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 coherently oscillating field. As an important example we consider simple models of chaotic inflation, where we find that today's ratio of energy density 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 ^4/4 model with inflaton self-coupling =10^-13, the maximal today's wavelength of gravitational waves 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 sensitivity of LIGO or TIGA at such frequencies. We discuss the possibility that in other models the interaction of classical waves can lead to an even stronger gravitational radiation background.