  We study conformally-coupled massive scalar field theory with the mass term
treated as a perturbation in the expanding half of de Sitter spacetime as a toy
model for comparing various perturbative formalisms. We point out that the
in-out perturbation theory breaks down due to infrared divergences coming from
the infinite future. We then clarify the relation between the free-theory
vacuum and the true vacuum using the Bogolubov transformation and show that the
discrepancy between the free-theory out-vacuum and the true vacuum causes
apparent pair creation of free-theory particles and makes the in-out two-point
function differ from the true two-point function. We also identify the infinite
Bogolubov coefficients as the cause of infrared divergences. We then examine
two alternative approaches: the Euclidean and in-in formalisms. We verify that
there are no infrared divergences in perturbation theory with the mass term
treated as a perturbation in either of these approaches and that the two-point
function of massive scalar field is reproduced correctly by these methods.
