STOCHASTIC GRAVITATIONAL BACKGROUND FROM INFLATIONARY PHASE-TRANSITIONS

We consider true vacuum bubbles generated in a first order phase trans ition occurring during the slow rolling era of a two field inflation: it is known that gravitational waves are produced by the collision of such bubbles. We find that the epoch of the phase transition strongly affects the characteristic peak frequency of the gravitational waves, causing an observationally interesting redshift in addition to the pos t-inflationary expansion. In particular it is found that a phase trans ition occurring typically 10-20 e-foldings before the reheating at kT similar or equal to 10(15) GeV may be detected by the next LIGO gravit y waves interferometers. Moreover, for recently proposed models capabl e of generating the observed large scale voids as remnants of the prim ordial bubbles (for which the characteristic wave lengths are several tens of Mpc), it is found that the level of anisotropy of the cosmic m icrowave background provides a deep insight upon the physical paramete rs of the effective Lagrangian. [S0556-2821(97)02120-6].