Hubble Space Telescope (HST) astronomers have come up with the best estimate yet for the age of the Universe - 12 billion years.

This is younger than many scientists believed, although not a huge surprise. Astronomers will be relieved that it is similar to the ages of the oldest stars.

Wendy Freedman announces the findings of the research

Team leader Wendy Freedman, from the Observatories of the Carnegie Institution of Washington, said: "Before Hubble, astronomers could not decide if the Universe was 10 billion or 20 billion years old.

"We are finally entering an era of precision cosmology. Now we can more reliably address the broader picture of the universe's origin, evolution and destiny," she added.

Eight years' work

The international group of 27 astronomers measured precise distances to far-flung galaxies for eight years before arriving at the figure.

This is the key to estimating the age of the Universe because it reveals the rate at which the Universe is expanding. The rate is called the Hubble constant, named like HST after Edwin Hubble, who discovered that the universe is expanding.

Over 800 Cepheid stars were found

Calculating the Hubble constant was one of the major goals of the HST. The new figure is 70 kilometres per second per megaparsec. (A megaparsec is 3.26 million light years.)

But measuring the distances was not easy. The astronomers observed 18 galaxies and 800 Cepheid "variable" stars. The latter are a rare class of pulsating star which provides a reliable yardstick for measuring cosmological distances.

The Cepheids have been called the mileposts of the Universe. Their brightness waxes and wanes on a regular timescale, which depends upon the actual brightness of the star.

The astronomers strain the HST's optics to the limit to find a dim Cepheid in a distant galaxy. By comparing the brightness they see with the actual brightness they calculate, it is possible to estimate its distance.

Other factors must be taken into account such as any obscuring gas or dust in front of the star.

All the evidence fits

Astronomers can also estimate the Hubble constant by observing other objects in space, such as exploding stars or Supernovae.

It is encouraging that these Hubble constant estimates are almost identical to the Cepheid approach.

Charles Lineweaver: "Astronomers' data is getting much more precise"

A very different approach to measuring the age of the Universe is presented in the journal Science by Charles Lineweaver of the University of New South Wales in Australia.

He used recent observations of the cosmic microwave background, radiation that fills the Universe and has been likened to an echo of the Big Bang.

His estimate is that the cosmos is 13.4 billion years old, give or take 1.6 billion years. This overlaps with the 10% uncertainty in the 12 billion-year age from the HST work.

The dark force

Astronomers say that their perception of the Universe has changed a great deal in the past decade or so. Observations of the most distant reaches of the cosmos suggest that things are not as simple as many once believed.

Also writing in the journal Science, Neta Bahcall of Princeton University and colleagues say that the evidence is forcing scientists to the conclusion that some strange "cosmic dark energy" exists that works against the expansion of the Universe.

This is because when astronomers look further and further into the depths of space, they see that the expansion of the Universe seems to be accelerating.

This means there must be an extra force that can be felt only on the largest scales, pushing galaxies away from each other. Understanding this force will be one of the major tasks of science in the new century.