Ghosts, Instabilities, and Superluminal Propagation in Modified Gravity Models

Details Ghosts, Instabilities, and Superluminal Propagation in Modified Gravity Models Ghosts, Instabilities, and Superluminal Propagation in Modified Gravity Models

2006-04-07

arxiv.org/abs/astro-ph/0604154v2

JCAP 0608 (2006) 005

Astronomy and Astrophysics

Astrophysics

17 pages, 1 figure, uses RevTeX. Added a few comments. Changed the figure

Scientific paper

10.1088/1475-7516/2006/08/005

We consider Modified Gravity models involving inverse powers of fourth-order curvature invariants. Using these models' equivalence to the theory of a scalar field coupled to a linear combination of the invariants, we investigate the properties of the propagating modes. Even in the case for which the fourth derivative terms in the field equations vanish, we find that the second derivative terms can give rise to ghosts, instabilities, and superluminal propagation speeds. We establish the conditions which the theories must satisfy in order to avoid these problems in Friedmann backgrounds, and show that the late-time attractor solutions are generically afflicted by superluminally propagating tensor or scalar modes.

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