ZEROING IN ON A COSMIC NUMBER

Astronomers have arrived at the most precise measurement ever of the Hubble constant, the key to the secrets of the universe

Astronomers have arrived at the most precise measurement ever of the Hubble constant, the key to the secrets of the universe
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 a number known as the Hubble constant that has led astronomers on a merry chase for three-quarters of a century.

8220;It is the most fundamental number in cosmology,8221; said Adam Riess, 38, an astronomer at the Space Telescope Science Institute and Johns Hopkins University, who discovered 10 years ago that 8220;dark energy8221; is speeding up the expansion of the universe. This spring, Riess announced that he and his comrade, Lucas Macri of the University of Texas, had used the Hubble Space Telescope to make the most precise measurement yet of this parameter.
Hubble constant, Riess reported, is 74 km 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 km per second faster.

The news was not in Riess8217; 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 per cent. Only 30 years ago, distinguished astronomers could not agree within a factor of two on the value of Hubble8217;s constant, leaving every other parameter in cosmology uncertain.

8220;I8217;m not saying we8217;re going to get to 1 per cent,8221; Riess said, 8220;but we might.8221; 8220;I think Adam8217;s work is nice,8221; said Freedman, who has led a large space telescope effort to measure the constant. But she and others added that some parts of Riess8217; scheme could be vulnerable to systematic errors.
The stakes are bigger than just dark energy. Cosmologists would like to know if the universe is in fact 13.7 billion years old, full of dark matter and dark energy, and speckled with galaxies that grew by gravity from random microscopic fluctuations in the Big Bang. That universe is described mathematically by half a dozen fundamental parameters, from which the Hubble constant can be calculated. But to test the model 8220;at a physically interesting level8221;8212;in the words of John Huchra of the Harvard-Smithsonian Center for Astrophysics8212;the Hubble constant needs to be actually measured to high accuracy.

Both the telescope and the 8220;constant8221; are named after Edwin Hubble, the Mount Wilson astronomer who discovered in 1929 that the universe was expanding. It is not really constant. Over cosmic time, gravity tries to slow the expansion while dark energy tries to speed it up. The history of the Hubble constant has seen many hopeful beginnings that have subsequently foundered. Allan Sandage, also of Carnegie Observatories, who inherited the universe when Hubble died in 1953 and has been measuring and remeasuring the Hubble constant ever since, likes to say that astronomy is an impossible science.
According to a compilation by Huchra, more than 500 values of the Hubble constant have been published over the years. In recent years the two main teams using the Hubble telescope to measure the constant, one led by Freedman and the other led by Sandage, have arrived at answers 15 per cent apart, 70 and 62, respectively, with 10 per cent error bars that slightly overlap.
_DENNIS OVERBYE, NYT