15 research outputs found
Black hole and de Sitter solutions in a covariant renormalizable field theory of gravity
It is shown that Schwarzschild black hole and de Sitter solutions exist as
exact solutions of a recently proposed relativistic covariant formulation of
(power-counting) renormalizable gravity with a fluid. The formulation without a
fluid is also presented here. The stability of the solutions is studied and
their corresponding entropies are computed, by using the covariant Wald method.
The area law is shown to hold both for the Schwarzschild and for the de Sitter
solutions found, confirming that, for the case, one is dealing with a
minimal modification of GR.Comment: 7 paages, latex fil
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
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
Topological Static Spherically Symmetric vacuum Solutions in Gravity
The Lagrangian derivation of the Equations of Motion for topological static
spherically symmetric metrics in -modified gravity is
presented and the related solutions are discussed. In particular, a new
topological solution for the model is found. The
black hole solutions and the First Law of thermodynamic are analyzed.
Furthermore, the coupling with electromagnetic field is also considered and a
Maxwell solution is derived.Comment: 12 pages, minor corrections, published versio
Static Spherically Symmetric Solutions in F(R) Gravity
A Lagrangian derivation of the Equation of Motion (EOM) for static
spherically symmetric metrics in F(R) modified gravity is presented. For a
large class of metrics, our approach permits to reduce the EOM to a single
equation and we show how it is possible to construct exact solutions in
-gravity. All known exact solutions are recovered. We also exibit a new
non trivial solution with non constant Ricci scalar.Comment: 8 pages, published version, some references added, a minor
modificatio
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
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