Massles multi gravity: Covariant, stable, two-derivative interacting   massless multi-graviton theories
  We present a general model of interacting metric fields with the sum of massless non-interacting spin 2 fields as the linear limit. In the non-interacting limit the model is reduced to a sum of general relativity actions, with the usual Einstein-Hilbert kinetic term for each metric field. Until now the accepted view was that such theories are inconsistent since it has been proven that ghost terms and/or discontinuity in the number of degrees of freedom at the zero interaction limit, are unavoidable. We do not refute these results, but instead prove by construction that they do not necessarily lead to inconsistencies. In particular, our suggested theories are: 1. Energetically stable, i.e. their Hamiltonian is bounded from below and the multi-Minkowski metric configuration is the unique ground state. 2. Continuous in the zero interaction limit so that general relativity solutions are restored smoothly. 3. Include no higher than two derivative Lagrangian terms. In addition, the dominant energy condition is maintained with respect to all metric fields for all field configurations. 
Idan Talshir
gr-qc