  In arXiv:hep-th/0011245, in the context of domain wall backgrounds, it was
shown that spontaneous breaking of diffeomorphism invariance results in
gravitational Higgs mechanism. Recently in arXiv:0708.3184 't Hooft discussed
gravitational Higgs mechanism in the context of obtaining massive gravity
directly in four dimensions, and pointed out some subtleties with unitarity. We
attribute these subtleties to breaking time-like diffeomorphisms, and discuss
gravitational Higgs mechanism with all but time-like diffeomorphisms
spontaneously broken. The resulting background is no longer flat but exhibits
expansion, which is linear in time. For space-time dimensions D <= 10 the
background is stable and has no non-unitary propagating modes. The absence of
non-unitary modes is due to the unbroken time-like diffeomorphism invariance.
The physical states correspond to those of a massive graviton. The effective
mass squared of the graviton is positive for D < 10, and vanishes for D = 10.
For D > 10 the graviton modes become effectively tachyonic. The special value
of D=10, which coincides with the critical dimension of superstring theory,
arises in our setup completely classically.
