  Metastable states decay at zero temperature through quantum tunneling at an
exponentially small rate, which depends on the Coleman-de Luccia instanton,
also known as bounce. In some theories, the bounce may not exist or its
on-shell action may be ill-defined or infinite, thus hindering the vacuum decay
process. In this paper, we test this possibility in modified theories of
gravity interacting with a real scalar field. We consider an Einstein-Hilbert
term with a non-minimally coupled scalar field and a quadratic Ricci scalar
contribution. To tackle the problem we use a new analytic method, with which we
prove that the scalar field on the bounce has a universal behaviour at large
Euclidean radii, almost independently of the potential. Our main result is that
the quadratic Ricci scalar prevents the decay, regardless of the other terms in
the action.
