Global structure of bigravity solutions
  We discuss the causal diagrams of static and spherically symmetric bigravity vacuum solutions, with interacting metrics $f$ and $g$. Such solutions can be classified into type I (or "non-diagonal") and type II (or "diagonal"). The general solution of type I is known, and leads to metrics $f$ and $g$ in the Schwarzschild-(Anti)de Sitter family. The two metrics are not always diagonalizable in the same coordinate system, and the light-cone structure of both metrics can be quite different. In spite of this, we find that causality is preserved, in the sense that closed time-like curves cannot be pieced together from geodesics of both metrics. We propose maximal extensions of Type I bigravity solutions, where geodesics of both metrics do not stop unless a curvature singularity is encountered. Such maximal extensions can contain several copies (or even an infinite number of them) of the maximally extended "individual" geometries associated to $f$ and $g$ separately. Generically, we find that the maximal extensions of bigravity solutions are not globally hyperbolic, even in cases when the individual geometries are. The general solution of type II has not been given in closed form. We discuss a subclass where $g$ is an arbitrary solution of Einstein's equations with a cosmological constant, and we find that in this case the only solutions are such that $f\propto g$ (with trivial causal structure). 
Diego Blas, Cedric Deffayet and Jaume Garriga