"After years of fractious debate, astronomers still don't know the answer."
Vol. 146 page 232
SEARCHING FOR COSMOLOGY'S HOLY GRAIL
HUBBLE TELESCOPE JOINS A CONSTANT BATTLE
By RON COWEN
Grown-ups love figures. When you tell them that you have made a new friend, they never ask you any questions about essential matters. They never say to you, "What does his voice sound like? What games does he love best? Does he collect butterflies?" Instead, they demand' 'How old is he?" - Antoine de Saint Exupery, The Little Prince
How old is the universe?
After years of fractious debate, astronomers still don't know the answer. Some believe the universe is 10 billion years old, others argue that it's closer to 20 billion. At the center of the controversy lies a number that has obsessed astronomers for decades - the Hubble constant.
The Hubble constant represents a measure of the rate at which the universe is expanding - how rapidly each object in the universe speeds away from any other object. Armed with this knowledge, scientists can estimate the age of the cosmos - how long since the Big Bang it has taken galaxies to reach their current locations.
The trouble is, no one can agree on the size of this constant. At best, astronomers have pinned the number down to within a factor of 2. Based on conflicting sets of observations and personal prejudices, two camps have sprung up since the 1970s. Several groups of researchers, using different measurement methods, favor a high value for the Bubble constant. This suggests a relatively small, young universe - one that began its expansion about 1O billion years ago. Others argue for a low Hubble constant, implying a cosmos about twice as big and twice as old.
Many researchers are hoping that the recent arrival of another Hubble - the Hubble Space Telescope - may resolve the controversy. Last December, the telescope got a new pair of eyeglasses and a new camera with built-in optics to correct for Hubble's notoriously flawed primary mirror. The corrective optics enable the telescope to produce sharp images of individual bright stars in galaxies 1O times farther from Earth than had been possible before. Three independent teams are now using the Earth orbiting observatory to measure the Bubble constant. On Oct. 27, one of the groups will report some of the first results.
No one, including members of that research group, expects the findings to quell the controversy anytime soon. But over the next few years, observations with the repaired telescope may bring the war over the Hubble constant to an end.
"The Hubble Space Telescope is going to put the debate on a whole new level," says astrophysicist Robert R Kirshner of Harvard University.
For a number that's the focus of so much controversy, the Hobble constant is conceptually simple.
In 1929, U.S. astronomer Edwin Hubble found compelling evidence that we live in an expanding universe. In particular, he made the remarkable discovery that when viewed from Earth, every distant galaxy appears to be moving away from our home galaxy, the Milky Way.
Hobble found that the more distant the galaxy the faster it is receding. For example, consider two galaxies, one of which lies twice as far from the Milky Way as the other. The galaxy that resides twice as far away will appear to move away twice as fast. (According to general relativity theory, the galaxies themselves don't move; rather, the fabric of space in which they are embedded expands.)