Homogeneous and Isotropic Universe from Nonlinear Massive Gravity

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The question of finite range gravity, or equivalently,
whether graviton can have a non-zero mass, has been one of the major challenges
in classical field theory for the last 70 years.

Generically, a massive gravity theory contains an extra
degree in addition to the 5 polarizations of massive graviton, which turns out
to be a ghost. Recently, de Rham, Gabadadze and Tolley constructed a nonlinear
theory of massive gravity, which successfully eliminates the ghost. Moreover,
the theory has also phenomenological relevance, since the graviton mass may
account for the accelerated expansion of the present universe, providing an
alternative to dark energy. I will present self-accelerating cosmological
solutions in the framework of this theory. The cosmological perturbations
around these backgrounds have an interesting behavior: instead of the 5 degrees
of freedom expected from a massive spin-2 field, only the 2 gravity wave
polarizations are dynamical, at linear level. However, nonlinear analysis of
the extra modes reveal the existence of ghost instabilities. This implies that
a consistent universe solution in this theory should be inhomogeneous and/or