Absence of ghost in a new bimetric-matter coupling

Understand

Interactions in bimetric theory, which can describe gravity in the presence of an extra spin-2 field, are severely constrained by the requirement of the absence of the Boulware-Deser ghost instability.

• Recently an interesting new matter coupling was proposed in terms of a composite metric but it was claimed to reintroduce the ghost.
• In this paper we carry out a nonlinear Hamiltonian analysis of this new matter coupling and show that it is indeed ghost-free.
• The analysis involves using a new set of variables that naturally appear in the relation between the metric and vielbein formulations of bimetric theory.