Article

# An overview of f(R) theories

07/2011;
Source: arXiv

ABSTRACT A brief introduction to theories of the gravitational field with a Lagrangian
that is a function of the scalar curvature is given. The emphasis will be
placed in formal developments, while comparison to observation will be
discussed in the chapter by S. Jor\'as in this volume.

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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.
05/2013;
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##### Article: Ghosts in pure and hybrid formalisms of gravity theories: a unified analysis
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ABSTRACT: In the first order formalism of gravitational theories, the spacetime connection is considered as an independent variable to vary together with the metric. However, the metric still generates its Levi-Civita connection that turns out to determine the geodesics of matter. Recently, "hybrid" gravity theories have been introduced by constructing actions involving both the independent Palatini connection and the metric Levi-Civita connection. In this study a method is developed to analyse the field content of such theories, in particular to determine whether the propagating degrees of freedom are ghosts or tachyons. New types of second, fourth and sixth order derivative gravity theories are investigated and the so called f(X) theories are singled out as a viable class of "hybrid" extensions of General Relativity.
Physical review D: Particles and fields 04/2013; 87(10).