hi, I'm currently reading and trying to make up my own mind about massive bigravity articles such as https://arxiv.org/pdf/1505.05146.pdf great effort is done to avoid the so-called BD ghost yet eventually the authors get a field equation (3.12) with both positive and negative energy sources coupling to gravity which i was always told is the perfect signature for the presence of ghost interactions !! If this antigravity physics is eventually allowed and harmless that would be really a revolution. that's why i want to be sure i understand the argument ! But i hardly find justifications : for instance in https://arxiv.org/pdf/1409.3834.pdf there is the sentence (page 2): "Of course quantum corrections will force every sector to couple to every metric, but such couplings will be highly suppressed and are thus unimportant for the rest of this discussion." Can this be the justification of the harmlessness of a ghost that appears like the nose on the middle of the face already in the first order field equation 3.12 ?! unbelievable ... but (and this is my second motivation) if true i would be glad since that would ensure the stability of my own theoretical proposal published many years ago... Was this already discussed somewhere in physis forum ?