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General boundary quantum field theory in de Sitter spacetime

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.