The gravitational wave detector VIRGO

VIRGO is a gravitational wave detector based on a Michelson interferometer with 3 km long arms. The aim of the experiment is the first detection of gravitational waves emitted by compact stellar objects such as neutron stars, black holes or supernova. The detector is being built near Pisa (Italy) by a French–Italian collaboration funded by INFN in Italy and CNRS in France. The test of the central part of the interferometer have been performed successfully this year. Civil engineering has also been completed this year and the detector final assembly will be finished early next year. The main scientific goals and detector issues are introduced. Some of the control and stabilization systems used to operate the interferometer are also discussed. Recent results and present status are presented. 1 SCIENTIFIC MOTIVATIONS Gravitational waves [1] are predicted by Einstein general relativity theory. According to Einstein theory, the gravitational force is a manifestation of space-time curvature. Any mass curves the space-time geometry around itself and, as a consequence, it curves the trajectory of a free falling test mass passing nearby. If this same mass starts oscillating the space-time curvature will also start oscillating and this oscillation will propagate across space-time at the speed of light as a wave propagating on a water surface. These small propagating perturbations of the space-time metric are gravitational waves. Their effect is to change the distance between two free falling masses and a detailed analysis of Einstein equations shows that they are spin 2 waves. As a consequence if a gravitational wave travels across a set of masses placed on a circle, the circle will be transformed in an ellipse with its axis pointing in the direction of the wave polarization (see Fig. 1). If L is the circle diameter, it will undergo a change h·L/2 where h<<1 is the gravitational wave amplitude. Any mass produces gravitational waves with amplitude h proportional to the second derivative of its quadrupole moment Q: