Gravitational Wave detectors

Gravitational Wave detectors

Since its inception the IGR has been involved with the design and construction of gravitational wave detectors, both at Glasgow and elsewhere. We research fundamental physics problems relevant to the detectors, and also manufacture critical components for them. The detectors we contribute to are described here.

Advanced LIGO

Advanced LIGO (aLIGO) consists of interferometric gravitational-wave detectors at two sites, one in Hanford (Washington State, USA) and one in Livingston (Louisiana, USA). We developed some critical elements of the design, including quasi-monolithic fused silica suspensions and signal recycling. We are part of the the Advanced LIGO UK project team, which developed, built and delivered all the main suspensions and associated sensors, actuators, electronics for the detectors. We are also involved in the continuing upgrading, commissioning and operating of the aLIGO detectors.

In 2015 Advanced LIGO opened the era of advanced gravitational wave detectors and carried out the first two observing runs "O1" and "O2" with GEO600. Virgo (described below) joined O2 in August 2017, improving triangulation of the source direction and allowing the polarisation of the waves to be measured.

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Advanced Virgo

Advanced Virgo is an interferometric gravitational-wave detector located near Pisa in Italy. We have contributed expertise in suspension fibers to this project, and we also analyse data produced (as part of the joint Virgo-LIGO data sharing agreement).

GEO600

GEO 600 was built near Hannover by a collaboration involving Glasgow, the University of Wales (Cardiff), the Albert-Einstein-Institute Hannover and the Albert Einstein Institute Golm. We contributed a great deal of fundamental and applied research to this instrument. Two of our most important contributions were quasi-monolithic fused silica suspensions and signal recycling, which were later implemented in the Advanced LIGO detectors.

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LISA and LISA Pathfinder

LISA is a future large-class mission (L3) of the European Space Agency (ESA). This will involve a formation of three spacecraft several million kilometres apart, forming the arms of a very long arm length interferometer. The long arm length and freedom from ground-based noise will allow sensitivity to low frequency (0.1 mHz to 1Hz) gravitational waves from sources such as supermassive black holes. To validate the technology required, LISA Pathfinder (a single spacecraft) was launched in 2015. The results from LISA Pathfinder not only exceeded expectations, but with some tuning met the requirements for the full LISA mission. LISA Pathfinder completed its mission in 2017.

We designed and built the the monolithic optical bench at the heart of LISA pathfinder. This was constructed by bonding (using Glasgow-developed techniques) glass optical components to a glass optical bench, to a precision measured in microns. This bench had to survive a rocket launch, and then remain stable down to picometer levels in use. In LISA Pathfinder it comfortably exceeded specifications.

For the main LISA mission we are designing the optical bench, and researching ways to partially automate the production process, due to the greater complexity of the benches and the higher numbers required.

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Future detectors

KAGRA and LIGO India are expected to join the observing network in the next few years providing even better sky position information, reaching into the distant Universe and allowing improved network up-time to catch the rarest events.

These advanced detectors apply the principles of laser interferometry to monitor the km-long distances between delicately suspended ultra-reflective mirrors. The performance of the advanced detectors is limited by a combination of thermal noise, including Brownian motion in the highly reflecting mirror coatings, and the quantum limits to measurement of the motion of the mirrors - the Heisenberg Uncertainty Limit due to the effect of fundamental disturbances in the hundreds of kW of laser light needed to perform the precision measurements.

IGR members are centrally involved in developing new technology needed for upgrades of Advanced LIGO early in the 2020s and for completely new detectors with order-of-magnitude enhanced performance planned for the 2030s.

The proposed "enhancement" of Advanced LIGO, called "A+" is at an advanced stage of planning. This project is built on a combination of improved mirror coatings and new interferometer technology to improve the observing range significantly.

More radical technological advances are required for the Einstein Telescope and Cosmic Explorer, both of which aim to reach ten-times deeper into the Universe than the advanced detectors. Cryogenic mirror and mirror-suspension technology is under investigation, as are revolutionary methods in interferometry required to push beyond the standard quantum limit.

As described previously, LISA, planned for launch into space in the 2030s, complements the ground-based detectors, extending the band of detectable signals to lower frequencies.