That Genesis Moment
It has been one of the most bitter debates in astronomy: at what speed is the universe expanding? Ever since Edwin Hubble discovered in 1929 that the universe is growing, its galaxies carried along in a rush of space-time like popcorn kernels in a rush of hot air, scientists have reacted with a, "Wow--how fast?" The rate of expansion is called the Hubble constant, and it became such a holy grail that the quest to discover it took on the overtones of a religious war. For decades the establishment asserted that the answer is about 50 (don't worry about the units for now), while upstarts arrived at an answer of 100. The smaller number suggested that the universe is 20 billion years old and the larger number implied that the cosmos is a mere stripling of 10 billion. Astronomers aligned with one camp or the other, depending on who their friends were; papers challenging the extremes were quashed; the chief defender of the orthodoxy accused his rival of "gimmicks," and the challenger asserted that if he had made as many mistakes as the grand old man he would have retired. And those were the friendly exchanges. "There are only a handful of people in this game," astronomer John Huchra of Harvard University said in the 1991 book "Lonely Hearts of the Cosmos," "and they all hate each other."
The game is still competitive, but last week astronomers took a big step toward shaking hands. Researchers led by Wendy Freedman of the Carnegie Observatories in Pasadena, Calif., announced that they had pointed the orbiting Hubble Space Telescope at 770 stars in 18 galaxies out to 65 million light years, and deduced a Hubble constant of 70. Now for those units: "70" means that, for every 3.26 million light years away from Earth, galaxies are speeding away at an extra 70 kilometers (42 miles) per second. The farther out you look, the faster the universe is expanding, like a humongous inflating balloon. The rate of expansion is the key to determining the age of the cosmos; a rate of 70 implies an age of 12 billion years. "This has been a marathon," says astronomer Jeremy Mould of Australian National University, a co-leader of the Hubble team. Now "we've [broken] the tape."
But it may be premature to name a winner, for the problems that turned the search for the Hubble constant into the Hubble War persist. To calculate the Hubble constant--which is, after all, just speed per distance--astronomers need to know how far away a bunch of celestial objects are, and how fast they are rushing away. Neither calculation is straightforward. Which fudge factors astronomers throw into their equations reflects art and personal predilection as much as science. "It becomes almost a matter of taste, of esthetics," says astrophysicist Keith Ashman of Baker University in Kansas--of how to account for cosmic dust that dims objects, for example. Although astronomers have come a long way from the days when one camp insisted the answer was 50 and its rivals wouldn't budge from 100, disputes remain. For instance, Freedman's group measured galaxies as far as 65 million light years away, "but that's only 10 percent of the universe," notes Robert Kirchner of Harvard. "They're getting only the local rate of expansion," which may differ from the expansion far away and long ago. If the universe has been speeding up lately--and observations of supernovas suggest so--then the Hubble constant is less than 70.
Calculating the age of the universe is even more of an art. The rate of expansion gives you a rough guide as to how long it has taken the cosmos to grow as big as it has; Freedman's group has settled on 12 billion years, give or take. But that calculation rests on hidden assumptions. One is that the universe has a certain density. Another is that it is not pervaded by a ghostly force that acts like antigravity. Different assumptions imply that the cosmos is not 12 billion but as much as 15 billion years old.
Still, arguing about 12 or 15 billion rather than 10 or 20 billion years is progress. But the quest for the moment of genesis is far from over. At the end of "The Hitchhiker's Guide to the Galaxy," the 1989 sci-fi classic by Douglas Adams, the supercomputer announces that the secret of "life, the universe and everything" is... 42. Now astronomers have a number: 70. The hard work is figuring out what it means for the ultimate fate of the universe.