19 June 2015

The hunt for gravitation

al waves—ripples in space-time—is the continuation of a search that began when a human first cupped a hand to their ear to hear a distant sound.

By tuning into the electromagnetic spectrum with telescopes we can now probe deep space, and glean information from gamma rays, light and radio waves that are from celestial objects in distant galaxies. 

But we are still deaf to the deep primordial boom of gravitational waves, with which the universe is predicted to resound. Gravitational waves contain the echoes of the very first fraction of a second after the big bang, and the shout of the most violent collisions of distant dying stars.

The Advanced Laser Interferometer Gravitational wave Observatory (aLIGO) is the most sensitive piece of equipment ever built in the hunt for these waves. Based in the USA, it is part of an international collaboration of researchers who have the expectation that this new way of sensing the universe will cut through the fog of light and give access to the raw bones at the core of the cosmos.

“Australia has been developing expertise in the detection of gravitational waves since the 1970s, beginning in David Blair’s group at the University of Western Australia (UWA) with early work using resonant bar detectors,” said Professor David McClelland, Head of the Department of Quantum Science at the Australian National University (ANU) and Chief Investigator of the Australian Advanced LIGO partnership.

The Australian Research Council (ARC) has directly supported Australia’s participation in the aLIGO project since 2009, through two Linkage Infrastructure Equipment and Facilities (LIEF) grants to the ANU and The University of Adelaide, which have provided a total of $2.79 million to the project.

“We now have partner status in Advanced LIGO through our expertise and these ARC LIEF grants,” Professor McClelland said.

Whereas most of the ARC’s LIEF grants are only for a single year, aLIGO is one of the few projects awarded multiple year funding. This has allowed the team time to install and fine tune equipment at the USA facilities in Washington State and Louisiana.

The extreme sensitivity of the instruments required for aLIGO has presented huge technical challenges for the researchers working on the project. aLIGO requires several complex noise reduction systems to detect the miniscule distortions in space-time. The movements are identified by reflecting laser beams off mirrors at the end of four-kilometre vacuum tunnels, in a technique called laser interferometry. 

“As a result of pioneering work conducted here in Australia and overseas, laser interferometry has gradually been improved to the point that motion is detectible to the size of 10^-19 metres—which is like being able to detect the sun moving by the width of a human hair in the distance to Alpha Centauri.”

“What we are actually measuring are distortions in space-time, but it is probably simpler just to say we are watching for a wobble in the mirrors.”

At sensitivity levels this high, there are all sorts of earth-based events that can make the mirrors wobble unpredictably.

“We can easily tell if someone drops a spanner on the floor,” said Dr Bram Slagmolen, ARC Future Fellow and Chairperson of the Australian Consortium for Interferometric Gravitational wave Astronomy (ACIGA)1.  Bram led the ANU team that designed and calibrated laser measurement equipment at the aLIGO. He is also working on other applications of gravitational force sensing for future versions of LIGO.

“Earthquakes will disturb the operations of the LIGO interferometers. This has led to an interesting offshoot of the research that we are doing with these amazingly sensitive interferometers. A new sensor will detect changes in the rock density at the quake’s epicentre, transmitted at the speed of light to our detector, before the actual seismic waves hit.”

“This buys us vital seconds which could have uses in shutting down nuclear reactors or train systems before the ground begins to shake, for instance. It also means we can calibrate LIGO to expect them and cancel out the signal or shutdown the experiment.”

A team at The University of Adelaide has developed a sensor to measure the deformations caused in the Advanced LIGO’s mirrors by 800kw of incident laser power. 

“We have increased the sensitivity of the instrument by turning up the power on the laser, but with the increased power, there are also heat effects which need to be counteracted,” Professor McClelland said.

Higher laser power also increases the jiggling of the mirrors that are under bombardment from photons that make up the beam. This can create parametric instabilities and increase quantum noise.

ARC funding through Discovery Projects grants has supported research at UWA into mitigating these instabilities and research at ANU into quantum noise reduction using ‘squeezed light’. Squeezed light technology is now being incorporated into interferometers and other highly sensitive optical devices around the world.

And what of the possibility of discovering gravitational waves?

“By the end of the year we think there is a 10% chance of something showing up in the range that we can detect; and as the instrument is improved to be more sensitive through next year, we should begin to see gravitational waves.”

“Gravitational waves will provide important new information about known astrophysical and cosmological sources, but what Ι really hope is that we discover something totally unexpected.”

For more information please contact Professor David McClelland.

1Membership of ACIGA includes ANU, University of Adelaide, UWA, and the Universities of Melbourne and Monash along with researchers from the CSIRO.


Image: The ANU system is installed at LIGO.
Image courtesy: LIGO.