The Cosmic Dating Game
FOR DECADES, COSMOLOGISTS HAVE failed to nail down the age of the universe more precisely than to say it's somewhere between 7 billion and 20 billion years-not a pretty sight in a profession that prides itself on knowing the ineffable down to the last decimal place. But now the researchers are closing in on the cosmic age. And things are looking even worse. The most recent age estimates are less than the age of some stars--which is like having 6 billion-year-old fossils in hand and then inferring that Earth is 4.8 billion years old. Last week Wendy Freedman of the Carnegie Observatories and 18 colleagues announced, in the journal Nature, that observations with the refurbished Hubble Space Telescope imply that the universe is 8 billion years young. Since some stars are 16 billion years old, says astronomer George Jacoby of the National Optical Astronomy Observatories in Tucson, Ariz., cosmology is "in a time of crisis."
Last week's low estimate is no fluke. Astronomers have battled over the age of the universe ever since Edwin Hubble discovered, in 1929, that the universe is expanding and so must have originated in a big bang. Since then, calculations of how long ago this explosion of space-time occurred have see-sawed between 7 billion and 20 billion years. Recent estimates have tended toward a young cosmos. Just last month six astronomers led by Michael Pierce of Indiana University reported observations of a galaxy in Virgo, using a telescope on Mauna Kea, which suggest that the universe is a mere 7.3 billion years old.
The "age crisis" means that at least one dearly held premise of astronomy is in trouble. It could be that the oldest stars aren't really as old as they appear. "But there is no evidence that there is a problem with our estimates of stellar ages," says Pierce. More likely, the relationship between the age of the universe and the expansion rate that the telescopes actually measure (diagram) is wrong; A fast expansion supposedly implies a youngish universe: if the universe is ballooning out rapidly, it took less time to reach its current size.
But what if the universe is pervaded by a ghostly force that acts like antigravity? The push from this force would fool astronomers into believing that the universe got to its present size in less time than it really took. The effect is akin to measuring the average speed of a race-walker without realizing that she sprinted when no one was looking. (Einstein once postulated such a "cosmological constant," then called it the worst idea of his life. But it would be just like Einstein that one of his few mistakes was to think he made a mistake.) Although cosmologists hate the idea of such a force- it mucks up their equations- "our job is to listen to what nature is telling us and not impose our own esthetics," says astronomer Sidney van den Bergh of Dominion Astrophysical Observatory in British Columbia. Invoking the cosmological force has another effect. It almost surely means that the universe will expand forever, its ultimate end not the fire of a big crunch but the ice of eternal, infinite space.
Don't look for a resolution to the age crisis tomorrow. As van den Bergh describes the quest, quoting Mark Twain: "The researches of many commentators have already thrown much darkness on this subject, and it is probable that, if they continue, we shall soon know nothing at all about it." Cosmology isn't that badly off, but there's sure to be more darkness before there is light.
To find out, astronomers combine observations and Inferences. They know the universe is expanding like an inflating balloon, so more distant regions are receding faster that closer ones. The faster the expansion, the less time it took for the universe to reach its present size. Thus measuring the expansion reveals the universe's age.
To find the expansion rate, astronomers determine the speed at which remote galaxies are fleeing. They do this by measuring the colors of light emitted by the galaxy. This spectrum looks like a simple grocery bar code. Astronomers compare the position of the bars to those in a known spectrum. The amount by which the bars are displaced is called the red shift. The shift is caused by the galaxy's recession: just as an ambulance siren decreases in pitch as it speeds away from a listener, so light from a galaxy speeding away from Earth also shifts. The size of the red shift is directly related to the recession speed.
Next, astronomers calculate the distance to the galaxy. With a telescope they measure the apparent brightness of rare pulsing stars called cepheids. They compare that to the cepheids' true brightness: there is a strict relationship between how often a cepheid pulses and how bright it is. Comparing true brightness to apparent brightness yields the distance.
Finally, they calculate the recession rate at the given distance. This is the Hubble constant (H[sub 0]), expressed as speed per unit of distance away. H[sub 0] is plugged into a formula to yield the universe's age. The higher the H[sub 0]--calculated last week as about 80-the younger the universe.