Superluminality in DGP

Abstract

We reconsider the issue of superluminal propagation in the DGP model of infrared modified gravity. Superluminality was argued to exist in certain otherwise physical backgrounds by using a particular, physically relevant scaling limit of the theory. In this paper, we exhibit explicit five-dimensional solutions of the full theory that are stable against small fluctuations and that indeed support superluminal excitations. The scaling limit is neither needed nor invoked in deriving the solutions or in the analysis of its small fluctuations. To be certain that the superluminality found here is physical, we analyze the retarded Green's function of the scalar excitations, finding that it is causal and stable, but has support on a widened light-cone. We propose to use absence of superluminal propagation as a method to constrain the parameters of the DGP model. As a first application of the method, we find that whenever the 4D energy density is a pure cosmological constant and a hierarchy of scales exists between the 4D and 5D Planck masses, superluminal propagation unavoidably occurs.

23 pages. Minor corrections. Version to appear in JHEP

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• When Matter Matters
• Deconstructing Dimensions and Massive Gravity
• Superluminality, Black Holes and Effective Field Theory
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• Cosmological perturbation in f(R,G) theories with a perfect fluid
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• The nonlinear dynamical stability of infrared modifications of gravity
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• Supercomputers against strong coupling in gravity with curvature and torsion
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