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Astronomers size up the universe

cosmology 6df

The AAO's 6dF galaxy survey has helped scientists calculate how fast the universe is expanding(Source: 'The International Centre for Radio Astronomy Research)

Size matters Scientists have gone back to the drawing board and come up with the most accurate measurement of how fast the universe is expanding.

The research reported in the Monthly Notices of the Royal Astronomical Society will help cosmologists better understand how the universe is evolving and how it will change over time.

PhD student Florian Beutler from the International Centre for Radio Astronomy Research at the University of Western Australia and colleagues calculated the cosmic expansion by measuring the Hubble constant with greater accuracy.

"The Hubble constant is a key number in astronomy because it's used to calculate the size and age of the Universe," says Beutler.

Calculating this constant is difficult because the universe is expanding with galaxies and clusters of galaxies moving away from each other. To complicate matters a little understood force called dark energy is accelerating that rate of expansion.

According to Beutler, "This means the Hubble constant can only tell us what is happening now".

Standard candles

Beutler and colleagues believe current measurements on a cosmic scale are based on well-established assumptions, but are prone to systematic errors.

Distances are worked out using 'standard candles', special types of stars and supernovae explosions, which are known to give off a set level of brightness for their size. The fainter they are the further away they must be.

Beutler says there are too many variables with this method, so he decided to tackle the problem using a different method.

"We used data from a survey of 126,000 galaxies carried out with the Australian Astronomical Observatory's UK Schmidt Telescope", says Beutler.

Called the 6dF Galaxy Survey, this is the biggest survey to date of relatively nearby galaxies, covering almost half the sky.

"Measuring the distribution of all the galaxies in the survey (called the baryon acoustic oscillation) allowed us to compare density fluctuations now, with the distribution of photons in the cosmic microwave background radiation, an imprint of the universe 300,000 years after the big bang," says Beutler.

"By comparing the two we can see how things like dark energy have affected the expansion of the universe. That gives us a more accurate measurement for the Hubble constant."

"The new figure provides a scale that scientists can use to measure the properties of the universe like how fast it's expanding, how old it is, and how it could end."

Tags: the-universe, physics, stars, galaxies