of the observed flatness of the universe compatible with a relatively short stage of slow-roll inflation inside an open universe. Therefore, it is not very improbable that $1-\Omega_{0}$ may be in the observable range. The goal of our paper is to study possible observational consequences of the open universe scenario with $1-\Omega_{0} \sim 10^{-2} - 10^{-3}$ for the cosmic microwave background (CMB) anisotropy. We will focus on tensor perturbations.

One should note that even though the basic idea of this scenario is pretty simple, it is not easy to find a realistic open inflation model in the single-field inflationary scenario. For example, the simplest potential which allows inflation at $\phi >M_{\rm pl}$ and has a metastable vacuum state at large $\phi$ can be written as a sum $m^{2}\phi^{2}/2 - \delta\phi^{3}/3 + \lambda\phi^{4}/4$, where $m^{2},\delta, \lambda > 0$. An investigation of inflation in this scenario demonstrated that $V'' < H^{2}$ in the vicinity of the barrier [@Linde:1995xm]. In this case the tunneling occurs not through the barrier, but to the top of the barrier, described by the so-called Hawking-Moss instanton [@Hawking:1981fz; @Jensen:1983ac; @b60; @Linde:1991sk; @Batra:2006rz]. Then we will have eternal inflation at the top of the barrier with $V' =
0$, 