Ghosts & Matter Couplings in Massive (bi-&multi-)Gravity

Understand

Recently, several works have investigated the coupling to matter in ghost-free massive (bi-&multi-)gravity and a new effective coupling to matter has been proposed.

• In this note we clarify some confusion on the existence and the implications of a ghost above the strong coupling scale.
• We confirm that the standard constraint which is otherwise typically present in this type of theories disappears on generic backgrounds as soon as this new coupling is considered.
• This implies the re-emergence of the Boulware-Deser ghost.