Prof. Michael Volkov (University of Tours)
I calculate the energy in the dRGT massive gravity. This requires resolving the constraints, which can be done in the spherically symmetric sector. The energy is found to be positive for globally regular and asymptotically flat fields constituting the "physical sector" of the theory. There are also solutions of the constraints whose energy is negative and unbounded from below and which show tachyons. However, these solutions are either not globally regular or not asymptotically flat. They cannot describe initial data triggering ghost instability of the physical sector. This allows one to conjecture that the physical sector is protected from the instability by a potential barrier separating it from superluminal and negative energy states. I also consider cosmological applications.