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Gravitational wave observatories to join forces

Detecting ripples in space-time is a step closer to reality now that the world's most sensitive observatories have joined forces. The collaboration boosts the chances that gravitational waves could be detected in the next four years.

Gravitational waves are ripples in space-time that expand outwards at the speed of light from violent events like supernovae and mergers of pairs of black holes and neutron stars.

Scientists have built sensitive experiments for detecting such waves. Using lasers, they measure the length between mirrored test masses hung inside two tunnels at right angles to each other. Gravitational waves decrease the distance between masses in one arm of the tunnel and increase it in the other by a tiny, but theoretically detectable, amount.

Now, the world's leading gravitational wave observatories are joining forces to improve their chances of making a detection.

The agreement brings together the Laser Interferometer Gravitational Wave Observatory (LIGO) based at Hanford, Washington, and Livingston, Louisiana, both in the US; the Virgo observatory, based near Pisa, Italy; and the GEO 600 observatory near Hannover, Germany.

Triangulating data

The agreement means the three projects will pool their data to analyse it jointly, boosting their chances of spotting a faint signal that might otherwise be hidden by detector noise, says LIGO scientist Peter Saulson of Syracuse University in New York, US.

Combining the data will also make it possible to triangulate to find the source of any gravitational waves detected.

None of the detectors have detected gravitational waves so far and it is difficult to calculate the chances for a detection with current observatories. A merger of a nearby pair of stellar-mass black holes would be detectable, but astronomers are not sure how often these events occur. Core-collapse supernovae should also produce gravitational waves, but physicists are very uncertain about how strong the resulting waves would be.

Astronomers do have a good handle on both the rate and the strength of gravitational waves for mergers of two neutron stars, however. They believe LIGO could see such an event out to a distance of about 50 million light years. But it appears that no such events have occurred that close since LIGO has been watching.

'Bunch of events'

A much improved version of LIGO, called Advanced LIGO, could extend the neutron star merger detection range out to about 650 million light years from Earth - "enough so that we do expect that a year's worth of observing would have a bunch of events in it", Saulson told New Scientist

Preparatory work for this upgrade will begin in 2008 if the US National Science Foundation's proposed 2008 budget, which includes $33 million for Advanced LIGO, is approved.

But the upgrade would not be completed until 2014. And at least a couple of years of calibrating and testing beyond that would likely be needed before science observations could begin, Saulson says.

By joining forces, LIGO and the two other projects are boosting the chances that gravitational waves will be detected before LIGO shuts down to begin the upgrade work. If funding goes ahead as planned, the shutdown would happen in late 2010 or early 2011.

Sensitivity upgrade

Virgo needs to finish some minor improvements to reach the same sensitivity as LIGO before the data sharing will begin, says Virgo scientist Carlo Bradaschia of the Istituto Nazionale di Fisica Nucleare (INFN) in Pisa, Italy.

"Starting in May, most likely, we think we will be able to reach LIGO [sensitivity] and start the effective full sharing of data," he told New Scientist.

More modest upgrades to both LIGO and Virgo planned in 2008 will boost the sensitivity of each by about a factor of two.

Another gravitational wave observatory called the Laser Interferometer Space Antenna (LISA) could launch late in the next decade. A joint mission of NASA and the European Space Agency, it would be sensitive to a different range of frequencies than any of the ground-based observatories. It would detect waves with frequencies of less than one cycle per second, allowing it to detect the merger of supermassive black holes at the centres of other galaxies.


The LIGO project's two observatories, including this one in Hanford, Washington, US, will join forces with the Virgo observatory in Italy and Germany's GEO 600 to hunt for gravitational waves (Image: LIGO Laboratory)


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