Ghosts, instabilities, and superluminal propagation in modified gravity models

By Antonio de Felice, M. Hindmarsh, M. Trodden. 2006

140 citations recorded by the source corpus.

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 generically exhibit superluminally propagating tensor or scalar modes.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/809b5eaf2834023bf777c594cef2243a0cc74990

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Citations in the corpus

f(R) Theories (2010, unknown): “For the Gauss– Bonnet (GB) curvature invariant defined by G ≡ R(2) − 4Rαβ R +Rαβγδ R, it is known that one can avoid the appearance of spurious spin-2 ghosts [572, 67, 302] (see also [98, 465, 153, 447, 110, 181, 109]).”
Modified Gravity and Cosmology (2011, background): “Not least of these is the presence of ‘ghosts’, or perturbative modes with negative norm, as well as tachyonic instabilities in massive modes [1187, 236, 606, 607, 293, 944, 392, 113, 990].”
Gauss–Bonnet cosmologies: crossing the phantom divide and the transition from matter dominance to dark energy (2007, background): “Recently, a number of authors [28, 38](11) discussed constraints on the field-dependent Gauss–Bonnet couplings with a single scalar field, so as to avoid the short-scale instabilities or superluminal propagation of scalar and tensor modes(12).”
A Bird's Eye View of f (R)-Gravity (2009, background): “A possible exception under certain conditions studied in [91] is provided by theories in which the extra terms are restricted to appear in the Gauss-Bonnet combination μ!"#”
f(R) Theories Of Gravity (2008, background): “In principle, several kinds of instabilities need to be considered to make sure that f(R) gravity is a viable alternative to GR (Calcagni et al., 2006; Chiba, 2005; De Felice et al., 2006; Sokolowski, 2007a,b,c; Wang, 2005).”
Approaches to understanding cosmic acceleration (2009, background): “It has recently been shown that around a FRW background with matter, the theories are ghost-free, but contain superluminally propagating scalar or tensor modes over a wide range of parameter space [176, 177].”