3 research outputs found
Cosmological perturbations in Palatini modified gravity
Two approaches to the study of cosmological density perturbations in modified
theories of Palatini gravity have recently been discussed. These utilise,
respectively, a generalisation of Birkhoff's theorem and a direct linearization
of the gravitational field equations. In this paper these approaches are
compared and contrasted. The general form of the gravitational lagrangian for
which the two frameworks yield identical results in the long-wavelength limit
is derived. This class of models includes the case where the lagrangian is a
power-law of the Ricci curvature scalar. The evolution of density perturbations
in theories of the type is investigated numerically. It is
found that the results obtained by the two methods are in good agreement on
sufficiently large scales when the values of the parameters (b,c) are
consistent with current observational constraints. However, this agreement
becomes progressively poorer for models that differ significantly from the
standard concordance model and as smaller scales are considered
Curvature singularities, tidal forces and the viability of Palatini f(R) gravity
In a previous paper we showed that static spherically symmetric objects
which, in the vicinity of their surface, are well-described by a polytropic
equation of state with 3/2<Gamma<2 exhibit a curvature singularity in Palatini
f(R) gravity. We argued that this casts serious doubt on the validity of
Palatini f(R) gravity as a viable alternative to General Relativity. In the
present paper we further investigate this characteristic of Palatini f(R)
gravity in order to clarify its physical interpretation and consequences.Comment: 15 pages. CQG in press. Part of the material moved to an appendix,
discussion on the meV scale predictions of Palatini f(R) gravity adde
The phase space view of f(R) gravity
We study the geometry of the phase space of spatially flat
Friedmann-Lemaitre-Robertson-Walker models in f(R) gravity, for a general form
of the function f(R). The equilibrium points (de Sitter spaces) and their
stability are discussed, and a comparison is made with the phase space of the
equivalent scalar-tensor theory. New effective Lagrangians and Hamiltonians are
also presented.Comment: 14 pages, 2 figures, published in Classical and Quantum Gravity;
references adde