For several years the European gravitational-wave detectors GEO600 (a collaboration between Germany and the UK), close to Hanover, and Virgo (a collaboration between Italy, France, the Netherlands, Poland and Hungary), close to Pisa, have been performing data-taking runs together with the LIGO detectors in the US. About a year ago the LIGO collaboration turned off its detectors to start an important upgrade, so this summer the European detectors joined forces to step up their search for gravitational waves in a last three-month data-taking run before Virgo also shuts down for its own upgrade.
GEO600 and Virgo had the good fortune to be on with an impressive 82% duty cycle at the time of the recent nearby supernova explosion (CERN Courier October 2011 p12). Unfortunately, the event on 24 August was too far away and of Type 1a, so releasing only a small amount of energy as gravitational waves. Analysis is nevertheless continuing at full speed.
These detectors are kilometre-scale Michelson laser-interferometers that work by measuring tiny changes caused by a passing gravitational wave in the lengths of their orthogonal arms. Laser beams sent down the arms are reflected from mirrors, suspended under vacuum at the ends of the arms, to a central photodetector. The periodic stretching and shrinking of the arms is then recorded as varying interference patterns.
The worldwide detector upgrades that are just starting will be a fundamental step forward. With current sensitivities, the probability of detecting a gravitational-wave burst in one full year of data-taking is estimated to be of the order of a few per cent. The upgrades aim to improve the sensitivities by a factor of 10 with respect to the present values, which should then extend the “listening” distance by a factor of 10. This will increase the volume of universe explored and the detection probability by a factor of 1000, offering the “certainty” of catching several gravitational-wave events a year.
The non-detection of gravitational waves so far has nevertheless allowed the derivation of several important scientific results. For example, important limits have been established on the production of gravitational waves of cosmological origin and by known pulsars. Improving the spin-down limit of the Crab and Vela pulsars should put limits on the ellipticity of the stellar mass-distributions, which are expected to be related to the magnetic asymmetries in these systems.
“Multimessenger” astrophysics has meanwhile begun, looking for coincidences of candidate gravitational-wave signals with gamma-ray bursts and signals from space-borne cosmic-ray detectors as well as neutrino and optical telescopes. Such clues will have paramount importance in studying the sources as soon as genuine gravitational-wave detection becomes routine after 2015, when detector upgrades are expected to be completed.