(ed.). Gravitational waves (IOP, 2001)(422s).

The relic graviton background 315the anisotropy of the CMB radiation, δT/T º 10 −5 . Thus, at low frequency,( ) T 2 G (t 0 ) º γ (t 0 )∼ 10 −14 . (16.108)T COBEIn string cosmology the spectrum is growing, the normalization is imposedat high frequency, and the peak value is controlled by the fundamental ratioM s /M p º 0.1. Thus( ) 2 Ms G (t 0 ) º γ (t 0 ) º 10 −6 . (16.109)M pThe graviton background obtained from the amplification of the vacuumfluctuations, in string cosmology and in standard inflation, is compared infigure 16.12 with other, more unconventional graviton spectra. In particular,the graviton spectrum obtained from cosmic strings and topological defects [73],from bubble collision at the end of a first order phase transition [74], and froma phase of parametric resonance of the inflaton oscillations [75]. Also shown infigure 16.12 is the spectrum from models of quintessential inflation [76], and athermal black body spectrum for a temperature of about one kelvin. All thesecosmological backgrounds are higher than the background expected from thevacuum fluctuations in standard inflation, but not in a string cosmology context.It may be interesting, at this point, to recall the expected sensitivities ofthe present, and near future, gravitational antennae, referred to the plots offigure 16.12.At present, the best direct, experimental upper bound on the energy of astochastic graviton background comes from the cross-correlation of the data ofthe two resonant bars NAUTILUS and EXPLORER [77]: G h 100 º 60, ν ≃ 907 Hz (16.110)(similar sensitivities are also reached by AURIGA [78]). Unfortunately, the boundis too high to be significant for the plots of figure 16.12. However, a much bettersensitivity, G ∼ 10 −4 around ν ∼ 10 3 Hz, is expected from the present resonantbar detectors, if the integration time of the data is extended to about one year. Asimilar, or slightly better sensitivity, G ∼ 10 −5 around ν ∼ 10 2 Hz, is expectedfrom the first operating version of the interferometric detectors, such as LIGOand VIRGO. At high frequency, from the kilohertz to the megahertz range, apromising possibility seems to be the use of resonant electromagnetic cavities asgravity-wave detectors [79]. Work is in progress [80] to attempt to improve theirsensitivity.The present, and near future, available sensitivities of resonant bars andinterferometers, therefore, are still outside the allowed region of figure 16.12,determined by the border line G h 100 ≃ 10 −6 . (16.111)