RELIC GRAVITATIONAL-WAVES PRODUCED AFTER PREHEATING

We show that gravitational radiation is produced quite efficiently in interactions of classical waves created by resonant decay of a coheren tly oscillating field. As an important example we consider simple mode ls of chaotic inflation, where we fmd that today's ratio of energy den sity in gravitational waves per octave to the critical density of the Universe can be as large as 10(-12) at the maximal wavelength of order 10(5) cm. In the pure lambda phi(4)/4 model with inflaton self-coupli ng lambda=10(-13), the maximal today's wavelength of gravitational wav es produced by this mechanism is of order 10(6) cm, close to the upper bound of operational LIGO and TIGA frequencies. The energy density of waves in this model, though, is likely to be well below the sensitivi ty of LIGO or TIGA at such frequencies. We discuss the possibility tha t in other models the interaction of classical waves can lead to an ev en stronger gravitational radiation background.