  In the de Sitter ambient space formalism the massless fields, which include
the linear gravity and massless minimally coupled scalar field, can be written
in terms of two separate parts: a massless conformally coupled scalar field and
a polarization tensor(-spinor) part. Therefore due to the massless conformally
coupled scalar field, there exist an unique Bunch-Davies vacuum state for
quantum field theory in the de Sitter space time. In the de Sitter ambient
space formalism one can show that the massless fields with spin $s\geq 1$ are
gauge invariant. By coupling the massless gauge spin-$2$ and the massless gauge
spin-$3/2$ fields and using the super-symmetry algebra in de Sitter ambient
space formalism, one can naturally construct a unitary de Sitter super-gravity
on Bunch-Davies vacuum state.
