1,709 research outputs found
On the consistency of universally non-minimally coupled theories
We discuss the consistency of a recently proposed class of theories described
by an arbitrary function of the Ricci scalar, the trace of the energy-momentum
tensor and the contraction of the Ricci tensor with the energy-momentum tensor.
We briefly discuss the limitations of including the energy-momentum tensor in
the action, as it is a non fundamental quantity, but a quantity that should be
derived from the action. The fact that theories containing non-linear
contractions of the Ricci tensor usually leads to the presence of pathologies
associated with higher-order equations of motion will be shown to constrain the
stability of this class of theories. We provide a general framework and show
that the conformal mode for these theories generally has higher-order equations
of motion and that non-minimal couplings to the matter fields usually lead to
higher-order equations of motion. In order to illustrate such limitations we
explicitly study the cases of a canonical scalar field, a K-essence field and a
massive vector field. Whereas for the scalar field cases it is possible to find
healthy theories, for the vector field case the presence of instabilities is
unavoidable.Comment: Minor changes, version published matching Phys. Rev. D 91 (2015)
104003. 15 pages, no figure
On tidal forces in f(R) theories of gravity
Despite the extraordinary attention that modified gravity theories have
attracted over the past decade, the geodesic deviation equation in this context
has not received proper formulation thus far. This equation provides an elegant
way to investigate the timelike, null and spacelike structure of spacetime
geometries. In this investigation we provide the full derivation of this
equation in situations where General Relativity has been extended in
Robertson-Walker background spacetimes. We find that for null geodesics the
contribution arising from the geometrical new terms is in general non-zero.
Finally we apply the results to a well known class of f(R) theories, compare
the results with General Relativity predictions and obtain the equivalent area
distance relation.Comment: 9 pages, 2 figure
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