Infrared instability of the de Sitter space
We continue to investigate various instabilities of the fixed backgrounds related to the de Sitter space. It is shown that in many cases the in/in perturbation theory contains IR/UV mixing and thus is non-renormalizible. The application of this result to the global de Sitter space leads to the conclusion that even massive particles generate IR divergence and the huge back reaction. The expanding universe is also unstable but in a weaker sense. We further discuss, the strange features of the Gibbons-Hawking radiation and its relation to the above instabilities. .