The Virgo detector, like GEO600, is a ground-based interferometer detector with orthogonal arms, each being 3km in length. It was built by a French-Italian collaboration and is located near Cascina in Italy. The detector has an optical path length of order 100km and is sensitive to gravitational wave frequencies in the range of 10-10,000Hz. This should allow it to see the coalescence of binary systems within the milky way and other galaxies such as the Virgo cluster, hence the name ‘Virgo’.

The detector as seen from inside the vacuum tube of the North Arm inside the housing.

Having reached design sensitivity during runs carried out between 2007 and 2011, the detector was taken offline for upgrades which are scheduled to be completed in 2016. The design sensitivity was of order 10-22 (i.e. it can detect a change in arm-length equivalent to the nearest star, Proxima Centuri, moving the width of a human hair), which compares to the 4km LIGO detectors in Hanford and Livingstone, USA. To achieve such high sensitivity the Virgo team has developed some of the most advanced techniques in the field of high power laser stability, high refelectivity mirrors, seismic isolation as well as position and alignment control. The mirrors are suspended from a complex series of pendulums and the light propagates in a ultra-high vacuum.

After the scheduled upgrades, it is hoped the improved detector (referred to as ‘Advanced Virgo’) will be 10 times more sensitive and thus able to observe ~1000 times the volume of its predecessor. Coincident data runs with the LIGO and GEO600 detectors would enable cross-examination of data, termed ‘coherent analysis’, which would dramatically improve sensitivity and the characterisation of detected gravitational wave sources.