36 research outputs found
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
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 evolution, future singularities, Little Rip and Pseudo-Rip in viable f(R) theories and their scalar-tensor counterpart
Modified f(R) gravity is one of the most promising candidates for dark
energy, and even for the unification of the whole cosmological evolution,
including the inflationary phase. Within this class of theories, the so-called
viable modified gravities represent realistic theories that are capable of
reproducing late-time acceleration, and satisfy strong constraints at local
scales, where General Relativity is recovered. The present manuscript deals
with the analysis of the cosmological evolution for some of these models, which
indicates that the evolution may enter into a phantom phase, but the behavior
may be asymptotically stable. Furthermore, the scalar-tensor equivalence of
f(R) gravity is considered, which provides useful information about the
possibility of the occurrence of a future singularity. The so-called Little Rip
and Pseudo-Rip are also studied in the framework of this class of modified
gravities.Comment: 20 pages. Extended version, new figures and additional analysis.
Version to be published in Class. Quant. Gra
Unifying inflation with dark energy in modified F(R) Horava-Lifshitz gravity
We study FRW cosmology for a non-linear modified F(R) Horava-Lifshitz gravity
which has a viable convenient counterpart. A unified description of early-time
inflation and late-time acceleration is possible in this theory, but the
cosmological dynamic details are generically different from the ones of the
convenient viable F(R) model. Remarkably, for some specific choice of
parameters they do coincide. The emergence of finite-time future singularities
is investigated in detail. It is shown that these singularities can be cured by
adding an extra, higher-derivative term, which turns out to be qualitatively
different when compared with the corresponding one of the convenient F(R)
theory.Comment: LaTeX 12 pages, typos are correcte
Screening of cosmological constant for De Sitter Universe in non-local gravity, phantom-divide crossing and finite-time future singularities
We investigate de Sitter solutions in non-local gravity as well as in
non-local gravity with Lagrange constraint multiplier. We examine a condition
to avoid a ghost and discuss a screening scenario for a cosmological constant
in de Sitter solutions. Furthermore, we explicitly demonstrate that three types
of the finite-time future singularities can occur in non-local gravity and
explore their properties. In addition, we evaluate the effective equation of
state for the universe and show that the late-time accelerating universe may be
effectively the quintessence, cosmological constant or phantom-like phases. In
particular, it is found that there is a case in which a crossing of the phantom
divide from the non-phantom (quintessence) phase to the phantom one can be
realized when a finite-time future singularity occurs. Moreover, it is
demonstrated that the addition of an term can cure the finite-time future
singularities in non-local gravity. It is also suggested that in the framework
of non-local gravity, adding an term leads to possible unification of the
early-time inflation with the late-time cosmic acceleration.Comment: 42 pages, no figure, version accepted for publication in General
Relativity and Gravitatio
Multiple CDM cosmology with string landscape features and future singularities
Multiple CDM cosmology is studied in a way that is formally a
classical analog of the Casimir effect. Such cosmology corresponds to a
time-dependent dark fluid model or, alternatively, to its scalar field
presentation, and it motivated by the string landscape picture. The future
evolution of the several dark energy models constructed within the scheme is
carefully investigated. It turns out to be almost always possible to choose the
parameters in the models so that they match the most recent and accurate
astronomical values. To this end, several universes are presented which mimick
(multiple) CDM cosmology but exhibit Little Rip, asymptotically de
Sitter, or Type I, II, III, and IV finite-time singularity behavior in the far
future, with disintegration of all bound objects in the cases of Big Rip,
Little Rip and Pseudo-Rip cosmologies.Comment: LaTeX 11 pages, 10 figure
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
The generalized second law of gravitational thermodynamics on the apparent horizon in f(R)-gravity
We investigate the generalized second law (GSL) of thermodynamics in the
framework of -gravity. We consider a FRW universe filled only with
ordinary matter enclosed by the dynamical apparent horizon with the Hawking
temperature. For a viable modified gravity model as , we examine the validity of the GSL during the early inflation and late
acceleration eras. Our results show that for the selected -gravity model
minimally coupled with matter, the GSL in the early inflation epoch is
satisfied only for the special range of the equation of state parameter of the
matter. But in the late acceleration regime, the GSL is always respected.Comment: 10 pages, accepted by Europhys. Lett. 201
Modified gravity with R-matter couplings and (non-)geodesic motion
We consider alternative theories of gravity with a direct coupling between
matter and the Ricci scalar We study the relation between these theories and
ordinary scalar-tensor gravity, or scalar-tensor theories which include
non-standard couplings between the scalar and matter. We then analyze the
motion of matter in such theories, its implications for the Equivalence
Principle, and the recent claim that they can alleviate the dark matter problem
in galaxies.Comment: typos corrected, minor changes, version published in CQ