SPHERES { OMNI - DIRECTIONAL MULTI - MODEGRAVITATIONAL - WAVE ANTENNAS FOR THE NEXTGENERATIONHo

The sensitivity of resonant-mass gravitational-wave antennas has now reached h 10 ?18 , where h is the dimensionless metric perturbation caused by the wave. In order to observe gravitational wave events such as coalescing neutron star binaries and colliding black holes from as far as the Virgo cluster of galaxies, the detector sensitivity must be improved by three orders of magnitude. With such an aim, construction of massive (40 ton) spherical antennas have been proposed by a number of groups world wide. The standard quantum limit of the sensitivity of such detectors will be h = 3 10 ?22. Unlike a cylinder, a sphere has ve degenerate quadrupole modes which interact with an incoming gravitational wave. By combining the responses of all these modes, one can determine the direction of the source and the polarization of the wave with a single spherical antenna. Summed over the modes, the sphere has a direction-independent absorption cross section, as expected from its symmetry, permitting full-sky coverage with a single antenna. These unique properties, combined with its enhanced sensitivity due 1 to its multi-mode nature and increased mass, make spherical detectors ideal new instruments for observational astronomy. Signiicant advances have been made recently on instrumentation of a spherical antenna. Mounting six identical resonant transducers on the six pentagonal faces in one hemisphere of a truncated icosahedral gravitational-wave antenna (TIGA) has been found to maintain the \spherical" symmetry in the coupled antenna-transducer system. The quadrupole modes of the sphere split into doublets with equal frequency splitting, as desired. Numerical calculations have shown that gravitational-wave signals from coalescing neutron star binaries in the Virgo cluster can be resolved with a spherical antenna with a near-quantum-limited sensitivity.