Garnavich points out. "But now it's much less, only about 15 percent of the mass of the universe needs to be dark."

Standard Candles

Garnavich, Astronomy Professor Robert Kirshner, and Peter Challis of the CfA worked with colleagues from 10 universities and observatories in four countries to make the tricky observations of deceleration. Another team from Lawrence Berkeley National Laboratory and the University of California, Berkeley, made similar measurements. The CfA group studied supernovae that blew up as long ago as 7.7 billion years ago. That is, they saw these exploding stars as they were then; the light has taken almost 8 billion years to reach Earth. Two others blew up approximately 5 billion years ago.

You might be surprised to find out that such outbursts are not hard to find. Astronomers may detect as many as five on a single night. Powerful telescopes in Chile and Hawaii take pictures of the same large regions of deep space three weeks apart. They then compare the two images to find bright dots of light that appear in the second photos but not the first.

The observers are particularly interested in exploding stars about the size of the sun. These so-called "white dwarfs" usually boast smaller companion stars. As the two orbit each other, the bigger twin's gravity pulls material out of the smaller. Eventually, the gravitational glutton eats more matter than it can hold. It collapses then explodes.

Scientists send location signals to the Hubble Space Telescope, orbiting from 300 to 400 miles above Earth. Circling far above city lights, clouds, and the shifting, polluted atmosphere, the space telescope snaps much clearer pictures of the explosions than are possible from the ground.

These supernovae serve as "standard candles" for those trying to judge distance in the universe. If you see a bright light in the distant sky, it's impossible to tell if it's a large object shining far away, or a small, brilliant one close by. Since all white dwarf explosions create about the same amount of light, their intrinsic brightness is known, and their apparent brightness depends on their distance.

To determine that distance, astronomers measure the shift in light caused by movement away from Earth. The light becomes redder, the faster (and thus farther) a star or galaxy is moving toward the edge of the universe. (Such light shifts toward blue when a heavenly object moves toward Earth.) It's like an ambulance or police siren that changes pitch as it moves away (lower pitch) or toward (higher pitch) you.

Putting together measurements of red shifts and intrinsic brightness, then adding the inevitable corrections, gave Garnavich and his colleagues a measure of how fast the universe was expanding 5 billion to 7.7. billion years ago. The comparison with younger, closer supernovae yielded the conclusions about less slower deceleration, older age, and less missing mass.

Garnavich and his colleagues stress, however, that their conclusions are preliminary, and they need more measurements to confirm them. Both the Harvard and California groups have about 30 other supernovae each under observation.