Alternative definitions of the Born approximation and the distorted-wave Born approximation within the framework of the configuration-space Faddeev equations are explored. The most natural definition does not correspond to the Born approximation derived from the Schrödinger equation, even though the exact T-matrices for both formalisms are equivalent. The Schrödinger form is optimal, although i...

We present a covariant framework to compute scattering amplitudes and potentials in de Sitter background. In this setting, we the potential of graviton-mediated process involving two very massive scalars at tree level. Although obtained reproduces Newtonian short distances, on Hubble-size length scales it is affected by constant curvature: effectively, yields repulsive force sub-Hubble distance...