Vacuum structure for scalar cosmological perturbations in Modified Gravity Models

Details Vacuum structure for scalar cosmological perturbations in Modified Gravity Models Vacuum structure for scalar cosmological perturbations in Modified Gravity Models

2009-04-14

arxiv.org/abs/0904.2092v2

JCAP 0906:034,2009

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

17 pages, 1 figure, uses RevTeX, references added

Scientific paper

10.1088/1475-7516/2009/06/034

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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