L. Di Fiore

The VIRGO Project: A wide band antenna for gravitational wave detection

Abstract The status of advancement of the VIRGO Project is presented: the first-generation results from the Pisa seismic noise super attenuator give an upper limit to the noise transfer function of 2 × 10 −8 at 10 Hz. The upper limit to the absolute noise of the 400 kg test mass at 10 Hz has been measured to be 1.5 × 10 −13 m/√Hz. The scheme and the related problems of the VIRGO interferometer, which is supposed to work down to 10 Hz, are also presented. At the 3rd Pisa Meeting in 1986 we presented the idea of what could be a very efficient seismic noise reduction system able to give a sensitivity h ∼ 10 −25 at 10 Hz, in a 3 km interferometer for 1 year integration time. Now we have two new facts to present: the first is that the attenuation has been built, is working in Pisa, and shows remarkable characteristics. The second is the Italian-French interferometer VIRGO [1,2], a 3 km long antenna for low and high frequency (10–1000 Hz) gravitational wave (GW) detection. These two items will be presented in this article.

The Virgo interferometer

The Virgo gravitational wave detector is an interferometer with 3 km long arms in construction near Pisa to be commissioned in the year 2000. Virgo has been designed to achieve a strain sensitivity of a few times at 200 Hz. A large effort has gone into the conception of the mirror suspension system, which is expected to reduce noise to the level of at 10 Hz. The expected signals and main sources of noise are briefly discussed; the choices made are illustrated together with the present status of the experiment.

The Virgo Project

The Virgo project is a Italian-French collaboration aiming at the construction of a long baseline interferometric antenna for the detection of gravitational radiation signals of cosmic origin. We describe the principles of the system, and high-light the technical challenges we need to overcome for reaching a sensitiity as low as 10−23Hz−1/2.The gravitational clustering of collisionless particles in an expanding universe is modelled using some simple physical ideas. I show that it is possible to understand the nonlinear clustering in terms of three well defined regimes: (1) linear regime; (2) quasilinear regime which is dominated by scale-invariant radial infall and (3) nonlinear regime dominated by nonradial motions and mergers. Modelling each of these regimes separately I show how the nonlinear two point correlation function can be related to the linear correlation function in hierarchical models. This analysis leads to results which are in good agreement with numerical simulations thereby providing an explanation for numerical results. Using this model and some simple extensions, it is possible to understand the transfer of power from large to small scales and the behaviour of higher order correlation functions. The ideas presented here will also serve as a powerful analytical tool to investigate nonlinear clustering in different models.

First results on the electronic cooling of the Pisa seismic noise super-attenuator for gravitational wave detection

Abstract The seismic noise excitation of the normal modes in the three-dimensional Pisa Super Attenuator, to be used in a long base interferometric gravitational antenna, produces large movements of the 400 kg test mass. In this paper it is shown, for the first time, that it is possible to damp, in a stable way, these normal modes using a six-dimensional damping system acting on the second suspended mass instead of the test mass itself, with the purpose not to reintroduce seismic noise. In this way the large displacement normal modes, including the verticals and the rotations, have been damped and the pendulum maximum displacement, originally varying from 15 to 30 μm according to the seismic noise intensity, has been reduced to less than 3.4 μm allowing locking to a fringe of the interferometer.

Vacuum fluctuation force on a rigid Casimir cavity in a gravitational field

We discuss the possibility of verifying the equivalence principle for the zero-point energy of quantum electrodynamics, by evaluating the force, produced by vacuum fluctuations, acting on a rigid Casimir cavity in a weak gravitational field. The resulting force has opposite direction with respect to the gravitational acceleration; the order of magnitude for a multi-layer cavity configuration is derived and experimental feasibility is discussed, taking into account current technological resources.

An improvement in the VIRGO Super Attenuator for interferometric detection of gravitational waves: The use of a magnetic antispring

We present a method of lowering below 2.5 Hz the vertical normal mode frequencies of the Pisa Super Attenuator by using permanent magnets which provide an antispring force. This method allows a more efficient suppression of the seismic noise decreasing the lower limit of the frequency region devoted to gravitational wave detection.

Status of the VIRGO experiment

The VIRGO experiment was approved in September 1993. The goal of the French-Italian collaboration is to detect gravitational waves using a 3 km arm-length Michelson interferometer. The construction of this detector, which will be installed in Pisa, is under way. The experiment is planned to take data, in a large bandwidth (10 Hz-10 kHz), at the beginning of the year 2000 with nominal sensitivity close to h = 3 X 10-‘3/&. The motivations, detection principle, main sources of noise and status of the experiment are presented.

Effects of misalignments and beam jitters in interferometric gravitational wave detectors

Abstract We present a calculation of the phase noise in a recycled interferometer with Fabry-Perot cavities in the arms, induced by the coupling of the geometrical fluctuations of the input laser with geometrical asymmetries of the interferometer. By comparison with the shot-noise limit planned for long-baseline interferometric gravitational wave detectors, upper limits in interferometer misalignments are established.

Analysis of contact binary systems - AA Ursae Majoris, V752 Centauri, AO Camelopardalis, and V 677 Centauri

In this paper we present a methodology for the solution of binary systems that allows the simultaneous use of all the available information. We give a statistical criterion to judge about the quality of the results. This methodology, based mainly on the Wilson-Prince procedure devised by our group for the solution of binary systems (by Barone et al. in 1988 and 1990 and Milano et al. in 1989), has here been applied to the analysis of four W UMa binaries for which existing solutions were, for different reasons, unsatisfactory. We show the cases for which more observational data are necessary or those for which a better analysis would be sufficient to give final answers

The VIRGO interferometer for gravitational wave detection.

The Virgo gravitational wave detector is an interferometer with 3 km long arms in construction near Pisa in Italy. The accessible sources at the design sensitivity and main noises are reviewed. Virgo has devoted a significant effort to extend sensitivity to low frequency reaching the strain level h = 10−21 Hz−1/2 at 10 Hz while at 200 Hz h = 3 · 10−23 Hz−1/2. Design choices and status of construction are presented.