Novel aspects of C-theories in Cosmology

Abstract

The field equations in FRW background for the so called C-theories are presented and investigated. In these theories the usual Ricci scalar is substituted with $f(\mathcal{R})$ where $\mathcal{R}$ is a Ricci scalar related to a conformally scaled metric ${\hat{g}}_{\mu \nu }=\mathcal{C}(\mathcal{R})g_{\mu \nu }$, where the conformal factor itself depends on $\mathcal{R}$. It is shown that homogeneous perturbations of this Ricci scalar around general relativity FRW background of a large class of these theories are either inconsistent or unstable.

18 pages, no figures

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