Vacuum structure for scalar cosmological perturbations in Modified Gravity Models
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We have found for the general class of Modified Gravity Models f(R,G) a new instability which can arise in vacuum for the scalar modes of the cosmological perturbations if the background is not de Sitter. In particular, the short-wavelength modes, if stable, in general have a group velocity which depends linearly in k, the wave number. Therefore these modes will be in general superluminal. We have also discussed the condition for which in general these scalar modes will be ghost-like. There is a subclass of these models, defined out of properties of the function f(R,G) and to which the f(R) and f(G) models belong, which however does not have this feature.
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Cited by 3 Pith papers
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Photon Sphere and Shadow of a Perturbative Black Hole in $f(R,\mathcal{G})$ Gravity
Perturbative f(R, G) corrections shift the photon-sphere radius and black-hole shadow size, with the Gauss-Bonnet sector contributing more than mixed terms.
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Photon Sphere and Shadow of a Perturbative Black Hole in $f(R,\mathcal{G})$ Gravity
Perturbative f(R,G) corrections shift the photon-sphere radius and shadow size, with the Gauss-Bonnet term dominating over mixed curvature contributions.
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Photon Sphere and Shadow of a Perturbative Black Hole in $f(R,\mathcal{G})$ Gravity
Perturbative higher-curvature corrections in f(R,G) gravity shift the photon-sphere radius and black-hole shadow size away from Schwarzschild values, with the Gauss-Bonnet sector contributing more than mixed terms.
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