Particle decays and CPT non-invariance in cosmology

The effect of the expansion of the universe upon the rate of decay of a massive scalar particle is investigated. In the limit of weak coupling to the spacetime curvature, an expression is given for the average decay rate of such a particle. It is shown that in general this process is not CPT invariant. Several explicit models for the expansion are considered, and the application of the results to the early universe is discussed. (orig.)