Causality Constraints on Massive Gravity
  The de Rham-Gabadadze-Tolley massive gravity admits pp-wave backgrounds on which linear fluctuations are shown to undergo time advances for all values of the parameters. The perturbations may propagate in closed time-like curves unless the parameter space is constrained to a line. These classical phenomena take place well within the theory's validity regime. 
Xi\'an O. Camanho, Gustavo Lucena G\'omez and Rakibur Rahman
hep-th astro-ph.CO gr-qc