The basic aim of the thesis is the study of the propagation of particles and
quasiparticles in non-trivial backgrounds from the quantum field theory point
of view. By "non-trivial background" we mean either a non-vacuum state in
Minkowski spacetime or an arbitrary state in a curved spacetime. Starting with
the case of a flat spacetime, the basic properties of the particle and
quasiparticle propagation are analyzed using two different methods other than
the conventional mean-field-based techniques: on the one hand, the quantum
state corresponding to the quasiparticle excitation is explicitly constructed;
on the other hand, the spectral representation of the two-point propagators is
analyzed. Both methods lead to the same results: the energy and decay rate of
the quasiparticles are determined by the real and imaginary parts of the
retarded self-energy respectively. These general results are applied to two
particular quantum systems: first, a scalar particle immersed in a thermal
graviton bath; second, a simplified atomic model, seizing the opportunity to
connect with other statistical and first-quantized approaches. In the second
part of the thesis the results are extended to curved spacetime. Working with a
quasilocal quasiparticle concept the flat-spacetime results are recovered. In
cosmology, within the adiabatic approximation, it is possible to go beyond the
flat spacetime results and find additional effects due to the universe
expansion. The cosmologically-induced effects are analyzed, obtaining that
there might be an additional contribution to the particle decay due to the
universe expansion. In the de Sitter case, this additional contribution
coincides with the decay rate in a thermal bath in a flat spacetime at the
corresponding de Sitter temperature.Comment: 269 pages, 17 figures. PhD thesis, Universitat de Barcelona. One
reference added, minor typos correcte
