Vacuum structure for scalar cosmological perturbations in modified gravity models
Abstract
We have found for the general class of Modified Gravity Models f(R, Script 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 shortwavelength 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 ghostlike. There is a subclass of these models, defined out of properties of the function f(R, Script G) and to which the f(R) and f(Script G) models belong, which however do not have this feature.
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 June 2009
 DOI:
 10.1088/14757516/2009/06/034
 arXiv:
 arXiv:0904.2092
 Bibcode:
 2009JCAP...06..034D
 Keywords:

 Astrophysics  Cosmology and Extragalactic Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Theory
 EPrint:
 17 pages, 1 figure, uses RevTeX, references added