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Moreover, the concept that underlies their calculations is utterly straightforward. Astronomers have known since Hubble's heyday in the 1920s that you need only two pieces of information to deduce the age of the universe: how fast the galaxies are flying apart and how far away they are. The ratio of these two numbers tells you how fast the cosmos is expanding (a rate known as the Hubble Constant; it's expressed, for those who insist on the proper terminology, in units of kilometers per second of recessional speed per megaparsec of distance). A simple calculation then tells you how long it's been since the expansion started. "There are these two loopholes, though," notes University of Oklahoma astrophysicist David Branch. "What's the right distance, and what's the right speed?"
These loopholes are big enough to drive the Starship Enterprise through. It's terrifically hard to measure how far away galaxies are. If they came in a standard brightness, like 100-W light bulbs, the astronomers could just figure that a dimmer galaxy was more distant than a bright one. Unfortunately, they don't. Edwin Hubble himself didn't realize this and triggered an earlier "age crisis" in the 1940s when he announced that the universe was 2 billion years old. Geologists already knew that Earth was older than that.
Astronomy's most reliable light bulb, or, to use the preferred and quainter term, standard candle, is a type of star called a Cepheid variable, whose inherent brightness can be easily calculated. But Cepheids can't be spotted more than a few galaxies away. And these nearby galaxies are virtually useless in filling in the other half of the equation-the expansion rate. Reason: in a universe that's expanding overall, neighboring galaxies are flying apart much more slowly than distantly spaced ones. Nearby galaxies are also subject to their neighbors' gravity. The Andromeda galaxy, for example, is being pulled closer to the Milky Way, despite the overall cosmic expansion.
Since accurate distances can be measured only nearby, while useful galaxies are found only deep in space, astronomers do the best they can to bridge the gap. They use the close galaxies to estimate distances to the faraway ones. But the method is inexact, which is why they haven't been able to agree on what the age actually is.
It's also why the Hubble Space Telescope was explicitly designed, at least in part, to find Cepheid-variable stars at greater distances than ground-based telescopes could. nasa and its scientific advisers figured that the deeper they could go into the universe before they had to switch from more to less accurate ways of gauging distance, the better. The Hubble's misshapen mirror delayed things for a while, but shortly after the spectacularly successful repair mission in December 1993, Freedman and her team focused the space telescope on a faraway galaxy called M100. "We could see right away that we'd be able to find Cepheids," she recalls.