Ultralow-temperature resonant gravitational wave detectors, present state and future prospects Original Research Article

Resonant gravitational wave detectors have evolved from the room temperature bars of J. Weber to the sub-Kelvin bars of the Rome and Padua groups. Their strain sensitivity h = ΔLL is now in the range of 10−19, thought to be enough to detect events like supernovae explosions within our Galaxy, events which are very rare. There is still room for improvement but in order to detect more frequent events, the future detectors must gain a factor 100 or more in h, thus 104 in energy sensitivity. For this, spherical (or spheroidal) detectors are being designed which will be cooled to 50 mK or less. Besides the higher planned sensitivity, a number of advantages relative to high directionality, the possibility of detecting both polarizations of the wave, and the possibility of putting upper limits on theories which predict scalar waves by measuring the monopole mode makes these antennas very attractive and an ideal complement to the interferometers. In the few-kilohertz range, low-cost spherical antennas can be built with sensitivities comparable to advanced interferometers working in narrow-band mode.