5 research outputs found
LambdaCDM epoch reconstruction from F(R,G) and modified Gauss-Bonnet gravities
Dark energy cosmology is considered in a modified Gauss-Bonnet model of
gravity with and without a scalar field. It is shown that these generalizations
of General Relativity endow it with a very rich cosmological structure: it may
naturally lead to an effective cosmological constant, quintessence or phantom
cosmic acceleration, with the possibility to describe the transition from a
decelerating to an accelerating phase explicitly. It is demonstrated here that
these modified GB and scalar-GB theories are perfectly viable as cosmological
models. They can describe the LambdaCDM cosmological era without any need for a
cosmological constant. Specific properties of these theories of gravity in
different particular cases, such as the de Sitter one, are studied.Comment: 14 page
Oscillations of the F(R) dark energy in the accelerating universe
Oscillations of the dark energy around the phantom divide line,
, both during the matter era and also in the de Sitter epoch
are investigated. The analysis during the de Sitter epoch is revisited by
expanding the modified equations of motion around the de Sitter solution. Then,
during the matter epoch, the time dependence of the dark energy perturbations
is discussed by using two different local expansions. For high values of the
red shift, the matter epoch is a stable point of the theory, giving the
possibility to expand the -functions in terms of the dark energy
perturbations. In the late-time matter era, the realistic case is considered
where dark energy tends to a constant. The results obtained are confirmed by
precise numerical computation on a specific model of exponential gravity. A
novel and very detailed discussion is provided on the critical points in the
matter era and on the relation of the oscillations with possible singularities.Comment: 23 pages, 11 figures, version to appear in EPJ
On thermodynamics second law in the modified Gauss Bonnet gravity
The second law and the generalized second law of thermodynamics in cosmology
in the framework of the modified Gauss-Bonnet theory of gravity are
investigated. The conditions upon which these laws hold are derived and
discussed.Comment: 9pages, typos corrected, references adde
Inflation and late-time cosmic acceleration in non-minimal Maxwell- gravity and the generation of large-scale magnetic fields
We study inflation and late-time acceleration in the expansion of the
universe in non-minimal electromagnetism, in which the electromagnetic field
couples to the scalar curvature function. It is shown that power-law inflation
can be realized due to the non-minimal gravitational coupling of the
electromagnetic field, and that large-scale magnetic fields can be generated
due to the breaking of the conformal invariance of the electromagnetic field
through its non-minimal gravitational coupling. Furthermore, it is demonstrated
that both inflation and the late-time acceleration of the universe can be
realized in a modified Maxwell- gravity which is consistent with solar
system tests and cosmological bounds and free of instabilities. At small
curvature typical for current universe the standard Maxwell theory is
recovered. We also consider classically equivalent form of non-minimal
Maxwell- gravity, and propose the origin of the non-minimal gravitational
coupling function based on renormalization-group considerations.Comment: 20 pages, no figure, JCAP versio
Finite-time future singularities in modified Gauss-Bonnet and gravity and singularity avoidance
We study all four types of finite-time future singularities emerging in
late-time accelerating (effective quintessence/phantom) era from
-gravity, where and are the Ricci scalar and the
Gauss-Bonnet invariant, respectively. As an explicit example of
-gravity, we also investigate modified Gauss-Bonnet gravity,
so-called -gravity. In particular, we reconstruct the -gravity and
-gravity models where accelerating cosmologies realizing the
finite-time future singularities emerge. Furthermore, we discuss a possible way
to cure the finite-time future singularities in -gravity and
-gravity by taking into account higher-order curvature
corrections. The example of non-singular realistic modified Gauss-Bonnet
gravity is presented. It turns out that adding such non-singular modified
gravity to singular Dark Energy makes the combined theory to be non-singular
one as well.Comment: 35 pages, no figure, published version, references adde