The Struggle to Measure Cosmic Expansion

By DENNIS OVERBYE
Published: August 18, 2008
Hoping to understand why the universe seems to be coming apart at its seams, a young astronomer and his colleagues have embarked on one of the oldest quests in cosmology, to measure how fast the universe is growing, how big it is and how old it is.

That information is encoded in the value of an elusive number known as the Hubble constant that has led astronomers on a merry chase for three-quarters of a century. “It is the most fundamental number in cosmology,” said Adam Riess, 38, an astronomer at the Space Telescope Science Institute and Johns Hopkins University, and one of the discoverers 10 years ago that some kind of “dark energy” is speeding up the expansion of the universe.

This spring, in what he called “a triumph of metrology,” Dr. Riess announced that he and his comrade, Lucas Macri of Texas A&M University, had used the Hubble Space Telescope to make the newest and most precise measurement yet of this parameter.

Expressed in the quaint terms astronomers favor, the Hubble constant, Dr. Riess reported, is 74 kilometers per second per megaparsec. It means that for every additional million parsecs (about 3.26 million light-years) a galaxy is from us, it is going 74 kilometers per second faster.

The news was not in Dr. Riess’s value, which, reassuringly, agreed roughly with the result from an earlier space telescope team led by Wendy Freedman, the director of Carnegie Observatories, and with calculations based on measurements of relic radiation surmised to be left from the Big Bang, but in the precision with which his group claimed to have measured it: an uncertainty of only 4.3 percent.

Only 30 years ago, distinguished astronomers could not agree within a factor of two on the value of Hubble’s constant, leaving every other parameter in cosmology uncertain by at least the same factor and provoking snickers from other fields of science.

But this is the age of so-called precision cosmology.

“I’m not saying we’re going to get to 1 percent,” Dr. Riess said, “but we might.”

more:

http://www.nytimes.com/2008/08/19/science/space/19hubble.html