Results 1 - 10 of 55
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[en] The t<0 branch of pre-big bang cosmological scenarios is subject to a gravitational wave instability. The unstable behaviour of tensor perturbations is derived in a very simple way in Hwang's covariant and gauge-invariant formalism developed for extended theories of gravity. A simple interpretation of this instability as the effect of an ''antifriction'' is given, and it is argued that a universe must eventually enter the expanding phase. (orig.)
[en] We discuss the validity, or lack thereof, of the Jebsen-Birkhoff theorem in scalar-tensor theories by generalizing it and regarding the Brans-Dicke-like scalar as effective matter. Both the Jordan and Einstein frames are discussed and an apparent contradiction between static spherical solutions of scalar-tensor gravity and Hawking's theorem on Brans-Dicke black holes is clarified. The results are applied to metric and Palatini f(R) gravity.
[en] We present a unified treatment of the phase space of a spatially flat homogeneous and isotropic universe dominated by a phantom field. Results on the dynamics and the late time attractors (big rip, de Sitter, etc) are derived without specifying the form of the phantom potential, using only general assumptions on its shape. Many results found in the literature are quickly recovered and predictions are made for new scenarios
[en] The Dolgov-Kawasaki instability discovered in the matter sector of the modified gravity scenario incorporating a -μ4/R correction to Einstein gravity is studied in general f(R) theories. A stability condition is found in the metric version of these theories to help ruling out models that are unviable from the theoretical point of view. For example, the theories f(R)=R+αμ2(n+1)/Rn, where α and n are real constants and n>0, are ruled out for any negative value of α
[en] Within the context of modified gravity and dark energy scenarios of the accelerated 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
[en] We revisit the issue of the correct Lagrangian description of a perfect fluid (L1=P versus L2=-ρ) in relation with modified gravity theories in which galactic luminous matter couples nonminimally to the Ricci scalar. These Lagrangians are only equivalent when the fluid couples minimally to gravity and not otherwise; in the presence of nonminimal coupling they give rise to two distinct theories with different predictions.
[en] In the context of a debate on the correct expression of the Hawking temperature of a cosmological black hole, we show that the correct expression in terms of the Hawking-Hayward quasilocal energy mH of the hole is T=(8πmH(t))-1. This expression holds for comoving black holes and agrees with a recent proposal by Saida, Harada, and Maeda
[en] It is shown that, when f''≠0, metric f(R) gravity is completely equivalent to an ω=0 scalar-tensor theory 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