Null Energy Condition violations in bimetric gravity
  We consider the effective stress-energy tensors for the foreground and background sectors in ghost-free bimetric gravity. By considering the symmetries of the theory, we show that the foreground and background null energy conditions (NECs) are strongly anti-correlated. In particular, the NECs can only be simultaneously fulfilled when they saturate, corresponding to foreground and background cosmological constants. In all other situations, either the foreground or the background is subject to a NEC-violating contribution to the total stress-energy. 
Valentina Baccetti (Victoria University of Wellington), Prado   Martin-Moruno (Victoria University of Wellington), and Matt ...
gr-qc hep-th