15 research outputs found
Class of viable modified gravities describing inflation and the onset of accelerated expansion
A general approach to viable modified gravity is developed in both the
Jordan and the Einstein frames. A class of exponential, realistic modified
gravities is introduced and investigated with care. Special focus is made on
step-class models, most promising from the phenomenological viewpoint and which
provide a natural way to classify all viable modified gravities. One- and
two-steps models are explicitly considered, but the analysis is extensible to
-step models. Both inflation in the early universe and the onset of recent
accelerated expansion arise in these models in a natural, unified way.
Moreover, it is demonstrated that models in this category easily pass all local
tests, including stability of spherical body solution, non-violation of
Newton's law, and generation of a very heavy positive mass for the additional
scalar degree of freedom.Comment: 15 pages, 2 figures, version to appear in Physical Review
One-loop f(R) Gravitational Modified Models
The one-loop quantisation of a general class of modified gravity models
around a classical de Sitter background is presented. Application to the
stability of the models is addressed.Comment: Latex, 8 pages, no figures. To appear in Journal of Physics A. Two
references adde
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
Equilibrium hydrostatic equation and Newtonian limit of the singular f(R) gravity
We derive the equilibrium hydrostatic equation of a spherical star for any
gravitational Lagrangian density of the form . The Palatini
variational principle for the Helmholtz Lagrangian in the Einstein gauge is
used to obtain the field equations in this gauge. The equilibrium hydrostatic
equation is obtained and is used to study the Newtonian limit for
. The same procedure is carried out for the more
generally case giving a good
Newtonian limit.Comment: Revised version, to appear in Classical and Quantum Gravity
One-loop f(R) gravity in de Sitter universe
Motivated by the dark energy issue, the one-loop quantization approach for a
family of relativistic cosmological theories is discussed in some detail.
Specifically, general gravity at the one-loop level in a de Sitter
universe is investigated, extending a similar program developed for the case of
pure Einstein gravity. Using generalized zeta regularization, the one-loop
effective action is explicitly obtained off-shell, what allows to study in
detail the possibility of (de)stabilization of the de Sitter background by
quantum effects. The one-loop effective action maybe useful also for the study
of constant curvature black hole nucleation rate and it provides the plausible
way of resolving the cosmological constant problem.Comment: 25 pages, Latex file. Discussion enlarged, new references added.
Version accepted in JCA
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
Cosmological entropy and generalized second law of thermodynamics in theory of gravity
We consider a spatially flat Friedmann-Lemaitre-Robertson-Walker space time
and investigate the second law and the generalized second law of thermodynamics
for apparent horizon in generalized modified Gauss Bonnet theory of gravity
(whose action contains a general function of Gauss Bonnet invariant and the
Ricci scalar: ). By assuming that the apparent horizon is in thermal
equilibrium with the matter inside it, conditions which must be satisfied by
are derived and elucidated through two examples: a quasi-de Sitter
space-time and a universe with power law expansion.Comment: 10 pages, minor changes, typos corrected, accepted for publication in
Europhysics Letter
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
Ghost-free braneworld bigravity
We consider a generalisation of the DGP model, by adding a second brane with
localised curvature, and allowing for a bulk cosmological constant and brane
tensions. We study radion and graviton fluctuations in detail, enabling us to
check for ghosts and tachyons. By tuning our parameters accordingly, we find
bigravity models that are free from ghosts and tachyons. These models will lead
to large distance modifications of gravity that could be observable in the near
future.Comment: Dedicated to the memory of Ian Kogan. Version to appear in Classical
and Quantum Gravit