Grothendieck topos, gerbes and lifting actions of group objects

Abstract: Let $C$ be a Grothendieck topos, $G$ and $H$ group objects of $C$. Let $p:P\rightarrow X$ be an $H$-torsor. Suppose that $X$ is endowed with an action of $G$. In this paper, we study the obstructions to lift the action of $G$ on $X$ to $P$ by using non commutative cohomology. Firstly, when a natural condition is satisfied, we associate to this problem an extension of groups objects in $C$ whose splittings correspond to the liftings of the action of $G$. We apply the results obtained to the categories of topological and differentiable manifolds, and to the category of schemes. For the categories of differentiable manifolds and affine varieties defined over a closed field, we use also another approach induced by the slice theorems of Koszul and Luna which enable to define Grothendieck topologies for $G$-invariant neighborhoods. This lifting problem has been studied in several categories by Brion, Hambleton, Hattori, Haussman, Lashof, May, Yoshida,... We recover and generalize some of their results