Modified gravity and the stability of de Sitter space

Within the context of modified gravity and dark energy scenarios of the accelerating universe, we study the stability of de Sitter space with respect to inhomogeneous perturbations using a gauge-independent formalism. In modified gravity the stability condition is exactly the same that one obtains from a homogeneous perturbation analysis, while the stability condition in scalar-tensor gravity is more restrictive.Comment: 5 pages, no figures, RevTeX, to appear in Phys. Rev. D (Rapid Communications section

Negative energy and stability in scalar-tensor gravity

Linearized gravitational waves in Brans-Dicke and scalar-tensor theories carry negative energy. A gauge-invariant analysis shows that the background Minkowski space is stable at the classical level with respect to linear scalar and tensor inhomogeneous perturbations.Comment: 9 pages, latex, to appear in Phys. Rev.

de Sitter space and the equivalence between f(R) and scalar-tensor gravity

It is shown that, when f'' is non-vanishing, metric f(R) gravity is completely equivalent to a scalar-tensor theory (with zero Brans-Dicke parameter) with respect to perturbations of de Sitter space, contrary to previous expectations. Moreover, the stability conditions of de Sitter space with respect to homogeneous and inhomogeneous perturbations coincide in most scalar-tensor theories, as is the case in metric f(R) gravity.Comment: 4 pages, revtex, to appear in Phys. Rev. D. Revised version contains additional and updated reference

Massive spin zero fields in cosmology and the tail-free property

Fields of spin $s \geq 1/2$ satisfying wave equations in a curved space obey the Huygens principle under certain conditions clarified by a known theorem. Here this theorem is generalized to spin zero and applied to an inflaton field in de Sitter-like space, showing that tails of scalar radiation are an unavoidable physical feature. Requiring the absence of tails, on the contrary, necessarily implies an unnatural tuning between cosmological constant, scalar field mass, and coupling constant to the curvature.Comment: To appear in "Cosmology and Quantum Vacuum", special issue of Symmetry edited by E. Elizald