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Ghosts, Instabilities, and Superluminal Propagation in Modified Gravity Models
journal contributionposted on 2023-06-07, 19:46 authored by Antonio De Felice, Mark HindmarshMark Hindmarsh, Mark Trodden
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.
JournalJournal of Cosmology and Astroparticle Physics
Department affiliated with
- Physics and Astronomy Publications
NotesCollaboration co-leader and supervisor of PDRA de Felice. We show that recently-proposed attempts to explain the cosmic acceleration by modifying gravity suffer from fundamental problems related to propagation instabilities of signals in the cosmos.
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