Oops ... Wrong Answer

By MICHAEL D. LEMONICK

The Hubble Space Telescope has performed some spectacular feats of science since a crew of astronauts heroically corrected the instrument's blurred vision last December. The orbiting observatory has snapped dramatic pictures & of the comet Shoemaker-Levy 9 smashing into Jupiter, discovered hundreds of what seem to be solar systems in the making, and provided proof that giant black holes lurk in the cores of galaxies. But all this was just a warm-up for the Hubble's most eagerly awaited mission: to gauge the age of the universe. The question of how old the heavens are is not only fascinating in its own right but also bears directly on just about every other major cosmic mystery from the universe's history to its eventual fate.

Last week the Hubble delivered its preliminary verdict: the universe is between 8 billion and 12 billion years old. That may seem imprecise, but it was specific enough to throw astrophysicists into a state of high anxiety. The problem: our own galaxy has stars believed to be as much as 14 billion to 16 billion years old. And it makes astronomers more than a little uncomfortable to try to explain how stars could have formed before the universe began. Says Alexei Filippenko, a University of California, Berkeley, astronomer who was on the team that made the Hubble observations: "This is fantastic stuff ... it might lead to a revolution in cosmology."

If the Hubble figure for the universe's age is confirmed by more studies, something has to give. Perhaps astrophysicists don't understand stars as well as they thought -- but that's considered unlikely. An alternative idea is downright shocking: something may be wrong with the revered theory that the universe began with a Big Bang and has been expanding ever since.

The team used a clever, indirect method of measuring the age of the universe. First the scientists determined the distance to a group of galaxies called the Virgo cluster. Then they took advantage of a trick first used by Edwin Hubble, the astronomer who discovered, back in the 1920s, that the universe is expanding -- and for whom the space telescope was named. If the whole cosmos is blowing up like a balloon, Hubble reasoned, then you can calculate backward to see when the balloon began to inflate.

You can, that is, as long as you know the rate of expansion. Formally known as the Hubble constant, this expansion factor is calculated by measuring the distance from our galaxy to other galaxies (something difficult to do) and determining how fast that distance is increasing (an easier task). Scientists look for particular kinds of stars, called Cepheid variables, because they know the inherent brightness of these stars. The fainter they appear here on earth, the farther away they are, and the distance can be roughly calculated. As researchers find Cepheids farther and farther away, calculations of the Hubble constant become more and more accurate.

The space telescope managed to spot Cepheids in M100, a particularly distant galaxy in the Virgo cluster, which enabled the Hubble scientists to estimate how far the cluster is from earth. According to their report in the current Nature, M100 is 56 million light-years away, and the Hubble constant is 80, leading to the conclusion that the universe is at most 12 billion years old. The uncertainty arises because it's unknown how tightly the universe is packed with matter; the gravity from a high density would have slowed the universe's expansion considerably by now, meaning that it could be closer to 8 billion years old. Most theorists think the density of matter is indeed high, though observers haven't been able to find most of it yet.

The scientists' method of calculating the universe's age is based on the assumption that it has been expanding ever since the Big Bang. If the age estimate is wildly wrong, then there could be a flaw -- possibly a fatal flaw -- in the Big Bang theory.

Several mitigating factors could prevent such a conceptual catastrophe. For one thing, the space-telescope astronomers acknowledge a significant margin of error in their calculations. It's not certain, for example, whether M100 lies right at Virgo's center or somewhere on the near or far edge. The Hubble numbers do broadly agree, however, with results announced last month by two other groups working with ground-based telescopes.

Another possible solution to the puzzle is that the cosmos contains much less matter than theorists like to think. But it's hard to understand how the galaxies and clusters of galaxies we now see could have evolved in a low-mass universe. There could also be, as some astrophysicists believe, a "cosmological constant," a sort of universal antigravity force that would make the universe look younger than it really is. Albert Einstein invented that concept as part of general relativity, then renounced it as "the greatest blunder of my life." It's still considered a long shot, but, says Princeton astrophysicist Ed Turner, "people are now going to start looking harder at cosmological constants again."

The task ahead is to make more precise estimates of the universe's age. If it seems close to 12 billion years, then reasonable adjustments to current theory may save the intellectual foundation of astrophysics. But if the age appears to be more like 8 billion, then the Big Bang may be shot.

With reporting by Dick Thompson/Washington