125 research outputs found
Linear growth of matter density perturbations in f(R,G) theories
We derive the equation of matter density perturbations on sub-horizon scales
around a flat Friedmann-Lema\^\i tre-Robertson-Walker background for the
general Lagrangian density f(R,\GB) that is a function of a Ricci scalar
and a Gauss-Bonnet term \GB. We find that the effective gravitational
constant generically scales as distance squared at small distances. The effect
of this diminishing of the gravitational constant might be important in the
gravitational dynamics of cosmic objects such as galaxies, which can be in
principle tested by observations. We also provide the general expressions for
the effective anisotropic stress, which is useful to constrain modified gravity
models from observations of large-scale structure and weak lensing. We also
find that there is a special class of theories which evade this unusual
behaviour and that the condition to belong to this special class is exactly the
same as the one for not having super-luminal modes with propagation speed
proportional to their wavenumber.Comment: Accepted for publication in Progress of Theoretical Physics,
references added and typos corrected, 13 page
Scalar mode propagation in modified gravity with a scalar field
We study the propagation of the scalar modes around a
Friedmann-Lemaitre-Robertson-Walker universe for general modifications of
gravity in the presence of a real scalar field. In general, there will be two
propagating scalar perturbation fields, which will have in total four degrees
of freedom. Two of these degrees will have a superluminal propagation--with
k-dependent speed of propagation--whereas the other two will travel with the
speed of light. Therefore, the scalar degrees of freedom do not modify the
general feature of modified gravity models: the appearance of modes whose
frequency depends on the second power of the modulus of the wave vector.
Constraints are given and special cases are discussed.Comment: 13 pages, 1 figure, uses RevTe
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