The lifetime of a massive particle in a de Sitter universe

We study particle decay in de Sitter spacetime as given by first-order perturbation theory in an interacting quantum field theory. We show that for fields with masses above a critical mass m(c) there is no such thing as particle stability, so that decays forbidden in. at spacetime do occur there. The lifetime of such a particle also turns out to be independent of its velocity when that lifetime is comparable with the de Sitter radius. Particles with lower mass are even stranger: the masses of their decay products must obey quanti. cation rules, and their lifetime is zero.