A group of astronomers announced today that they have finally nailed down the so-called Hubble constant, the rate at which the universe is currently expanding. Combined with the other object of cosmologists' search, the universe's density of matter and energy, the Hubble constant gives the age of the universe. The findings will be described in this week's issue of Science.
Astronomers have been trying to measure the Hubble constant since 1929, when Edwin Hubble found the first evidence that the universe is expanding. By comparing crude estimates of galaxies' distances with their velocities--easily measured from the "redshift" of their light--he found that the galaxies farthest from us are moving away the fastest. Hubble later estimated that the universe expands at 558 kilometers per second per megaparsec (3.26 million light-years) of distance. Unfortunately, that would make it younger than Earth's rocks. In recent decades better distance measures have led to more plausible results, but some estimates still pointed to a universe younger than its oldest stars, leading to talk of a cosmic "age crisis."
The Key Project of the Hubble Space Telescope aimed to settle the differences by remeasuring the Hubble constant using various methods. Each method relies on objects with known true brightness, called standard candles; their apparent brightness as seen from Earth then indicates their distance. The best standard candle is a kind of precisely varying star called a Cepheid variable, which brightens and dims in a period that depends on its true brightness. Cepheids can't be seen at distances great enough to give a good measure of cosmic expansion, but the Key project used them to calibrate other, longer range standard candles, such as Type Ia supernovae and certain types of galaxies. Combining all the candles "gets the systematic errors to cancel out," says one of the team leaders, Wendy Freedman of the Carnegie Observatories in Pasadena, California, and gives an overall Hubble constant of 70. "The uncertainty is 8%--it's what we designed the Key Project to do."
Not everyone is persuaded. "There's still a controversy," says Alan Sandage, also at the Carnegie Observatories, who has spent a distinguished career looking for the value and is holding out for a slightly slower expansion rate. Whether the number is 70 or a bit lower, it won't provoke another age crisis. Astronomers now believe that the universe's density of matter is low and its expansion is speeded up by an energy, called the cosmological constant, that pervades empty space. Both factors would push up the age of the universe with a Hubble constant of between 60 and 70 to around 13 billion years. The oldest stars are between 11 and 14 billion years old. Says Freedman, "There's still some tension, but there's no crisis."