Australian researchers played a critical role in Gravitational Wave Discovery

Feb 11
Robert Ward

Scientists from the Australian Consortium for Interferometric Gravitational Wave Astronomy (ACIGA) made significant contributions to the discovery of gravitational waves from a binary black hole merger– both to the development of the detector technologies and to the sophisticated data analysis. The six ACIGA institutions each made individual contributions as described below.


The ANU contributed a critical piece of hardware, called the Arm Length Stabilisation system. This system is required to “turn the detector on”. To sense the gravitational waves we attach mirrors to two masses separated by 4 km and bounce laser light between them. We use the laser light to measure where the mirrors are with respect to each other, and this lets us record any changes in their separation induced by a gravitational wave. There is a problem though, in that our system to measure the separations, while incredibly precise, only works when the mirrors are very near (closer than the size of a single atom) to their ideal ‘operating’ positions. Since the ground, and indeed everything around, is already moving much more than that, we must actively, gently hold the masses in place such that we can measure their movements while letting them remain free enough to respond to gravitational waves. In fact the problem is much harder than this. If the masses are not already very very near their ‘operating’ positions, our system to measure where they are, and which we would use to hold them in place, doesn’t work at all! Initially the masses are nowhere near their ‘operating’ positions, and so we must find a way to locate them precisely in order to bring them to their operating point. In LIGO there are five relevant mirror separations to measure, so this is a very complex and coupled control problem. The technical term for this is lock acquisition. At ANU we solved this by introducing a second, different coloured laser beam. The main (science) laser used to record the signal is infrared (1 micron). The laser we use to determine the position of all the optics in the first place is green (532 nm). The green laser system is not precise enough to measure gravitational waves directly, but it allows us to lock mirrors in groups of 2 and then deterministically move them all into ‘resonance’ with the science laser. This process has been automated so that now should something like a major earthquake cause the mirrors to drop out of this ‘locked state’, our green system clicks into action and sets up the system again. Because we can get back into the ‘locked state’ quickly, this maximises the ‘duty’ cycle, ie how many hours a day our interferometer is searching for gravitational waves. This was also one of the reasons why the commissioning of aLIGO has been so much quicker than its predecessor initial LIGO – knowing that the complicated lock acquisition process is fully automated makes it easier to perform the necessary tests to improve sensitivity.

Funded by the Australian Research Council (ARC), we designed, constructed and tested the system in the ANU facility. ANU researchers, working with onsite staff installed and commissioned this system on the LIGO interferometers in the USA, culminating in the first lock being achieved in May 2014, with the first automated lock shortly thereafter in June 2014.

Also funded by the ARC, ANU also designed, built, installed and commissioned 30 small optics steering mirrors for routing the signal laser beam around inside the detector.

The ANU group took a central role in the LOOCUP project, which has now evolved into the electromagnetic follow-up program, comprising the LSC and Virgo with international collaborators. The aim is to search for burst gravitational wave sources using a multi-messenger astronomy approach, providing electromagnetic confirmation of a gravitational wave event. The group has been involved in the important quest to analyse coincident event triggers from the LIGO-Virgo interferometer network in close to real-time and has contributed to the construction of the data pipeline. Their emphasis is on using the SkyMapper optical transient telescope which played an important role in the recent first observing run of Advanced LIGO.

The future:

Advanced LIGO is now close to a level of performance where how well we can measure a GW induced change will be limited by inherent quantum noise accompanying the laser light used for sensing! In the quantum world, trying to measure or observe an object disturbs its state reducing the accuracy of the measurement. Again funded by the ARC, we at the ANU are in the process of developing a system called a ‘squeezed light generator’, which can be installed on Advanced LIGO in the future. With this system we can beat even these quantum limits, opening up the observable GW universe and thus the GW event rate by another factor of 30.

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Researchers at UWA determined, before Advanced LIGO was constructed, that a runaway processs known as a ‘parametric instability’ could be a problem that would limit the performance of the detectors, especially at high laser powers. UWA researchers investigated solutions and undertook detailed modeling and simulation to ensure that parametric instabilities would not limit Advanced LIGO. The parametric instabilities have been seen in Advanced LIGO, and thanks to UWA researchers, solutions were ready to be deployed. UWA researchers also make significant contributions to the data analysis. We ran an independent analysis of the data to verify the signals, and we searched the sky with our Zadko robotic telescope to see if there was any explosion visible in light. We have also developed the most sophisticated models and methodology for studying a stochastic background from binary black holes – the situation that would occur if there where many, many binary black holes, with most of them being too small to observe directly, but the sum of all them contributing to a background ‘noise’.

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The Advanced LIGO detectors must use very powerful lasers to detect the minute signals produced by gravitational waves. Unfortunately, even though the mirrors in the detectors are the best available, they still absorb a very small part of the laser power, which heats the part of the mirror through which the laser beam propagates. This part expands slightly and the speed with which light travels through it decreases slightly, which results in distortion of the laser beam. Since the LIGO detectors need to detect minute changes in the position of the mirrors, they are also very sensitive to these small distortions. So, the distortions need to be measured and their effect removed.

The University of Adelaide developed an ultra-high precision Hartmann wavefront sensor that can measure these distortions, and can do so when the distorted mirrors are buried deep within the LIGO vacuum envelope.

Using funding from the Australian Research Council (ARC), we developed, constructed and installed 16 Hartmann wavefront systems at the LIGO detectors. These systems include a large computer program that communicates with the CCD cameras, and records and processes the images from the camera to produce maps of the distortion. This information is then be used to drive an adaptive optics system within the interferometer, thus compensating for the effect of the distortion.

The future:

As the sensitivity of the Advanced LIGO detectors is improved, the detectors will become even more sensitive to beam distortion. The University of Adelaide group is currently funded by the ARC to develop next-generation wavefront sensors that could be used to more fully understand the beam distortion within the interferometer and adaptive optics systems that can be used for compensation. Such systems will be essential for the continued improvement of gravitational wave detectors and the realisation of gravitational astronomy.

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Monash University played a leading role in the design and implementation of key hardware and software components associated with the detection and interpretation of GW150914: the first ever observation of gravitational waves. Monash researchers also made critical contributions to the way we understand and model fundamental thermal noise, one of the most important noise sources that limits the sensitivity of gravitational wave detectors.

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The University of Melbourne analysed LIGO data on massive supercomputers to hunt for persistent signals from neutron stars, some of the most extreme objects in the Universe.

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Charles Sturt University has contributed to detector characterisation, validation of the calibration of the instruments and development of the detection pipeline for the stochastic background of gravitational waves.

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