2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/203916We explore the adiabatic particle excitations of an interacting field in a cosmological background. By following the time-evolution of the quantum state corresponding to the particle excitation, we show how the basic properties characterizing the particle propagation can be recovered from the two-point propagators. As an application, we study the background-induced dissipative effects on the propagation of a two-level atom in an expanding universe.20 pages, 2 figures. A contribution to the Proceedings of the International Workshop on Cosmology and Gravitation, Peyresq 12, June 16-22, 2007, Peyresq, France. Accepted for publication in Clas. Quant. GravGeneral Relativity and Quantum CosmologyHigh Energy Physics - Phenomenologytext