Physicists in the UK are ready to start construction of a major part of an advanced new experiment, designed to search for elusive gravitational waves. They are already part of two experiments: the UK/German GEO 600 project and the US LIGO experiment (Laser Interferometer Gravitational-Wave Observatory), both in their commissioning phases. By bringing GEO 600 technology to LIGO, they and their German colleagues from the Albert Einstein Institute are now set to become full partners in Advanced LIGO, a more sensitive observatory that once fully operational should be able to detect a gravitational wave event a day.
Gravitational waves should be created when massive objects, such as black holes or neutron stars in astronomical binaries interact and spiral-in towards, and eventually collide with, each other emitting a strong burst of gravitational radiation or when a star, at the end of its long evolutionary phase, collapses due to its own gravity resulting in a supernova with the core forming a neutron star or a black hole. Rapidly rotating neutron stars or pulsars with tiny deformities in their spherical shape, and newly formed neutron stars, are continuous emitters of the radiation. There should also be background "noise" made up from a population of such events and, possibly, phase transitions in the early Universe and the echoes of the Big Bang itself.
First predicted by Einstein's Theory of Relativity, gravitational waves have never been observed, but indirect evidence of their existence has been obtained by measuring the effect of their emission by a binary pulsar system (two neutron stars orbiting each other). The observed effect was found to match predictions.
Professor Ken Strain, Institute for Gravitational Research at the University of Glasgow, explains "Gravitational waves are ripples in the fabric of space-time, produced by the acceleration of mass. Because the gravitational interaction is very weak, large masses and high accelerations are needed to produce gravitational waves of significant amplitude. These are the very conditions that occur during violent astrophysical events such as supernovae or when neutron stars coalesce."
The detection and study of gravitational radiation will be of great scientific importance. It will open up a new window on the universe through which may come unique information about a variety of astrophysical systems -supernova explosions, black hole formation, pulsars and coalescing compact binary objects. It is also possible that totally unexpected discoveries will be made, in much the same way as has occurred in radio and x-ray astronomy.
Gravity waves regularly pass through the Earth unnoticed, as Dr Chris Castelli of Birmingham University explains: "As gravity waves pass through, they contract or expand by tiny amounts in a plane perpendicular to the direction they are moving, usually too small to notice. If we split a laser signal and send it off in perpendicular directions before bouncing the light back off test masses and recombining it, we can measure whether the light has travelled the same distance in each direction. If a gravity wave has interacted with the system, it will have changed the relative distance between the test masses forming the two perpendicular arms."
The longer the baseline of the detector, the more sensitive it is. However, as practical constraints limit the size of experimental facilities, GEO 600 has come up with new ways of improving sensitivity using triple suspended test masses, advanced optics and specialised control electronics. Sharing this technology with Advanced LIGO is granting full partner status to GEO 600 and will contribute to enhancing LIGO to Advanced LIGO, with a factor of ten increase in sensitivity.
Mr. Justin Greenhalgh, of CCLRC Rutherford Appleton Laboratory explains the benefits of the GEO 600 technology: "The UK team will provide quadruple pendulum suspensions developed from the GEO 600 triple design. The extra stage provides enhanced isolation against seismic noise and noise from the control systems that are required to allow Advanced LIGO to achieve extreme sensitivity at low observation frequencies. The suspension design incorporates ultra-low mechanical loss techniques pioneered in GEO 600 to meet the exacting requirements set by the science goals for Advanced LIGO" Grants totalling £8.6 million have been made by the Particle Physics and Astronomy Research Council (PPARC) for Glasgow and Birmingham Universities to carry out the work. Much of the construction work, and overall management of the UK programme, will be done by CCLRC Rutherford Appleton Laboratory.
The new funding announced by PPARC consists of £7.2million to the University of Glasgow and £1.4million to the University of Birmingham. Funding for the Cardiff University participation in this project is included in their rolling grant that also covers other aspects of their work.
GEO 600 is a joint UK/ German project consisting of the Universities of Glasgow and Cardiff from the UK and in Germany, the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) Potsdam and Hannover, Max Planck Institute for Quantum Optics, Garching and the University of Hannover. See http://www.geo600.uni-hannover.de for more information. The GEO German partners are also expected to strengthen their position in the Advanced LIGO project.
LIGO is a US project, involving the LIGO Hanford Observatory, LIGO Livingston Observatory, California Institute of Technology, Massachusetts Institute of Technology. The detectors are sited at Hanford and Livingston. See http://www.ligo.caltech.edu/ for more information.
A world-wide network of detectors Gravitational wave detectors are not direction sensitive; each receives signals from much of the sky. Therefore to identify where a signal is coming from, it needs to be detected at several locations. The difference in time when it arrives at each detector can be used to calculate the signal location and also rule out earth based interference that could give false positives – such as earthquakes! In addition to GEO 600 and LIGO, there is TAMA 300 in Japan and a French/Italian detector, VIRGO, about to start operation this year.
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