Primordial gravitational waves from open inflation

We calculate the spectrum of gravitational waves generated during inflation in open $({\Omega }_0<1)$ inflationary models. In such models an initial epoch of old inflation solves the horizon and flatness problems, and during this first epoch of inflation the quantum state of the graviton field rapidly approaches the Bunch-Davies vacuum. Then the old inflation ends by the nucleation of a single bubble, inside of which there is a shortened epoch of slow-roll inflation giving ${\Omega }_0<1\mathrm{}$ today. In this paper we reexpress the Bunch-Davies vacuum for the graviton field in terms of the hyperbolic modes inside the bubble and propagate these modes forward in time into the present era. We derive the expression for the contribution from these gravity waves to the cosmic microwave background anisotropy including the effect of a finite-energy difference across the bubble wall.