Massive Gravity in de Sitter Space via Gravitational Higgs Mechanism
  In this paper we discuss massive gravity in de Sitter space via gravitational
Higgs mechanism, which provides a nonlinear definition thereof. The Higgs
scalars are described by a nonlinear sigma model, which includes higher
derivative terms required to obtain the Fierz-Pauli mass term. Using the
aforesaid non-perturbative definition, we address appearance of an enhanced
local symmetry and a null norm state in the linearized massive gravity in de
Sitter space at the special value of the graviton mass to the Hubble parameter
ratio. By studying full non-perturbative equations of motion, we argue that
there is no enhanced symmetry in the full nonlinear theory. We then argue that
in the full nonlinear theory no null norm state is expected to arise at the
aforesaid special value. This suggests that no ghost might be present for lower
graviton mass values and the full nonlinear theory might be unitary for all
values of the graviton mass and the Hubble parameter with no van
Dam-Veltman-Zakharov discontinuity. We argue that this is indeed the case by
studying full nonlinear Hamiltonian for the relevant conformal and helicity-0
longitudinal modes. In particular, we argue that no negative norm state is
present in the full nonlinear theory.
