The coupling to matter in Massive, Bi- and Multi-Gravity
  In this paper we construct a family of ways in which matter can couple to one or more `metrics'/spin-2 fields in the vielbein formulation. We do so subject to requiring the weak equivalence principle and the absence of ghosts from pure spin-2 interactions generated by the matter action. Results are presented for Massive, Bi- and Multi-Gravity theories and we give explicit expressions for the effective matter metric in all of these cases. 
Johannes Noller, Scott Melville
hep-th astro-ph.CO gr-qc hep-ph