Gravitational Wave Production At The End Of Inflation

We consider gravitational wave production due to parametric resonance at the end of inflation, or ``preheating''. This leads to large inhomogeneities which source a stochastic background of gravitational waves at scales similar to (or smaller than) the comoving Hubble horizon at the end of inflation. We confirm previous conjectures that the present amplitude of these gravitational waves need not depend on the inflationary energy scale. In particular, we analyze explicit models, where the inflationary energy scale is ~10^9 GeV. These yield a signal close to the sensitivity of Advanced LIGO. This signal highlights the possibility of a new observational ``window'' into inflationary physics, and provides significant motivation for searches for stochastic backgrounds of gravitational waves in the Hz to GHz range, with an amplitude on the order of Omega_{gw}h^2 ~ 10^{-10}. Finally, the strategy used in our numerical computations will be applicable to the gravitational waves generated by many inhomogeneous or turbulent processes in the early universe