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.

27 pages important comments added. references added