Massive gravitons as dark matter and gravitational waves
  We consider the possibility that the massive graviton is a viable candidate
of dark matter in the context of bimetric gravity. We first derive the
energy-momentum tensor of the massive graviton and show that it indeed behaves
as that of dark matter fluid. We then discuss a production mechanism and the
present abundance of massive gravitons as dark matter. Since the metric to
which ordinary matter fields couple is a linear combination of the two mass
eigenstates of bigravity, production of massive gravitons, i.e. the dark matter
particles, is inevitably accompanied by generation of massless gravitons, i.e.
the gravitational waves. Therefore, in this scenario some information about
dark matter in our universe is encoded in gravitational waves. For instance, if
LIGO detects gravitational waves generated by the preheating after inflation
then the massive graviton with the mass of $\sim 0.01$ GeV is a candidate of
the dark matter.
