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"Brane Induced Gravity" is regarded as a promising framework for addressingthe cosmological constant problem, but it also suffers from a ghost instabilityfor parameter values that make it phenomenologically viable. We carry out adetailed analysis of codimension > 2 models employing gauge invariant variablesin a flat background approximation. It is argued that using instead a curvedbackground sourced by the brane would not resolve the ghost issue, unless avery specific condition is satisfied (if satisfiable at all). As for otherproperties of the model, from an explicit analysis of the 4-dimensionalgraviton propagator we extract a mass, a decay width and a momentum dependentmodification of the gravitational coupling for the spin 2 mode. In the flatspace approximation, the mass of the problematic spin 0 ghost is instrumentalin filtering out a brane cosmological constant. The mass replaces a backgroundcurvature that would have had the same function. The optical theorem is used todemonstrate the suppression of graviton leakage into the uncompactified bulk.Then, we derive the 4-dimensional effective action for gravity and show thatgeneral covariance is spontaneously broken by the bulk-brane setup. Thisprovides a natural realization of the gravitational Higgs mechanism. We alsoshow that the addition of extrinsic curvature dependent terms has no bearing onlinearized brane gravity.

Brane Induced Gravity, its Ghost and the Cosmological Constant Problem