  Relevant deformations of gravity present an exciting window of opportunity to
probe the rigidity of gravity on cosmological scales. For a single-graviton
theory, the leading relevant deformation constitutes a graviton mass term. In
this paper, we investigate the classical and quantum stability of massive
cosmological gravitons on generic Friedman backgrounds. For a Universe
expanding towards a de Sitter epoch, we find that massive cosmological
gravitons are self-protected against unitarity violations by a strong coupling
phenomenon.
