The Cosmology of Interacting Spin-2 Fields
  We investigate the cosmology of interacting spin-2 particles, formulating the
multi-gravitational theory in terms of vierbeins and without imposing any
Deser-van Nieuwenhuizen-like constraint. The resulting multi-vierbein theory
represents a wider class of gravitational theories if compared to the
corresponding multi-metric models. Moreover, as opposed to its metric
counterpart which in general seems to contain ghosts, it has already been
proved to be ghost-free. We outline a discussion about the possible matter
couplings and we focus on the study of cosmological scenarios in the case of
three and four interacting vierbeins. We find rich behavior, including de
Sitter solutions with an effective cosmological constant arising from the
multi-vierbein interaction, dark-energy solutions and nonsingular bouncing
solutions.
