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Puzzles Plague the Quest for Age, Size of Universe : Cosmos: New tools let scientists probe farther back in time. But they create once unimaginable conundrums.

Rethinking Cosmic Questions. SECOND OF TWO PARTS

March 07, 1995|K.C. COLE | TIMES SCIENCE WRITER

As cosmologists struggle to get a handle on the universe, their biggest adversary is time.

Probing the whys and wherefores of the universe turns on getting information from sources that are millions (or even billions) of light-years away. That means they are also millions (or billions) of years back in time.

Deciphering starlight is like reading a letter from a friend who writes that he's sitting at a window looking at the crocuses in bloom. By the time you receive it, he could be buried two feet deep in snow.

This is only one of the problems confronting scientists as they take on the ultimate questions of cosmology: How old is our universe? How big? What is our cosmic fate?

But new tools are enabling astronomers to go farther--and farther back--than they ever have before. Since looking out in space is also looking back in time, they are calculating the age of objects so distant they could scarcely see them before.

While this has triggered an explosion of new information about the universe, it has created conundrums that no one had imagined.

In the most confounding of recent findings, data seems to suggest that the universe may be younger than its oldest stars. Although this incongruity has scientists scratching their heads, they are still encouraged because they believe they are homing in on the true age of the universe. Measures of size (and therefore age) of the universe indicate it is 8 billion to 25 billion years old. (The oldest stars should have taken at least 15 billion years to mature into their present state.)

"It means we're narrowing the range of right answers," said University of Michigan theorist Katherine Freese.

Nevertheless, peering this far back into the history of time puts astronomers in uncharted territory. "Observational cosmology is different from anything I've ever done in that very, very basic things are simply not known," said the University of Washington's Christopher Stubbs. "Even the alleged facts contradict each other."

Theoretically, solving the puzzle of the universe should be relatively simple. Since the universe exploded from a single point in space and time, and has been expanding outward and onward ever since, it should be possible to trace our steps backward by looking outward.

In the 1920s, Edward Hubble noticed that the farther away galaxies were, the faster they were moving. He also realized that there is a direct relationship between how far and how fast (a ratio now known as the Hubble constant).

So if a galaxy is moving away from Earth twice as fast as another galaxy, it is also twice as far away. Simply by knowing how fast a galaxy is being swept away by the expansion of space-time, one should be able to calculate how far away it is and therefore how long ago it all began. The trouble is, no one knows for sure how fast the galaxies are moving.

The question is complicated by the way matter is distributed throughout the universe: Amid vast voids of empty space, astronomers find stars clumped into galaxies, galaxies gathered into clusters and clusters congregated along huge sheets that in turn bend into bubbles millions of light-years across. On the surface, the speed of galaxies should be easily measurable. Since the spectra of colors emitted by stars stretch like taffy along with the expansion of space, the amount of stretch indicates their apparent velocity. (The stretch is known as red shift because the light waves stretch toward the red end of the spectrum.)

But there are complications. For example, a red shift may be the result of either the expansion of the universe as a whole, or the gravitational pull of a local galaxy cluster.

Since the universe is as full of gravitational currents and eddies as Van Gogh's "Starry Night," it can be difficult to tell. "It's like the tide coming in and filling the harbor," said astronomer Robert Kirshner of Harvard. "There are little swirlings around the foot of the pier that are not part of the big story."

If one's goal is to find out the expansion rate of the universe as a whole--the better to deduce its size and age--these local currents can be deceiving.

One way to get around the problem is to look deep enough into space so that local currents don't affect the red shift. But it turns out that because the universe expands more and more slowly as time goes on, looking back in time also gives a different expansion rate.

"When you look farther back in time, (you find that) the universe is not static," said Stubbs. "It's decelerating."

The Hubble constant, in other words, isn't constant. The expansion rate that an astronomer might measure today is not necessarily the same as the expansion rate billions of years ago.

The universe was almost certainly expanding much faster right after the Big Bang than it is today. The question is: How much faster?

In part, the answer depends on the amount of matter dragging the universe down, because it is the gravitational pull of matter that has slowed the expansion.

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