Fewer than 70 years have passed since astronomers discovered that the universe is expanding. It’s been only 30 since they first detected the cosmic microwave background--the feeble, universe-wide glow that is the main evidence for the Big Bang. Yet for most of that time it has seemed as if they were making steady progress. Like the universe itself, our understanding of it, having begun with a bang in the twentieth century, seemed to be expanding constantly, rapidly, and inexorably. One could almost imagine a time when we would have figured it all out.
These days that goal looks distant indeed; these days cosmology seems to be collapsing in on itself. In the past few years two new discoveries have come along, observations so disconcerting that they threaten to shake the field to its foundation. The first is that galaxies in a huge chunk of the universe, a region at least 1 billion light-years across that includes our own Milky Way, appear to be moving, all in the same direction, at about 435 miles per second, or 1.56 million miles per hour. Even if you’re a cosmologist and are used to such ridiculous distances and speeds, you have trouble explaining that observation. The only force that could set such a large fraction of the universe in motion is gravity--and there just isn’t enough mass around to generate that much gravity. Even when cosmologists resort to their favorite deus ex machina and say that 90 percent of the universe is made of some invisible dark matter--identity unknown but full of gravity--they can’t get that many galaxies to move that fast.
The second observation, made with the newly repaired Hubble Space Telescope, is even more startling. The Hubble has allowed astronomers to make the most credible measurement to date of the age of the universe--of how long it’s been since the Big Bang. They have found that the universe is somewhere between 8 and 12 billion years old. Yet there is almost no doubt that the oldest stars in the Milky Way, which live in the globular star clusters that orbit the galaxy’s central bulge, are at least 14 billion years old, and probably even more ancient than that. A universe younger than the stars it contains is, to say the least, a fundamental contradiction.
Too little mass, too little time--either problem alone would be disturbing. Taken together they raise the specter of a scientific revolution, a shift in the cosmological worldview in which some fundamental assumptions in cosmological theory--perhaps even the Big Bang itself--will have to give. It would be premature to panic, says David Weinberg, an astronomer at Ohio State University. But if these results are confirmed, we theorists will be in trouble. We really have no good ways of explaining these observations.
It would be one thing if the observers in question were inexperienced or untrustworthy. In fact, they’re quite the opposite. Take the age-of-the-universe measurement. The lead astronomer on the 14-member team that made the discovery is Wendy Freedman, of the Carnegie Observatories in Pasadena, California. Few observers in the business are more respected for care and competence than she is--and several of those are on her team.
