  This paper explores infrared quantum effects in the de Sitter space. The
notion of "eternal manifolds" is introduced and it is shown that in most cases
the de Sitter space doesn't belong to this class. It is unstable under small
perturbations which may cause a breakdown of the de Sitter symmetry. The de
Sitter string sigma model is discussed. It is argued that the gauge theory at
the complex coupling is dual to the matrix elements of vertex operators in the
de Sitter space, taken between the Bunch - Davies vacuum and the "out" state
without particles.The described infrared effects are likely to screen away the
cosmological constant.
