Effective graviton mass in de Sitter space

With the use of the non-stationary Keldysh-Schwinger diagrammatic technique, we find the one-loop effective action for graviton that interacts with the scalar field on the background of de Sitter space. We show that the graviton does not acquire mass for the most symmetric Bunch-Davies state in one-loop effective theory. However, we have shown that even in this case, there is a nontrivial modification of the theory at one loop in the scalar sector of gravity.