All resonant bar detectors currently in operation, as well as the more advanced bar and interferometric detectors presently in the planning and proposal stage, are sensitive in frequency bands whitin the range - . These detectors were developed or designed with the main purpose to detect impulsive radiation, as that coming from a collapsing star or from the fall of large masses into a black hole, or other types of gravitational waves, like those due to pulsars or to coalescence of binary systems (Thorne 1987). Recently, it has been pointed out (Ferrari 1996) that a different source of gravitational waves (g.w.), the stochastic background, could be one of the most interesting as it might give information on the very early stages of the Universe and its formation. In particular a new source for stochastic background, based on the string theory of matter, has been more deeply investigated (Brustein et al. 1995). On the other hand several sources of stochastic background of g.w. have been considered in the past years (Flanagan, 1993), but all the models tend to predict g.w. in the frequency range below , lower than the operating frequency of the present detectors already in operation (resonant bars) or entering in operation in the next years (long-arm interferometers).
The interesting feature of this model based on string cosmology, from the observer's point of view, is that it predicts relic g.w. whose energy spectral density increases, in a certain range, with the frequency to the third power. This energy density remains below the limit imposed by nucleosynthesis considerations that put an upper limit to the stochastic g.w.'s background (Brustein et al. 1995).
The limit is usually expressed in terms of the fraction of the cosmological closure density that is in gravitational wave energy per unit logarithmic frequency interval,
where H is the Hubble's constant. As the predictions of the new string cosmology models depend on a number of parameters, then the results obtained with a measurement, even an upper limit, would help very much in delineating the exact model.
A recent paper (Astone et al. 1996) reported the experimental upper limits at about for the g.w. stochastic background, obtained with the Explorer and Nautilus resonant detectors. Here we report the corresponding upper limit at about , using the data collected with the cryogenic resonant detector ALTAIR .
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999