  Recently, the construction of ghost-free nonlinear massive spin-2
interactions solved a long outstanding problem in classical field theory. The
consistent formulation requires a second rank-two tensor (or metric) and
therefore has the form of a bimetric theory. Besides providing the solution to
an old problem in field theory, ghost-free bimetric theory also has an
interesting phenomenology: Its homogeneous and isotropic background solutions
can reproduce the expansion history of the universe without any input of vacuum
energy.
