Opening Up the Dark Side of the Universe

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.