News in Science

Biggest ever dark matter survey about to begin in Australia

Measuring the universe Astronomers are about to embark on a project to measure the movement of a million galaxies, building the largest map of dark matter ever attempted.

The TAIPAN galaxy survey will measure the galaxies at an unprecedented speed, also providing scientists with the most accurate measurement of the rate of expansion of the universe.

"This will be about ten times larger than our best previous survey," says Professor Andrew Hopkins of the Australian Astronomical Observatory.

"By measuring the movement of individual galaxies, we can measure the gravitational influences they're subjected to and we can use that to measure the distribution of dark matter in the universe."

Scientists don't know what dark matter is, however they know it exists because they can see its gravitational influence.

TAIPAN will also allow astronomers to develop the most precise measurement yet of the expansion of the universe, known as the Hubble constant.

"The Hubble constant is the speed at which galaxies appear to be receding from us," says Hopkins.

Despite it's name, the Hubble constant is not constant. The current best measurement for it is about 72 kilometres-per-second, per-megaparsec. One megaparsec is about 3.2 million light years.

This means a galaxy 3.2 million light years away is moving away from us at 72 kilometres per second, and a galaxy 6.4 million light years away is moving away from us at 144 kilometres per second, and so on.

"Our best current measurements of the Hubble constant are accurate to within three to four per cent," says Hopkins.

TAIPAN will allow Hopkins and colleagues to measure more galaxies more quickly, allowing them to get a more precise figure for the Hubble constant.

"We think we can get our accuracy down to an error rate of just one per cent," says Hopkins.

"All our measurements of the most distant parts of the universe, the shape of the universe if you will, depend on assuming a value for the present day expansion of the universe.

"So this is a very important measurement because it helps to tie all the other cosmological parameters very precisely."

Measuring the effect of dark energy

The Hubble constant is constantly changing because of the effects of a mysterious force called dark energy.

"Those two parameters, the expansion rate, and the acceleration of the expansion rate, play a significant role in the way the universe is measured," says Hopkins.

From the middle of next year TAIPAN will begin its survey program measuring a million galaxies and two million stars.

TAIPAN is a new instrument developed at the Australian Astronomical Observatory for the UK Schmidt Telescope at Siding Spring.

TAIPAN measures the light coming from 150 individual galaxies simultaneously, using tiny robots called starbugs attached to the focal plane of the telescope.

"The starbugs move around independently of each other, and each places an optical fibre cable on the light from a galaxy which is projected onto the telescope's focal plane," says Hopkins.

"It takes just 15 minutes to gather the data and move on to the next 150 galaxies, depending on the type of measurements being taken.

"Earlier generations of surveys used a pick and place robot which took one fibre at a time and positioned it in turn, taking several hours for each measurement."

TAIPAN will dramatically speed this process up allowing astronomers to conduct their survey in just four to five years.