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arXiv · hep-th/0604086

DGP Specteroscopy

Christos Charmousis·Ruth Gregory·Nemanja Kaloper·Antonio Padilla
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Abstract

We systematically explore the spectrum of gravitational perturbations in codimension-1 DGP braneworlds, and find a 4D ghost on the self-accelerating branch of solutions. The ghost appears for any value of the brane tension, although depending on the sign of the tension it is either the helicity-0 component of the lightest localized massive tensor of mass $0<m^2 < 2H^2$ for positive tension, the scalar `radion' for negative tension, or their admixture for vanishing tension. Because the ghost is gravitationally coupled to the brane-localized matter, the self-accelerating solutions are not a reliable benchmark for cosmic acceleration driven by gravity modified in the IR. In contrast, the normal branch of solutions is ghost-free, and so these solutions are perturbatively safe at large distance scales. We further find that when the $\mathbb{Z}_2$ orbifold symmetry is broken, new tachyonic instabilities, which are much milder than the ghosts, appear on the self-accelerating branch. Finally, using exact gravitational shock waves we analyze what happens if we relax boundary conditions at infinity. We find that non-normalizable bulk modes, if interpreted as 4D phenomena, may open the door to new ghost-like excitations.

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Christos Charmousis, Ruth Gregory, Nemanja Kaloper, Antonio Padilla. 2006-05-17. DGP Specteroscopy. https://doi.org/10.1088/1126-6708%2F2006%2F10%2F066

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