Antonio De Felice et al JCAP08(2006)005 doi:10.1088/1475-7516/2006/08/005
Antonio De Felice1, Mark Hindmarsh1 and Mark Trodden2
Show affiliationsWe 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 generically exhibit superluminally propagating tensor or scalar modes.
E-print Number: astro-ph/0604154
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Issue 08 (August 2006)
Received 3 May 2006, accepted for publication 21 July 2006
Published 16 August 2006
Antonio De Felice et al JCAP08(2006)005
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