  The process of reheating the universe after hybrid inflation is extremely
violent. It proceeds through the nucleation and subsequent collision of large
concentrations of energy density in bubble-like structures, which generate a
significant fraction of energy in the form of gravitational waves. We study the
power spectrum of the stochastic background of gravitational waves produced at
reheating after hybrid inflation. We find that the amplitude could be
significant for high-scale models, although the typical frequencies are well
beyond what could be reached by planned gravitational wave observatories like
LIGO, LISA or BBO. On the other hand, low-scale models could still produce a
detectable stochastic background at frequencies accesible to those detectors.
The discovery of such a background would open a new window into the very early
universe.
