Extending applicability of bimetric theory: chameleon bigravity
  This article extends bimetric formulations of massive gravity to make the
mass of the graviton to depend on its environment. This minimal extension
offers a novel way to reconcile massive gravity with local tests of general
relativity without invoking the Vainshtein mechanism. On cosmological scales,
it is argued that the model is stable and that it circumvents the Higuchi
bound, hence relaxing the constraints on the parameter space. Moreover, with
this extension the strong coupling scale is also environmentally dependent in
such a way that it is kept sufficiently higher than the expansion rate all the
way up to the very early universe, while the present graviton mass is low
enough to be phenomenologically interesting. In this sense the extended
bigravity theory serves as a partial UV completion of the standard bigravity
theory. This extension is very generic and robust and a simple specific example
is described.
