> We remark that A_\mu also contains a pure vector mode perturbation which is expected to behave similarly as in the Einstein-Æther theory [90, 91]"
Their [91] is Jacobson & Mattingly https://arxiv.org/abs/gr-qc/0007031 whose §VII (DISCUSSION) contains this, which I struggle to see as helpful for them: "With the action adopted in this paper the aether vector generically develops gradient singularities even when the metric is perfectly regular. We take this as a sign that the theory is unphysical as an effective theory". (That doesn't stop Jacobson from investigating things like (time-independent) black hole solutions https://arxiv.org/abs/gr-qc/0604088 "It is a plausible conjecture that nonsingular spherically symmetric initial data will evolve to one of the regular black holes whose existence has been demonstrated here, but this has certainly not been shown", and worse they show that the aether does not obey the Raychaudhri equation, so the relativistic MOND authors seem to need more ghosts).
Their [91] is Jacobson & Mattingly https://arxiv.org/abs/gr-qc/0007031 whose §VII (DISCUSSION) contains this, which I struggle to see as helpful for them: "With the action adopted in this paper the aether vector generically develops gradient singularities even when the metric is perfectly regular. We take this as a sign that the theory is unphysical as an effective theory". (That doesn't stop Jacobson from investigating things like (time-independent) black hole solutions https://arxiv.org/abs/gr-qc/0604088 "It is a plausible conjecture that nonsingular spherically symmetric initial data will evolve to one of the regular black holes whose existence has been demonstrated here, but this has certainly not been shown", and worse they show that the aether does not obey the Raychaudhri equation, so the relativistic MOND authors seem to need more ghosts).
For the life of me, I can't figure out the relevance of their reference [90] which I believe is https://www.jstor.org/stable/2414316
I wonder who their Reviewer 2 was.