  We study the effects of gravitationally-driven decoherence on tunneling
processes associated with false vacuum decays, such as the Coleman--De~Luccia
instanton. We compute the thermal graviton-induced decoherence rate for a wave
function describing a perfect fluid of nonzero energy density in a finite
region. When the effective cosmological constant is positive, the thermal
graviton background sourced by a de Sitter horizon provides an unavoidable
decoherence effect, which may have important consequences for tunneling
processes in cosmological history. We discuss generalizations and consequences
of this effect and comment on its observability and applications to black hole
physics.
