  We quantize a massive scalar field in de Sitter spacetime and derive the
S-matrix for the general interacting theory. Using the general boundary
formulation of quantum field theory, we also propose a new type of S-matrix
derived from the asymptotic limit of the amplitude associated with a spacetime
region bounded by one connected and timelike hypersurface. Based on previous
works in Minkowski spacetime, we call this region the hypercylinder region. We
show how the new S- matrix coincides with the usual one by constructing an
isomorphism between the space of temporal asymptotic states of the traditional
setting and the space of spatial asymptotic states defined on the asymptotic
hypercylinder.
