Cosmic expansion histories in massive bigravity with symmetric matter
  coupling
  We study the cosmic expansion history of massive bigravity with a viable
matter coupling which treats both metrics on equal footing. We derive the
Friedmann equation for the effective metric through which matter couples to the
two metrics, and study its solutions. For certain parameter choices, the
background cosmology is identical to that of LCDM. More general parameters
yield dynamical dark energy, which can still be in agreement with observations
of the expansion history. We study specific parameter choices of interest,
including minimal models, maximally-symmetric models, and a candidate
partially-massless theory.
