It's only precise, of course, if what astronomers believe they know about Cepheid variables holds up, since that was the yardstick used.

But evidence from other research appears to confirm the number. Which is good news. No more of 'Hey, how come there are stars around that are older than the universe???' That bogeyman has, hopefully, been finally put to rest.

Read on. Interesting, especially if you're one of those cosmology types always intrigued by the big picture.

http://www.nature.com/nsu/010607/010607-8.html

space: Eleven years of constant searching is over
TOM CLARKE

'The Hubble Space Telescope (HST) has completed its measurement of the Hubble constant, a measure of how fast the Universe is expanding and a cornerstone of our understanding of the age and size of the Universe. The final value of the constant seems to be just about spot-on.

'The finding marks the attainment of one of the HST's primary goals. "We are literally done," says Wendy Freedman, who has led the HST Hubble constant project since the telescope was launched 11 years ago.

'The US astronomer Edwin Hubble noticed in 1929 that the further a galaxy is from Earth, the faster it appears to be moving away. This idea of an expanding universe underpins the Big Bang theory, which says that the Universe began with an intense burst of energy and has been expanding ever since.

'Measuring the rate of the Universe's expansion requires calculating the acceleration of objects both near and far from us, and measuring the difference. Before the HST, the best estimates of the Hubble constant put the figure at either 50 or 100 kilometres per second per megaparsec. This number describes Hubble's discovery — an object's speed increases with increasing distance from the observer.

'The lower of these values meant that some objects in the Universe would be older than the Universe itself. But most astronomers believed that the higher figure was too big.

'Measuring the Hubble constant was one of the HST's primary aims because it can detect dim, distant objects that are not visible from Earth, such as many so-called 'Cepheid variable' stars. These stars 'pulse' between being dim and bright and their pulse rate is linked to their maximum brightness. Because this unique property of Cepheids makes their distance easier to confirm, they make ideal cosmic yardsticks with which to measure the Hubble constant.

'"Our figure falls right in between" the previous upper and lower estimates, says Freedman.

'After surveying Cepheids in 18 galaxies at various distances using the HST, and combining their data with other measurements from other estimates of the constant, her team calculated the Hubble constant to be 72 kilometres per second per megaparsec. "It's laid to rest a historic discrepancy and it's nice to have that behind us," comments Freedman.

'What's more, says Freedman, her team's result agrees well with other recent measures of the size and shape of the Universe, such those based on cosmic microwave background radiation. This approach has come up with an estimate of the expansion rate of the Universe by measuring the faint microwave afterglow of the Big Bang. "At last there's actually a consistent picture with a lot of different lines of evidence converging at the same point," says Freedman.

'According to Edward Kolb, a cosmologist at the University of Chicago in Illinois, the HST's analysis is widely accepted as being "the best and most reliable measure". But, he warns, "being consistent is not the same as being correct".

'Because the technique relies on Cepheid stars, if there turns out to be a problem with this approach, it could seriously weaken the HST's estimate of the Hubble constant. "It all rests on the same foundation," says Neil Tanvir, an astronomer at the University of Hertfordshire in Hatfield, UK. "If there's a systematic error the whole thing could move".'

Freedman, W. L. et al. Final results from the Hubble space telescope key project to measure the Hubble constant. Astrophysical Journal 533, 47–72 (2001).