Research of the group
The work of this group, supported by STFC funding, is targeted at the development of detectors and signal analysis methods to search for gravitational waves from astrophysical sources. Gravitational waves - waves in the curvature of space-time - are a prediction of General Relativity. In recent years there has been considerable progress towards the detection of these waves. Indirect confirmation of their existence has come from observations of the orbital motion of the binary pulsar PSR 1913+16, for which work Hulse and Taylor were awarded the 1993 Physics Nobel Prize. This evidence and the recognition of its importance gave a significant boost to the efforts of physicists worldwide in the gravitational wave field. The group in Glasgow has been involved in both experimental development and data analysis for around 35 years.
(Introductory leaflet from the STFC)
Potential sources of gravitational waves include the coalescence of two compact stars such as neutron stars and/or black holes in binary systems and the collapse of massive stars in supernova explosions. The detection of these waves should give us unique new information about some of the most violent astrophysical processes in the universe. Detection techniques being developed at Glasgow and elsewhere rely on using laser interferometry to sense the minute changes (of order 10-18 m or less) induced by a gravitational wave in the relative separation of neighbouring test masses. In order to reach the sensitivity required to start making serious astrophysical observations, separations of kilometre-scale are required. Developments and operation of such long baseline detectors are currently being undertaken by several groups worldwide, including LIGO in the USA, VIRGO, a joint French/Italian collaboration, GEO 600 (Germany, UK) and TAMA 300 (Japan).
GEO 600 and LIGO
GEO 600 is a detector of arm length 600m, built in Hannover by a collaboration involving Glasgow, the University of Wales (Cardiff), the Albert-Einstein-Institute Hannover and the Albert Einstein Institute Golm. Although of shorter arm length than the LIGO or VIRGO detectors, GEO has some unique technical features that have enabled it to take part in a series of five major data taking runs with the larger detectors. These features include:
- quasi-monolithic fused silica suspensions of the test masses to allow a reduction in thermal noise
- the use of signal recycling - recycling of the signal sidebands on the output light back into the interferometer in a resonant way - to enhance signal size
The main areas focussed on by the Glasgow group are the development of precision novel interferometric techniques, and the development of systems of ultra low mechanical loss for the suspensions of mirror test masses. This latter area encompasses the development of multiple pendulum systems with the final stages using silica fibres for supporting the test masses. The group is also working on a novel bonding technology - hydroxy-catalysis bonding - which exhibits very low mechanical loss and is compatible with ultra high vacuum. To help with this work a JIF grant for the development of new laboratories, clean areas and interferometers was awarded to the group in 1999, and a Strategic Research Development Grant from the Scottish Funding Council in 2005, supporting the ongoing work of the IGR funded by STFC. These optical and mechanical technologies, as well as being used in GEO, are being transferred to the next 'Advanced' generation of the US laser interferometer system LIGO, and the GEO group is playing a crucial role in the development and construction of Advanced LIGO.
Advanced LIGO was approved for construction by the National Science Board in the US. Glasgow, Birmingham and the Rutherford Appleton Laboratory (RAL), in collaboration with Strathclyde University were funded by STFC to supply the mirror suspensions and part of the optics. With construction of the two Advanced LIGO detectors nearing completion, the focus is moving to commissioning. Currently, Glasgow, Birmingham, Cardiff, RAL and Sheffield are funded by STFC to install and commission the suspensions we delivered and to prepare data analysis computing systems in readiness for engineering and science data runs scheduled to start by 2015.
The benefits of cooling to reduce noise in suspension systems is an area of research to be pursued over the next few years, of relevance to future detector upgrades and designs, and the group plays a leading part in a pan-European consortium carrying out a design study for a potential future '3rd generation' gravitational wave detector - the 'Einstein Telescope' - which may employ this, and other novel technologies.
The Glasgow team are actively involved in the international efforts to fly a spaceborne gravitational wave observatory. The current baseline mission is called eLISA. Such a mission will provide detailed observations of gravitational wave sources in the richly populated low frequency band, where sources include inspiraling massive black holes, extreme mass ratio inspirals and binary systems in our own galaxy. eLISA will complement the observations expected from ground-based gravitational wave detectors that will give information about higher-frequency gravitational wave sources.
The eLISA design has a high level of design and technical maturity. The observatory consists of a triangular constellation of three spacecraft with million kilometre baselines and with laser interferometry used to monitor relative positions of 'test masses' within spacecraft at opposite ends of an arm with picometre precision. Gravitational waves passing through the system will result in variations in the measured distances between these test masses. The approach is similar to that used in the ground based detectors, but for eLISA the signals will occur over timescales of thousands of seconds. The technological requirements for spaceborne gravitational wave detectors are many and varied, and at Glasgow we specialise in the optical sensing aspects. The challenges for a spaceborne detector were significant enough to warrant flying a technology demonstrator: LISA Pathfinder. This mission, due for launch in 2015, is demonstrating - both during its development, and by its eventual flight - crucial aspects for spaceborne gravitational wave detector missions, and also technologies that could find uses in other areas on the ground and in space.
Supported by UKSA and ESA funding, the group in Glasgow have built and tested the flight hardware optical bench for LISA Pathfinder, and is now utilising this experience to develop and prototype aspects of the optical metrology for eLISA. This has involved developing the hydroxide catalysis bonding process originally created at Stanford University - and subsequently used within GEO - to build ultra-precise optical sensing assemblies suitable for space flight.
The route ahead to a spaceborne gravitational wave observatory has recently become clear, with the adoption by ESA of the "Gravitational Universe" as their theme for the third L-class mission due to launch in 2034. The call for mission proposals to address this theme will come later this decade. This timescale is very compatible with the Pathfinder flight and the positive results expected, and with the anticipated period of targeted development to complete the technological readiness of the full eLISA design.
Spin-off and related work
As well as its work in gravitational wave detectors, the Glasgow group works on applications of the hydroxy-catalysis bonding techniques for other experiments and projects in astronomy and space science.
A brief summary of our research can be found in this pdf flyer.