  We show that gravitational interactions between massless thermal modes and a
nucleating Coleman-de Luccia bubble may lead to efficient decoherence and
strongly suppress metastable vacuum decay for bubbles that are small compared
to the Hubble radius. The vacuum decay rate including gravity and thermal
photon interactions has the exponential scaling $\Gamma\sim\Gamma_{CDL}^{2}$,
where $\Gamma_{CDL}$ is the Coleman-de Luccia decay rate neglecting photon
interactions. For the lowest metastable initial state an efficient quantum Zeno
effect occurs due to thermal radiation of temperatures as low as the de Sitter
temperature. This strong decoherence effect is a consequence of gravitational
interactions with light external mode. We argue that efficient decoherence does
not occur for the case of Hawking-Moss decay. This observation is consistent
with requirements set by Poincare recurrence in de Sitter space.
