10 research outputs found
Stealth Acceleration and Modified Gravity
We show how to construct consistent braneworld models which exhibit late time
acceleration. Unlike self-acceleration, which has a de Sitter vacuum state, our
models have the standard Minkowski vacuum and accelerate only in the presence
of matter, which we dub ``stealth-acceleration''. We use an effective action
for the brane which includes an induced gravity term, and allow for an
asymmetric set-up. We study the linear stability of flat brane vacua and find
the regions of parameter space where the set-up is stable. The 4-dimensional
graviton is only quasi-localised in this set-up and as a result gravity is
modified at late times. One of the two regions is strongly coupled and the
scalar mode is eaten up by an extra symmetry that arises in this limit. Having
filtered the well-defined theories we then focus on their cosmology. When the
graviton is quasi-localised we find two main examples of acceleration. In each
case, we provide an illustrative model and compare it to LambdaCDM.Comment: 32 pages, 5 figure
Second-order matter density perturbations and skewness in scalar-tensor modified gravity models
We study second-order cosmological perturbations in scalar-tensor models of
dark energy that satisfy local gravity constraints, including f(R) gravity. We
derive equations for matter fluctuations under a sub-horizon approximation and
clarify conditions under which first-order perturbations in the scalar field
can be neglected relative to second-order matter and velocity perturbations. We
also compute the skewness of the matter density distribution and find that the
difference from the LCDM model is only less than a few percent even if the
growth rate of first-order perturbations is significantly different from that
in the LCDM model. This shows that the skewness provides a model-independent
test for the picture of gravitational instability from Gaussian initial
perturbations including scalar-tensor modified gravity models.Comment: 15 pages, 1 figure, version to appear in JCA
Low energy effective theory on a regularized brane in 6D gauged chiral supergravity
We derive the low energy effective theory on a brane in six-dimensional
chiral supergravity. The conical 3-brane singularities are resolved by
introducing cylindrical codimension one 4-branes whose interiors are capped by
a regular spacetime. The effective theory is described by the Brans-Dicke (BD)
theory with the BD parameter given by . The BD field is
originated from a modulus which is associated with the scaling symmetry of the
system. If the dilaton potentials on the branes preserve the scaling symmetry,
the scalar field has an exponential potential in the Einstein frame. We show
that the time dependent solutions driven by the modulus in the four-dimensional
effective theory can be lifted up to the six-dimensional exact solutions found
in the literature. Based on the effective theory, we discuss a possible way to
stabilize the modulus to recover standard cosmology and also study the
implication for the cosmological constant problem.Comment: 12 pages, 1 figur
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