12 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
Consistent perturbations in an imperfect fluid
We present a new prescription for analysing cosmological perturbations in a
more-general class of scalar-field dark-energy models where the energy-momentum
tensor has an imperfect-fluid form. This class includes Brans-Dicke models,
f(R) gravity, theories with kinetic gravity braiding and generalised galileons.
We employ the intuitive language of fluids, allowing us to explicitly maintain
a dependence on physical and potentially measurable properties. We demonstrate
that hydrodynamics is not always a valid description for describing
cosmological perturbations in general scalar-field theories and present a
consistent alternative that nonetheless utilises the fluid language. We apply
this approach explicitly to a worked example: k-essence non-minimally coupled
to gravity. This is the simplest case which captures the essential new features
of these imperfect-fluid models. We demonstrate the generic existence of a new
scale separating regimes where the fluid is perfect and imperfect. We obtain
the equations for the evolution of dark-energy density perturbations in both
these regimes. The model also features two other known scales: the Compton
scale related to the breaking of shift symmetry and the Jeans scale which we
show is determined by the speed of propagation of small scalar-field
perturbations, i.e. causality, as opposed to the frequently used definition of
the ratio of the pressure and energy-density perturbations.Comment: 40 pages plus appendices. v2 reflects version accepted for
publication in JCAP (new summary of notation, extra commentary on choice of
gauge and frame, extra references to literature
Curing singularities in cosmological evolution of F(R) gravity
We study modified gravity models which are capable of driving the
accelerating epoch of the Universe at the present time whilst not destroying
the standard Big Bang and inflationary cosmology. Recent studies have shown
that a weak curvature singularity with can arise generically in
viable models of present dark energy (DE) signaling an internal
incompleteness of these models. In this work we study how this problem is cured
by adding a quadratic correction with a sufficiently small coefficient to the
function at large curvatures. At the same time, this correction
eliminates two more serious problems of previously constructed viable DE
models: unboundedness of the mass of a scalar particle (scalaron) arising in
gravity and the scalaron overabundance problem. Such carefully
constructed models can also yield both an early time inflationary epoch and a
late time de Sitter phase with vastly different values of . The reheating
epoch in these combined models of primordial and present dark energy is
completely different from that of the old inflationary
model, mainly due to the fact that values of the effective gravitational
constant at low and intermediate curvatures are different for positive and
negative . This changes the number of e-folds during the observable part of
inflation that results in a different value of the primordial power spectrum
index.Comment: Discussion expanded, references added, results unchanged, accepted
for publication in JCAP. A minor typo in Eq. (2.14) has been correcte
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