  Stability about cosmological background solutions to the bi-metric
Hassan-Rosen theory is studied. The results of this analysis are presented, and
it is shown that a large class of cosmological backgrounds is classically
unstable. This sets serious doubts on the physical viability of the
Hassan-Rosen theory - and in turn also of the de Rham-Gadabaze-Tolley model, to
which the mentioned theory is parent. A way to overcome this instability by
means of curvature-type deformations is discussed.
