  de Sitter space-time has a one complex parameter family of invariant vacua
for the theory of a free, massive scalar field. For most of these vacua, in an
interacting scalar theory the one loop corrections diverge linearly for large
values of the loop momentum. These divergences are not of a form that can be
removed by a de Sitter invariant counterterm, except in the case of the
Euclidean, or Bunch-Davies, vacuum.
